Week4 Handout

University of Technology, Sydney 31275 Mobile Networking

Week 4: GSM

Dr Peter Leijdekkers 4.1

Peter Leijdekkers UTS – Mobile Networking (31275) 4.1

Overview of GSM

Week 4

Peter Leijdekkers


Influence of mobile communication to the layer model

service location

new applications, multimedia

adaptive applications

congestion and flow control

quality of service

addressing, routing, device location

hand-over

SDMA, FDMA, TDMA, Aloha

Reservation schemes

Collision avoidance, MACA

Polling

Frequencies

Signals

Antenna

Signal propagation

Multiplexing Spread spectrum

Application layer

Transport layer

Network layer

Data link layer

Physical layer


Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


Week 4: GSM



3G wireless Standards

3rd Generation mobile systems

3G: Wireless Wide Area Networks (WWANs) are the broadest range wireless networks. Most widely deployed today in the cellular voice infrastructure - also have the ability to transmit data. Includes UMTS (Universal Mobile Telecommunications Service), EV-DO Evolution-Data Optimized wireless high-speed data network. Applications: Multimedia such as full-motion video, video conferencing and Internet access. High-speed data transmissions of 144Kbps and higher (2Mbit/s).

http://www.youtube.com/watch?v=hX0i6pXzsLE&NR=1

5


Performance characteristics of GSM (wrt. analog sys.)

Communication

mobile, wireless communication; support for voice and data services

Total mobility

international access, chip-card enables use of access points of different providers

Worldwide connectivity

one number, the network handles localization

High capacity

better frequency efficiency, smaller cells, more customers per cell

High transmission quality

high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains)

Security functions

access control, authentication via chip-card and PIN

http://www.youtube.com/watch?v=hX0i6pXzsLE&NR=1


Week 4: GSM



Disadvantages of GSM

There is no perfect system!!

no end-to-end encryption of user data

no full ISDN bandwidth of 64 kbit/s to the user

reduced concentration while driving

abuse of private data possible

roaming profiles accessible

high complexity of the system

several incompatibilities within the GSM standards

limited access to American market


Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


Mobile Services

GSM offers

several types of connections

voice connections, data connections, short message service

multi-service options (combination of basic services)

Three service domains

Bearer Services

Telematic Services

Supplementary Services

GSM-PLMN

transit

network

(PSTN, ISDN)

source/

destination

network

TE TE

bearer services

tele services

R, S (U, S, R)Um

MT

MS

TE: Terminal MT: Mobile Termination PSTN: Public Switched Telephone Network

MS: Mobile Station PLMN: Public Land Mobile network ISDN: Integrated Services Digital Network


Week 4: GSM



Bearer Services

Telecommunication services to transfer data between access points

Specification of services up to the terminal interface (OSI layers 1-3)

Different data rates for voice and data (original standard)

data service (circuit switched)

synchronous: 2.4, 4.8 or 9.6 kbit/s

asynchronous: 300 - 1200 bit/s

data service (packet switched)

synchronous: 2.4, 4.8 or 9.6 kbit/s

asynchronous: 300 - 9600 bit/s

Today: data rates of approx. 50 kbit/s possible


Tele Services I

Telecommunication services that enable voice communication via mobile phones

All these basic services have to obey cellular functions, security measurements etc.

Offered services

mobile telephonyprimary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz

Emergency numbercommon number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)


Tele Services II

Non-Voice-Teleservices

group 3 fax

voice mailbox (implemented in the fixed network supporting the mobile terminals)

electronic mail (MHS, Message Handling System, implemented in the fixed network)

...

Short Message Service (SMS)alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS

Multimedia Message Service (MMS)


Week 4: GSM



Supplementary services

Similar to ISDN services besides lower bandwidth due to the radio link

May differ between different service providers, countries and protocol versions

Important services

User Identification

Call Redirection or forwarding

suppression of number forwarding

automatic call-back

conferencing with up to 7 participants

locking of the mobile terminal (incoming or outgoing calls)

...


Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


Cellular Network Reference Model

BTS: Base Transceiver Station BSC: Base Station ControllerMS: Mobile Station AC: Authentication Centre EIR: Equipment Identity Register HLR: Home Location Register VLR: Visitor Location Register

MSC: Mobile Services Switching Center EIR: Equipment Identity RegisterAC: Authentication Centre OMC: Operation & Maintenance Center

MSCHLR

VLR

MS

VLR

EIR

AC

MSC PSTN ISDN

UmA

B

C

D

G

H

E

AiDi

F

NSS/OSS (Network & Switching Subsystem)

OMC

BTS

BTS

BSC


Week 4: GSM



NNS: Network and Switching subsystem

NSS: the main component of the public mobile network GSM Connects wireless network to fixed networks Performs handovers between different BSCs Functions to worldwide localisation of users Charging, accounting functions Roaming of users between different providers in different countries

Mobile Services Switching Center (MSC)

High performance digital ISDN switches Manages several BSCs in a geographical region Connects to other networks (PSTN,ISDN,X25) Uses standard signalling system SS7

Databases (important: scalability, high capacity, low delay) Home Location Register (HLR)

central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)

Visitor Location Register (VLR)local database for a subset of user data, including data about all user currently in the domain of the VLR. Each MSC has a VLR database


Mobile Services Switching Center (MSC)

The MSC plays a central role in GSM

switching functions

additional functions for mobility support

management of network resources

interworking functions via Gateway MSC (GMSC)

integration of several databases

Functions of a MSC

specific functions for paging and call forwarding

termination of SS7 (signaling system no. 7)

mobility specific signaling

location registration and forwarding of location information

provision of new services (fax, data calls)

support of short message service (SMS)

generation and forwarding of accounting and billing information


Operation subsystem

The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems Communication with other components via SS7 signaling

Authentication Center (AC) generates user specific authentication parameters on request of a VLR authentication parameters used for authentication of mobile terminals and

encryption of user data on the air interface within the GSM system

Equipment Identity Register (EIR) registers GSM mobile stations and user rights stolen or malfunctioning mobile stations can be locked and sometimes even

localized

Operation and Maintenance Center (OMC) traffic monitoring status reports of network entities accounting and billing …


Week 4: GSM



Base Station System and Mobile Station

MS

Um

MSC

A

BSC

MS

UmAbis

Abis

Base Station System

BTS

BTS

Radio cell

Radio cell

Base Transceiver Station (BTS):

• radio components (sender, receiver,

antenna)

• if directed antennas are used one BTS can

cover several cells

Base Station Controller (BSC):

• switching between BTSs,

• controlling BTSs,

• managing of network resources,

• mapping of radio channels (Um) onto

terrestrial channels (A interface)

• Handles handover between BTSs

• Paging of MS


possible radio coverage of the cell

idealized shape of the cellcell

segmentation of the area into cells

Radio Cells

use of several carrier frequencies

not the same frequency in adjoining cells

cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc.

hexagonal shape of cells is idealized (cells overlap, shapes depend on geography)

if a mobile user changes cells handover of the connection to the neighbor cell


Mobile Station (MS)

Terminal for the use of GSM services

A mobile station (MS) comprises several functional groups

MT (Mobile Terminal):

offers common functions used by all services the MS offers

corresponds to the network termination (NT) of an ISDN access

end-point of the radio interface (Um)

TA (Terminal Adapter):

terminal adaptation, hides radio specific characteristics

TE (Terminal Equipment):

peripheral device of the MS, offers services to a user

does not contain GSM specific functions

R SUm

TE TA MT


Week 4: GSM



Identifiers Used in GSM

SIM (Subscriber Identity Module) personalization of the mobile terminal, stores user parameters

IMSI (international mobile subscriber identification) IMSI identifies a subscriber worldwide. Mobile country code + mobile network node + ms identification nr E.g 240 + 23415 + 123456

TMSI (temporary mobile subscriber identification): TMSI identifies a subscriber. Used over the air only to hide identity of the user. Used by VLR is only valid for a certain period

MSISDN (mobile station ISDN number) MSISDN is a dialable number assigned to a given subscriber. +61 4 2456 1842

IMEI (international mobile station equipment identity) IMEI identifies a particular handset.

This is also called mobile identity number (MIN).


Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


The GSM Radio Interface

MOBILE

BASE TRANSCEIVER STATION


Week 4: GSM



Air Interface

Radio Transmission Techniques

FDMA

TDMA

CDMA

Channels

Physical channels

Logical channels


FDMA

Time

Frequency

Channel


TDMA

Time

Frequency

0 1 2 3 4 5 6 7

Channel

Time Slot


Week 4: GSM



GSM uses paired radio channels

TDMA

GSM900: 124 radio carriers, inter carrier spacing 200khz.

890 to 915mhz mobile to base - UPLINK

935 to 960mhz base to mobile - DOWNLINK

0 124 0 124

890MHz 915MHz 935MHz 960MHz


1 2 3 4 5 6 7 8

higher GSM frame structures

935-960 MHz

124 channels (200 kHz)

downlink

890-915 MHz

124 channels (200 kHz)

uplink

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguard

space S user data tailguard

space

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA


Burst and Frames

The information contained in one time slot on the TDMA frame is called a burst.

Five types of burst

Normal Burst(data transmission)

Frequency Correction Burst(correct local oscillator of MS to avoid interference with neighboring channels)

Synchronization Burst (training sequence to synchronize Ms with BTS)

Access Burst (initial connection setup between MS and BTS)

Dummy Burst (No Data is available for that slot)


Week 4: GSM



Channels

Physical Channels

Associated with frequency bands, time slots, codes

Physical channels transfer bits from one network element to another

Logical Channels

Distinguished by the nature of carried information and the way to assemble bits into data units

Three types

one-to-one: traffic channels between a BTS and a MS

one-to-many: synchronization signals from BTS to MSs in a cell

many-to-one: from MSs to the same BTS


Logical Channels

Control Channels

Broadcast Channels (BCH)

Common Control Channels (CCCH)

Dedicated Control Channels (DCCH)

Traffic Channels (TCH)

Full Rate (TCH/F)

Half Rate (TCH/H)


Control Channels

Broadcast Channels (BCH)

Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)

Broadcast Control Channel (BCCH)

Common Control Channels (CCCH)

Paging Channel (PCH)

Random Access Channel (RACH)

Access Grant Channel (AGCH)

Dedicated Control Channels (DCCH)

Stand alone Dedicated Control Channel (SDCCH)

Cell Broadcast Channel (CBCH)

Slow Associated Control Channel (SACCH)

Fast Associated Control Channel (FACCH)


Week 4: GSM



Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


Mobile Terminated Call (MTC)

PSTNcalling

stationGMSC

HLR VLR

BSSBSSBSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

1316

10 10

11 11 11

14 15

17

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR

6: forward responsible MSC to GMSC

7: forward call to

current MSC

8, 9: get current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

16, 17: set up connection

BSS: Base Station System MS: Mobile Station HLR: Home Location Register VLR: Visitor Location RegisterGMSC: Gateway Mobile Services Switching Center


Mobile Originated Call (MOC)

PSTN GMSC

VLR

BSS

MSC

MS1

2

6 5

3 4

9

10

7 8

1, 2: connection request

3, 4: security check

5-8: check resources (free circuit)

9-10: set up call


Week 4: GSM



MTC/MOC call setup

BTSMS

paging request

channel request

immediate assignment

paging response

authentication request

authentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acknowledge

data/speech exchange

BTSMS

channel request

immediate assignment

service request

authentication request

authentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acknowledge

data/speech exchange

Mobile Terminated Call Mobile Originated Call


Overview

Market

GSM Services

GSM components

Air Interface

Call Setup

Handover


4 types of handover

MSC MSC

BSC BSCBSC

BTS BTS BTSBTS

MS MS MS MS

12 3 4

1. Intra cell handover

2. Inter cell, intra BSC handover

3. Inter-BSC, intra MSC handover

4. Inter MSC handover


Week 4: GSM



Handover decision

receive level

BTSold

receive level

BTSold

MS MS

HO_MARGIN

BTSold BTSnew


Handover procedure

HO access

BTSold BSCnew

measurement

result

BSCold

Link establishment

MSCMSmeasurement

report

HO decision

HO required

BTSnew

HO request

resource allocation

ch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear command

clear complete clear complete

Week4 Handout

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