Networks and Protocols CE00997-3

Week 7a

Network technologies

1st & 2nd generationGSM

First Generation

•1st Generation devices– Introduced in the UK by Vodafone and Cellnet

• January 1985

– UK Technology (and Italy)• Total Access Cellular System (TACS)

– This was based on the American design of AMPS

• Used the 900MHz frequency range

– Europe• Germany adopted C-net• France adopted Nordic Mobile Telephone (NMT)

•Operates–Frequency Division Multiple Access (FDMA)

• Covered in next slide–Operates in the 900MHz frequency range

• Three parts to the communications– Voice channels– Paging Channels– Control Channels

First Generation

•FDMA– Breaks up the available frequency into 25 KHz channels

• Allocates a single channel to each phone call• The channel is agreed with the Base station before transmission

takes place on agreed and reserved channel• Separate channels are allocated for uplink and downlink

– This means no sharing of the medium is required• The device can then transmit on this channel

– No other device can share this channel even if the person is not talking at the time!

– A different channel is required to receive• The voice/sound is transmitted as analogue data, which means that

a large than required channel has to be allocated.

First Generation

•Voice calls– Are transferred using Frequency modulation– The rate at which the carrier wave undulates is changed

• Encoding information• More resistant to interference than AM radio

(www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0030280.html, 2004)

First Generation

•Each of the mobile devices need to operate on a unique frequency– This is given to the devices by the base station when communications

are initially requested– The base station will give the phone a frequency in the range

• 890-915 MHz uplink• 935-960 MHz downlink• The downlink will then be allocated by the mobile device by

adding 45Mhz to the uplink• So

– 890 MHz uplink will be 935 MHz downlink

First Generation

•Three kinds of channels for communications

– Fixed channels (always the same)• Paging Channels

– Constant transmission by the BS– Incoming Call Signal– The device monitors this to see if another BS has a stronger signal

» If it does a handover takes place• Control Channels

– Information sent over this link would include» Device wishes to make a call» Carry out a hand over» Frequency to communicate upon

– Dynamic channels• Voice/traffic channels

– These are allocated as discussed previously by the BS as required– If a channel is not available the phone will wait a random time interval and try again

First Generation

•1G infrastructure

Mobile Switching Centre

PSTN

First Generation

FMC – First Generation

• Infrastructure– Base Station

• Carries out the actual radio communications with the device

• Sends out paging and control signals

– MSC• Takes responsibility

– Controls all calls attached to this device– Maintains billing information– Switches calls (Handover)

•Cellular Architecture continued–Cellular architecture requires the available frequency to be distributed between the cells

• If 2 cells next to each other used the same frequency each would interfere with each other

Cell

Cell Cell

Cell

Cell

Frequency 900

First Generation

•Cellular Architecture continued–There must be a distance between adjoining cells–This distance allows communications to take place

Cell

Cell Cell

Cell

Cell

Cell

Cell

Cell Frequency 900

Frequency 920

Frequency 940

Frequency 960

First Generation

•Radio Planning–Logically we picture a cell as being a Hexagon

• In reality the shape of a transmission will change depending on the environment

• In this diagram of a cell you can see this– The building are the rectangles in dark green– The darker the shade of green the stronger

the signal

Cell

Cell Cell

Cell

Cell

First Generation

– First Generation

• Radio Planning– Planning needs careful thought– You must cover the entire area with the minimum of base stations

• Base stations cost the company money• They also make the potential for radio problems greater

– Simulations can be used but accurate models of the area is required• Best solution is to measure the signals at various points

– From this a decision can be made

Cell

Cell Cell

Cell

– First Generation

• Cellular infrastructure why ??– Cells with different frequencies allow devices to

move between these cells• The device just informing what frequency they are

communicating at

– Cellular communications can only travel a certain distance• Cell sizes are flexible

– Examples in the TUK TACS system were up to 50 Miles!

– First Generation

• Cellular infrastructure– Once you get to the ‘edge’ of a cell you will need a

handover• Handover allows the user to move between cells

– After a certain distance the amount of data which is sent in error becomes greater than the data sent correctly at this point you need to connect to a new cell which is closer.

