Download - GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

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Page 1: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM

bull What is GSM

bull Evolution of GSM

bull Fundamental of GSM

bull Architecture of GSM

bull Establishment of Network

bull Fundamental of Radio Propagation

bull Call flow

History of Wireless Communication

bull 1906mdashHuman voice was successfully transmitted over RADIO

bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio

bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved

audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over

200 million

GSM

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 2: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull What is GSM

bull Evolution of GSM

bull Fundamental of GSM

bull Architecture of GSM

bull Establishment of Network

bull Fundamental of Radio Propagation

bull Call flow

History of Wireless Communication

bull 1906mdashHuman voice was successfully transmitted over RADIO

bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio

bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved

audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over

200 million

GSM

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 3: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

History of Wireless Communication

bull 1906mdashHuman voice was successfully transmitted over RADIO

bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio

bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved

audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over

200 million

GSM

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 4: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull 1906mdashHuman voice was successfully transmitted over RADIO

bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio

bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved

audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over

200 million

GSM

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 5: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 6: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European

Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in

three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible

ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)

ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360

billionyear worldwide

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 7: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Performance characteristics of GSM

Communication mobile wireless communication support for voice and data

services

Total mobility international access Worldwide connectivity

High capacity better frequency efficiency smaller cells more customers per cell

High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone

calls at higher speeds (eg from cars trains)

Security functions access control authentication

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 8: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GENERATION OF GSM

bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT

bull 2nd generation- digital mobile technologies - GSM CDMA

bull 25generation- Enhancement of GSM- GPRS

bull 3rd generation- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 9: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

YEAR EVENT TOOK PLACE

1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 EEC supports GSM standards throughout community

1987 Digital Technology standards set for TDMA speech coding channel and modulation method

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install

system in 1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US amp Canada

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 10: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM

EDGE

25 G

GPRS

2G

25 g+

UMTS 3G WCDMA

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 11: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Evolution

1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 12: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM Evolution for Data Access

1997 2000 2003 2003+

GSM

GPRS

EDGE

UMTS

96 kbps

115 kbps

384 kbps

2 Mbps

GSM evolution 3G

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 13: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash

Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication

system)Uses WCDMA technologies Over Air interface (5MHz)

bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel

bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 14: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)

bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 15: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz

GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 16: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 17: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Spectrum

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 18: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz

UPLINK

DOWNLINK

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 19: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Technology

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 20: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

bull FDMA (Frequency division Multiple Access)

bull TDMA (Time division Multiple Access)

FDMA (Frequency division Multiple Access) In FDMA signals from various

users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 21: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Frequency multiplex

bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the

spectrum for the whole timebull Advantages

ndash no dynamic coordination necessary

ndash works also for analog signalsbull Disadvantages

ndash waste of bandwidth if the traffic is distributed unevenly

ndash inflexiblendash guard spaces

k2 k3 k4 k5 k6k1

f

t

c

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 22: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

TDMA (TIME DIVISION MULTIPLE ACCESS)

In a TDMA system data from each user is conveyed in time

intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels

Advantage of TDMA

1) TDMA permits a flexible bit rate

2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates

3) TDMA transmits each signal with sufficient guard time between time slots

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 23: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

f

t

c

k2 k3 k4 k5 k6k1

Time multiplexbull A channel gets the whole spectrum for a certain

amount of timebull Advantages

ndash only one carrier in themedium at any time

ndash throughput high even for many users

bull Disadvantagesndash precise

synchronization necessary

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 24: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

f

Time and Frequency Multiplex

bull Combination of both methodsbull A channel gets a certain frequency band for a certain

amount of time

t

c

k2 k3 k4 k5 k6k1

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 25: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

f

Time and Frequency Multiplexbull Example GSM bull Advantages

ndash Better protection against tapping

ndash Protection against frequency selective interference

ndash Higher data rates compared tocode multiplex

bull But precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC

[exchange] understands only 64kbps

NSS Functions (1)

bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after

the call is over

bull Mobility management ndash taking care of the location of the subscribers before

during and after a call

bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip

NSS Functions (2)

bull Transfer the acquired charging information to the

Billing centre

bull Signalling with other networks and BSS through

the different interfaces

bull Subscriber data handling

ndash Data storage permanently or temporarily in some

databases

MOBILE SWITCHING CENTER (MSC) -

Mobile services switching center (MSC)

performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs

Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station

and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities

HOME LOCATION REGISTER (HLR) -

The Home Location Register (HLR) contains

the identities of mobile subscribers their service parameters and their location information

Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station

VISITOR LOCATION REGISTER (VLR) -

The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

Functions of the VLR include -

bull Identity of mobile subscriber

bull Any temporary mobile subscriber identity (TMSI) allocation

bull Location area where the mobile station is registered

bull Copy of the subscriber data from the HLR

AUTHENTICATION CENTER (AUC) -

The Authentication center (AUC)

bull Contains subscriber authentication data called authentication key (Ki)

bull Generates security related parameters needed to authorize service using Ki

bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data

AUC Contdhellip

AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number

If result sent by MS is same as AUC Then MS is authenticated

EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is

accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent

This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under

observationbull Black or prohibited list ndash list of mobiles for which service is barred

OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance

Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS

IWF

bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include

bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]

OTHER NETWORK ELEMENTS

Other optional network elements that the MSC can interface include

Operations and Maintenance Support

Includes database management traffic metering and measurementmanndashmachine interface

Internetwork Interworking

Manages the interface between the GSM network and the PSTN

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections

Billing Center and Service Center

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)

CHANNELS

Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement

Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH

channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the

BTSrsquos BSICBCCH

Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero

Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)

Dedicated Control Channels (DCCH)

DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned

NMC

INTERFACE

Um INTERFACE (MS- BTS)

The interface between the MS and the BSS

A-BIS INTERFACE (BSC - BTS)

When the BSS consists of a Base Station Controller

(BSC) and one or more Base Transceiver Stations

(BTS) this interface is used between the BSC and

BTS to support the services offered to the GSM

users and subscribers The interface also allows

control of the radio equipment and radio frequency

allocation in the BTS

B INTERFACE (MSC - VLR)

The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired

C INTERFACE (HLR and the MSC)

The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber

D INTERFACE (HLR - VLR)

This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E INTERFACE (MSC ndash MSC)

When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC

F INTERFACE (MSC and EIR)

This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station

G INTERFACE (VLR ndash VLR)

When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR

H INTERFACE (HLR - AuC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized

GSM Voice amp Channel Coding

Here we will consider two forms of coding

techniques used within the GSM system

Firstly the process used to convert human

speech into a digital equivalent and

Secondly the coding processes for

compressing and protecting the data for

transmission over the air interface

GSM Voice amp Channel Coding Sequence

Speech Coding

bull GSM transmits using digital modulation - speech must be

converted to binary digits Coder and decoder must work to

the same standard Simplest coding scheme is Pulse Code

Modulation (PCM)

bull Sampling every 125 μs

bull Requires data rate of 64 kbps

bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors

bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors

The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading

Inter leaving

So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe

Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems

HANDOFF

bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained

Needs two mechanisms

1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements

1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel

Types of Handoff

042123 101

IV Handover

bull radio links are not permanently allocated for the duration of a call

bull Handover (handoff) switching of on-going call to a different channel

bull Radio Resource layer performs execution and measurements required for

handover

bull 4 types of handover in the GSM system

1 transferring a call between channels (time slots) in the same cell

2 transferring a call between cells (BTS) under the same BSC

3 transferring a call between BTS under different BSCs but same MSC

4 transferring a call between Cells under the control of different MSCs

Hard amp Soft handover

042123 103

internal handovers involve only one BSC

bull managed by the BSC without involving the MSC for efficiency

bull MSC is notified at the completion of the handover

external handovers are handled by the MSC(s) involved

bull original MSC (anchor) remains responsible for most call-related functions

bull new MSC (relay) handles subsequent inter-BSC handovers

MSC

BSC BSCMSC

BSC BSC

23 4

1

Call Flow

CALL MANAGEMENT

The Following Steps Took Place while a Mobile call is originated and Terminated-

bull Mobile-to-Land Scenario

bull Land-to-Mobile Scenario

bull Mobile-to-Mobile Scenario

Mobile-to-Land CALL Scenario

The following table lists the phases of a Mobile-to-Land call

STAGE DESCRIPTION

1 Request for services the MS requests to setup a call

2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated

3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity

5 Call Setup the MSC establishes a connection to the MS

6 Handover (s)

7 Call release the speech path is released

Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario

Land-to-Mobile CALL Scenario

The following table lists the phases of a Land-to-Mobile CALL Scenario

STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from

the HLR and VLR

2 Paging the MSC initiates a communication with the MS

3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated

4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered

5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation

6 Call setup the MSC establishes a connection to the MS

7 Handover (s)

8 Call release the speech path is released

Mobile-to-Mobile Scenario

Phases of a Mobile-to-Mobile Call

The Mobile-to-Mobile call phases can be sub divided in two parts

1 The Originating Mobile

2 The Terminating Mobile

ORIGINATING MOBILE

The phases of originating mobile arebull Request for services

bull Authentication (optional)

bull Ciphering (optional)

bull Call setup

bull Release

Mobile-to-Mobile Scenario

TERMINATNG MOBILE

The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release

  • GSM
  • Slide 2
  • History of Wireless Communication
  • Slide 4
  • GSM
  • Slide 6
  • Performance characteristics of GSM
  • GENERATION OF GSM
  • Slide 9
  • Slide 10
  • Slide 11
  • GSM Evolution for Data Access
  • Slide 13
  • Slide 14
  • GSM STANDARDS
  • Slide 16
  • Spectrum
  • Slide 18
  • Slide 19
  • Slide 20
  • GSM uses paired radio channels
  • Technology
  • Slide 23
  • Frequency multiplex
  • Slide 25
  • Time multiplex
  • Time and Frequency Multiplex
  • Slide 28
  • Various Access Method
  • Cells
  • Slide 31
  • Representation of Cells
  • Cell size and capacity
  • Cell structure
  • capacity
  • Slide 36
  • Sectorization
  • Slide 38
  • Slide 39
  • Slide 40
  • Slide 41
  • Slide 42
  • GSM Architecture
  • Slide 44
  • Slide 45
  • Slide 46
  • Mobile Station
  • Types of MS
  • SIM
  • Information stored in a SIM card (1)
  • BASE STATION SYSTEM (BSS) - (BSC+BTS)
  • BSS
  • Slide 53
  • Slide 54
  • Slide 55
  • BTS Scheme
  • BTS
  • BTS Functions
  • BSC Functions
  • Transcoder
  • NSS Functions (1)
  • NSS Functions (2)
  • Slide 63
  • Slide 64
  • Slide 65
  • Slide 66
  • IWF
  • OTHER NETWORK ELEMENTS
  • CHANNELS
  • Slide 71
  • Traffic Channels (TCH)
  • Broadcast Channels (BCH)
  • Common Control Channels (CCCH)
  • Dedicated Control Channels (DCCH)
  • NMC
  • Slide 77
  • INTERFACE
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82
  • Slide 83
  • Slide 84
  • Slide 85
  • GSM Voice amp Channel Coding
  • Slide 87
  • GSM Voice amp Channel Coding Sequence
  • Slide 89
  • Slide 90
  • Slide 91
  • Slide 92
  • Slide 93
  • Antenna types
  • Slide 95
  • Slide 96
  • Slide 97
  • Slide 98
  • Slide 99
  • Types of Handoff
  • Slide 101
  • Hard amp Soft handover
  • Slide 103
  • Slide 104
  • Slide 105
  • Slide 106
  • Call Flow
  • CALL MANAGEMENT
  • Slide 109
  • Slide 110
  • Mobile-to-Mobile Scenario
  • Slide 112
Page 26: GSM What is GSM. Evolution of GSM. Fundamental of GSM Architecture of GSM Establishment of Network Fundamental of Radio Propagation Call flow.

Various Access Method

CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments

bull Cell horizontal radius varies depending on

bull antenna height

bull antenna gain

bull geographical conditions(landscape)

bull propagation conditions from a couple of hundred meters to several tens of kilometers

bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

bull Capacity within cell limited by available bandwidth and operational requirements

bull Each network operator has to size cells to handle expected traffic demand

Cell structurebull Implements space division multiplex base station covers a certain

transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures

ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc

locallybull Problems

ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells

bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies

capacity

Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to

handle expected traffic demand

Sectorizationbull The problem with employing omni-directional cells (Radiate waves

to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand

bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell

bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller

area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo

bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs

GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10

TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits

MSISDN ndash Mobile Subscriber ISDN number

The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places

MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area

LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal

placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique

BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs

GSM Architecture

GSM Architecture

Mobile Station MOBILE STATION (MS) - The mobile station (MS)

represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI

The SIM may be a removable module while the equipment identity is not linked to a particular subscriber

Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for

multi-path distortion on the received signal

Types of MS

SIMbull It is basically

ndash a removable smart card in compliance with the ISO 7816 standard

ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod

bull It includes a microprocessor with all the subscriber-related information

bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal

Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it

Information stored in a SIM card (1)

bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering

information ndash A3 and A8 algorithmndash Ki Kc

bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)

BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components

1) The Base Transceiver Station ndash BTS

The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS

2) The Base Station Controller ndash BSC

The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs

3) The Transcoder ndash XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over

theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)

BSS

bull BSS includes the network elements taking care of the radio cellular resources within the GSM network

bull On one side it is directly linked to the MSs through the radio interface (Air interface)

bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and

hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)

Characteristics of the Base Station System (BSS) are

The BSS is responsible for communicating with mobile stations in cell areas

One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS

handover

- Each PLMN Should have at least one HLR

- One VLR is connected to several MSC but one MSC has one VLR

BSS

BTS Scheme

helliphellip

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mbs

TXTX RX

Signal Processi

ngTRXControll

erTRX

one TX antenn

aSplitter

TXTX RX

Signal Processin

gTRXControll

erTRX

two RX antenna

s

helliphelliphelliphelliphellip

BTSbull BTS is a network element with transmission

and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air

interface managementbull It can be considered as a complex radio

modem controlled by the BSCbull It is involved also in the transmission and

reception with the BSC through the A-bis interface

bull It has just executive functions (no management)

BTS Functionsbull Broadcastreceive tofrom the MS either

signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be

broadcastedreceived through the Air interface radio channel

bull Multiplex the information to be transmitted over each carrier

bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels

bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface

BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and

the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired

by the BTSs over the downlink and uplink channels

bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS

TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channe