– TACS carries this out by monitoring the amplitude of the voice signal

– First Generation

• Cellular infrastructure– Communicating with BS1

• Moving towards BS2

BS2BS1

Transmission BS2Transmission BS1

– First Generation

• Cellular infrastructure– Power of signal now weakening from BS1

BS2BS1

– First Generation

• Cellular infrastructure– Signal stronger so hand over to the new BS2

BS2BS1

– First Generation

• Handover– Once a handover is decided upon by the BS

• The MSC is informed– All BS in the area of the current location are informed to start

paging the device– The BS with the strongest signal is then handed over to– The call can continue– In reality a lot of calls were dropped whilst waiting for a handover

to take place

– Ending a call• A 8Khz tone is sent for 1.8 seconds

– The phone then returns to an idle state

•TACS (Total Access Communication System)– Problems

• Roaming was not applicable outside of the UK– All of Europe was using different standards

» Different frequencies» Different frequency spacing» Different encoding technologies

• Security– Calls were easily ‘listened’ upon– Limited capacity of the available spectrum– Analogue signal meant a larger than required amount of the frequency had to be allocated to

each call– Expansion of the network was difficult

• From the point of view of this course – there was very little computing capabilities within the devices

– Simply put the electronics were not readily and cheaply available in addition there was not the demand for computing services

– TACS was designed for voice services alone– In 1985 there were 1,961 hosts on the Internet (http://www.isc.org/index.pl?/ops/ds/host-count-history.php)

• This was unacceptable– GSM was introduced

First Generation

Second Generation

•Why was 2G needed– The number of phones & users increased

• The user base which was originally mostly business related started to change to include other groups

– This increased the number of users which were on the networks– Became a supply and demand problem

» If current providers could not provide for the demand users would move away from the technology.

– As the user base increased in size users in peak areas would find that they could not connect to the network

•Second Generation (2G) Technology– Mobile phones became popular and requirements

changed• Users wanted more from the phones

– The frequency for the phones to use was limited and better use of this frequency was required

– Guarantee that a call was possible when needed– Privacy was needed as the phones may be used for business or

personal conversations.– The phones needed to be smaller for ease of carrying– Improved battery life

Second Generation

•Global System for Mobile Communications (GSM)– 1982 the European Commission requested that 900

MHz be reserved for the use of GSM• Before the use of TACS and NMT !!!!

– 1989 ETSI defined the standard which was GSM– European Telecommunication Standards Institute (ETSI)

• Originally called “Groupe Spéciale Mobile” later changed to English

Second Generation

•There was a recognition of the problems with 1G and compatibility issues

– To deal with this 2 approaches were taken• Force adoption of a standard

– European Commission decided that all member countries should use one system GSM

• Market forces dictating– United States of America adopted this approach

» In the US the 2G system had to use the same spectrum as 1G» AMPS – became DAMPS – Digital AMPS» Code division Multiple access (CDMA) (also know as IS-95) was

introduced

Second Generation

– 2nd Generation World

• GSM due to its standards based approach– Now dominates the cellular market– 80.79% of all mobile phones operate on the GSM standards

• Just under 3 Billion users– http://www.gsmworld.com/news/statistics/pdf/gsma_stats_q2_08.pdf – Although a slow down is now being seen in the number of phones being sold

» http://www.vnunet.com/vnunet/news/2226669/per-cent-world-mobile-2011

» This is most likely due to saturation of the market– In some countries there are now more phones than people– Users having more than one phone for each of there roles– http://www.guardian.co.uk/world/2008/feb/27/nokia.mobilephones

» More phones than people in the UK and Italy

– 2nd Generation World

http://www.gsmworld.com/news/statistics/pdf/gsma_stats_q2_08.pdf

• Subscriber statistics end Q2 2008– World 3,665,389,343– cdmaOne 7,919,081 0.22%– CDMA2000 1X 292,479,194 7.98%– CDMA2000 1xEV DO ‐ 101,171,640 2.76%– CDMA2000 1xEV DO Rev. A ‐ 2,349,544 0.06%– GSM 2,961,292,242 80.79%– WCDMA 216,434,662 5.90%– WCDMA HSPA 43,416,405 1.18%– TDMA 3,042,671 0.08%– PDC 9,150,409 0.25%– iDEN 27,610,798 0.75%– Analog 522,697 0.01%

•GSM Services– Intended primarily as a voice service

• Although now we can see the benefit of data, the increasing use of voice calls was the issue addressed by this standard

• GSM does support some limited data services– Three services are offered by GSM

• Teleservices• Bearer Services• Supplementary Service

Second Generation

•Teleservices– Telephony

• Codecs for voice and data• Encryption of voice data

– Emergency Calls• These calls must have the highest priority• Directing you to the nearest connection point

– Voice Mail– Fax Mail– Facsimile Group 3– Short Messaging Service (SMS)

• Including the SMS – Cell Broadcast (SMS-CB)

Second Generation

•Bearer Services– Bearer services are for the transport of data

• 9600 bits per second maximum– Synchronous and asynchronous– Alternate data and speech

• Transparent – Fixed delay in data transfer– No guarantees of data integrity

• Non transparent– Variable Delay– Guaranteed data integrity via Automatic repeat request (ARQ)

Second Generation

•GSM Operates– Using Time Division Multiplex Access (TDMA) & FDMA

• This allow the frequency to be broken up into slots– The frequencies used are GSM 900 , GSM 1800 and GSM 1900

» Possible selling off though of the 900 Mhz range in the uk» http://www.theinquirer.net/en/inquirer/news/2007/09/21/ofcom-wants-to-sell-900-mhz-bandwidth

– Separate frequencies are used for the uplink and downlink» 890-915MHz uplink, 935-960MHz downlink for example» Uplink and Downlink are 45 MHz apart

– 200KHz spacing on the frequency– 124 pairs of channels – These channels are then divided into 8 time slices

» For GSM each slice is 577 ms (0.000577 seconds)– Total channels available is 8 * 124 = 992 (in a theoretical context)

• This means that there is eight times the capacity as before in one part of the frequency

– Assuming the same rate is used

Second Generation

•TDMA & FDMA

Frequency

Time

Frequencies890 MHz - 960 MHz – Europe1710 – 1880 MHz - Europe1850 MHz – 1950 MHz - America

Second Generation

Second Generation• TDMA & FDMA

– Same diagram in a different view– Four devices all communicating at an exact time

• All of the devices will act as though they are the only device transmitting/receiving

Frequency – Uplink

Time

Second Generation• GSM Operates

– One of the features which allowed the increase in the number of users is digitisation of voice• The voice is sampled using a (analogue to Digital Converter) ADC

– 8KHz / second, with an 8 bit result• The voice information is then transferred using one of these methods

– Enhanced Full Rate» Best quality speech» Uses a full slot» Marginal increase in processor overhead

– Full Rate» Improved speech quality» Takes a full slot to transfer

– Half Rate» This mode allows for a doubling of capacity to a base station» Loss in speech quality, but not significant» Half a slot to transfer

Second Generation

• Analogue to Digital Convertor (ADC)– Regular samples are taken from the analogue signal

and converted into a digital value• The more samples you take with a higher level of precision

the more accurate the representation of the original signal– 8Khz with a 8 bit result

» 8000 samples taken per second with a value of 0 to 255255

0Time

Second Generation

• Digitisation– Of voice allows for a number of features

• Takes less bandwidth to transfer the voice• The data once digital

– Can be encrypted– Can be recovered even with the attenuation effects– Additional bits can be added to allow for errors within transport

» Without the need for retransmission – Forward Error Checking (FEC)

– The digital voice data can then be place inside of a packet for transport

Time Division Multiple Access (TDMA)

Guard Time: Interval between bursts used to avoid overlappingPreamble: First part of the burstMessage: Part of burst that includes user data Postamble: Last part of burst – used to initialise following burst

Slot 1 Slot 2 Slot 8 Slot 8Slot 1

Frame 1 Frame 26

Multiframe

Guard Time Preamble Message Postamble Guard Time

Slot

Slot i ……..

GSM System – Multiple Access

•Subscriber Identity Module (SIM) Card– Essential for the GSM network– The SIM is located within the Mobile station (MS)

• Basically the phone handset you use– Contains

• Subscriber Authentication key– 128 bit encryption key

• International Mobile Subscriber Identity (IMSI)– A unique international none dialable number which is attached to your device

• Temporary Mobile Subscriber Identity • Mobile Station Integrated Service Digital Network (MSISDN)

– Basically this is YOUR phone number• PIN to secure the card• SMS messages• Personal data, phone numbers, Phone settings etc

Second Generation

•SIM continued– Each one is unique

• e-commerce– Purchasing items from your phone, with certainty that your device bought

the item• Authentication encryption is used called the A3/A8

– This is used to authenticate your device using challenge/response» A random number is sent» The A3/A8 algorithm then works on the number and returns a 32-bit

response.» If this matches the one which the network has calculated the device is

authenticated• A5 Encryption is used for the voice calls

– http://www.gsmworld.com/using/algorithms/index.shtml • Using your mobile as an electronic wallet

– http://technology.timesonline.co.uk/tol/news/tech_and_web/the_web/article2631026.ece

Second Generation

LocationGSM consists of three major systems:

Base-Station System (BSS) Network Switiching Sub-System (NSS) Operation and Support System (OSS)

The Switching System performs call processing and subscriber related functions

The system contains the following functional units Home Location Register (HLR) Mobile Switching Center (MSC) Visitor Location Register (VLR) Authentication Center (AUC) Equipment Identity Register (EIR)

BSS Base Station System (BSS)

This is the part of the network which connects the MS to the network via the radio interface

Base Station (BS) Know as (Base Transceiver Station (BTS) ) in official GSM

documentation This is the radio transceiver the area of transmission is referred

to as the cell

BS

Theory

BS

Reality

Transmission distance

Second Generation

•Base station controller– This node takes overall control of a number of base

stations a point of control for the BS• Other functionality

– Traffic concentration– Switching operations

» Data based and voice– Network management to all BS attached– Radio channel management for all BS attached– Conversion of air interface rates to network applicable rates

» 13 Kbps to 64 Kbps– Handover control

» This is for base stations which it has control of

BSS Base station Controller

BS

BS

BS

BS

Base Station Controller

GSM - NSS•Network Switching Sub-System (NSS)

– Responsible for the transfer of information from the handsets to the correct part of the infrastructure

– Functions• Handovers between BSC’s• Allow communication between mobile and PSTN networks• Allows for roaming between networks• A number of nodes are a part of the NSS

•NSS nodes consist of– Mobile Switching Centre (MSC)– Home Location Register (HLR)– Visitor Location Register (VLR)– Authentication Centre (AuC)– Equipment Identity Register (EIR)

GSM - NSS•HLR is the most important database

– Storage and management of subscriptions– Permanent data includes:

• Subscriber's service profile • Subscriber's location information• Subscriber‘s activity status

– Subscribing to a particular provider‘s service registers you in the HLR of that provider

•The MSC performs the telephony switching functions of the network– Controls call to and from other telephone and data systems

• Also performs functions such as – Toll ticketing– Network interfacing– Common Channel signalling

GSM - NSS•VLR contains data on visiting (roaming) subscribers

– Integrated with the MSC– When a roamer enters the service area the VLR queries the appropriate HLR– If a roamer makes a call the VLR will already have the information it needs for call

setup •The AUC verifies the identity of the user and ensures and ensures the confidentiality of each call

– By provide authenticity and encryption parameters for every call– Protects network operators from fraud– Assures a certain level of security for the content of each call

•The EIR is a database that includes info solely about the identity mobile equipment – Prevents calls from stolen, unauthorised or defective mobile devices

• Since 2002 all UK based operators have shared the contents of the EIR database• This is also true for countries which have roaming agreements with UK operators• If a phone is stolen anywhere in the world, it can now be registered as stolen and traced

within 24 hours

GSM - OSS

•Operation and Support System– This part of the network allows monitoring of the

network– Ensures the smooth running and involves management

tasks• Accounting and Billing• Status Reports• Traffic monitoring• In addition if a BSC can not carry out all of its tasks the OSS

will take control of the BSC’s tasks– The OSS is logically connected to all equipment in the

infrastructure

•GSM Infrastructure–The specifications created by ETSI do not require any of the following infrastructure but recommends it !

• All mobile operators have it VLRMSC

VLR MSC

HLR

MSC Mobile Switching Center

VLR Visitor Location Register

HLR Home Location Register

OSS

AUC

Second Generation

GSM

•The infrastructure which has been discussed– Is ever expanding– As new functionality is required this is built into the

infrastructure• This gives GSM the ability to remain future proof without

large investment in new infrastructure when new technology becomes available

• This will be seen more in the future lectures when we can see that new technology uses GSM as a basis and essential part

– Even for 3G/4G communications

GSM Network Features

Roaming

•Major feature of GSM is automatic world wide location of users using the same phone number•HLR always contains data about MS location•As soon as user moves location, HLR transmits data to appropriate VLR•You are only roaming if you connect to a network which is not your own via the VLR

– If you move around for example your own country and are always connected to your network this is not roaming, this is simply handover

Roaming•To locate MS requires

– Mobile Station ISDN number (MSISDN)• Consists of

– country code (CC)– National Destination Code (NDC) usually the number of the network

provider– Subscriber number (SN) the phone number allocated to the SIM

– International Mobile Subscriber Identity (IMSI)• Consists of

– Mobile country code (MCC)– Mobile network code (MNC)– Mobile Subscriber Identification Number (MSIN)

Roaming•To locate MS requires

– Temporary Mobile Subscriber Identity (TMSI)• Used to hide the IMSI over the air interface to

protect their identity– Mobile Station Roaming Number (MSRN)

• Temporary address generated by the VLR containing the Visitor country code (VCC) and Visitor National Destination Code (VNDC)

•Access to this internal network operator data is restricted

• Only operators with roaming agreements provide access to the data

Handover

•Crossing from one cell to another requires that the network update user location data, etc. Process is called handover•GSM aims at maximum handover duration of 60ms•Two primary reasons for handover

– Network cannot guarantee QOS due to distance from current BTS

– Loading on one BTS may necessitate transfer to another, load balancing

Handover

•BTS and MS perform periodic tests on the quality of uplink & downlink (approx every 0.5s)•The values are compared to a handover margin (HO_MARGIN)•Dependent upon difference between the current value and the HO_MARGIN handover decision is made by the BSC

– MSC is notified and it manages the connection to the new BSC/BTS– MS has to drop existing connection once new one is established– This is an example of a hard handover

• If terminal makes connection QoS maintained• If not connection is lost