2. GSM networks: services, architecture, functional

Mobile communications

2. GSM networks: services, architecture, functional

description

2.1 Introduction

❑initial vision was stated by GSM (Groupe Spéciale Mobile), a working group

established at Conférence Européenne des Administrations des Poste

et des Télécommunications (CEPT) -1982

❑later standardization was confined to a workgroup

SMG (Special Mobile Group) inside ETSI (European Telecommunications

Standards Institute)

-initial frequency bands were stated by the GSM working group:

890-915 MHz (uplink) and 935-960 MHz (downlink)

❑1991 GSM becomes Global System for Mobile Communications

includes both DCS1800 and PCS1900

❑major role in the standardization process :

GSM Memorandum of Understanding (MoU)

❑GSM standardization towards 3G is carried out by 3GPP – 3rd Generation

Partnership Project that unites several standard development organizations


❑the GSM standard is described in more than 6000 pages included

in technical specifications (recommendations)

Ex: GSM 05.03 –

channel coding on

the radio path,

GSM 11.10 – tests

on MS before

commercialization

http://www.3gpp.org


2.2 Categories of services. Standardization phases

Categories of services:

- phased approach: Phase 1 (1991), Phase 2 (1991-1995), Phase 2+

(specifications on yearly basis –releases Release 96, Release 97, Release 98)

-teleservices: no additional equipment besides a mobile station is needed

-bearer services: transport of digital data between standardized interfaces,

external equipment needed (laptop computer for example)


-external connections are provided to/from the following networks

- PSTN – Public Switched Telephone Network

- ISDN – Integrated Services Digital Network

- CSPDN – Circuit Switched Public Data Network – dedicated data

networks compliant with the International Telecommunication

Union –Telecommunications (ITU-T) X.21 standard

- PSPDN – Packet Switched Public Data Network – dedicated data

networks compliant with the ITU-T X.25 standard

- Internet (only through PSTN or ISDN)


-a network operator may opt to implement all bearer services or

only some of them – the 9.6 kbit/s asynchronous bearer service

the most popular

-supplementary services: additional features that are available or

not, depending on the operator’s choice to implement them, free of

charge or paid.

-except GPRS all of the GSM’s bearer service are based on circuit

switching CSD (circuit switched data)


Some GSM supplementary services:

- network improvement is carried out also using a phased

approach: CAMEL, SOR etc.


2.3 Mobility support in GSM networks

-GSM operates on a cellular structure

- GSM services are of circuit switched type: a voice or data call

uses dedicated resources till its completion

- a mobile station involved in an active call or using dedicated

resources for signaling purposes is said to be in dedicated mode

- a mobile station not have a physical channel allocated is in idle

mode- a mobile station that is switched off is said to be detached

- a mobile station that is switched on is said to be attached

❑ Terminal mobility (e.g. continuity for ongoing calls, connectivity

for following communications)

- handovers

- location management proceduresHard handovers are employed for ensuring continuity of ongoing

calls (the mobile station is in dedicated mode)


- the handover decision is always taken by the network based on signal

strength measurements taken both by the network and the mobile station

(MAHO- Mobile Assisted HandOver) or based on load balancing algorithms

- during a handover process the mobile station stops transmitting on the

initial frequency and starts emitting again on a new channel allocated by

the network (usually) on another frequency

Location management procedures are employed for ensuring connectivity for

following communications (the mobile station is in idle mode)

- three methods can be employed in cellular systems for locating

the position of a mobile subscriber

-the mobile station indicates each change of cell to the network; its

position is known by the network on a cell per basis

-the mobile station indicates each change of position only when it

passes from a group of cells to another group of cells

-the position of the mobile station is not known


- in GSM the position of a mobile station in idle mode is known on a

group of cells per basis (location area)

Location area = the group of cells receiving a paging message

signaling an incoming call

Location updating = the procedure used by the mobile station to indicate

a change of the location area; location and subscription related data is

stored on a database

❑ Service mobility (i.e a particular service (voice, data, multimedia) will

still be accessible to a given user even if the user changes its mobile

device or its mobile network)

- change of mobile device - user and terminal identities are split in GSM

- user identity is kept by the SIM (Subscriber Identity Module) card /

subscribed services are also related to the SIM card

- change of mobile network – referred as roaming in GSM terminology


- national roaming

- roaming within the GSM space

- non GSM roaming – the SIM card can be plugged into a non

GSM terminal; visited network must support SIM cards

The functions dealing with mobility are grouped in GSM in

broader contexts:

- radio resource management functions – including besides

handover and paging functions, specific channel

allocation/release functions.

- mobility management functions including besides location

updating some security related functions i.e. authentication

and ciphering.


2.4 Technical characteristics of the GSM system

Frequency bands [MHz] GSM: 890-915 (uplink), 935-960 (downlink)

DCS: 1710-1785 (uplink),

1805-1880 (downlink)

EGSM : 880-915 (uplink), 925-960 (downlink)

Number of carriers/carrier spacing 124/200KHz

(DCS – 374/200KHz)

Multiple access scheme Combined FDMA/TDMA (8 timeslots defined on each

carrier)

Duplex distance 45 MHz/ (DCS 1800-95 MHz)

Data rate for digitized speech 13kbps(full rate)/6.5kbps (half rate)

Traffic channels gross data rate 22.8 kbps (full rate) /11.4kbps (half rate)

Max. user data rate 9.6kbps (14.4 kbps)

Voice encoder (full rate) Linear predictive coding /Long term

prediction/Regular Pulse Excitation –

LPC/LTP/RPE

Minimal /Maximal cell radius 100 m/35km (up to 70 km –extended cells)

Modulation Gaussian Minimum Shift Keying - GMSK


MSC

2.5 The architecture of a GSM network

BTS

BTS

BSC MSC

BSC

VLR HLR AUC

EIR

GMSC

BSS

NSS

OMC

OMS

Um

interface

Abis

interface

A interface

X25

PSTN

ISDN

PSPDN

CSPDN

PLMN

IWF

MS

data/voice

signaling

MS

❑ SubsystemsBSS – Base Stations Subsystem

NSS – Network and Switching SubsystemOMS – Operation and Maintenance Subsystem

- each subsystem is composed of functional units (different equipments)

connected through standardized interfaces

BSS – several BTSs (Base Transceiver Station)

- a BSC (Base Station Controller)

- a TRAU (Transcoder and Rate Adaptation Unit)

NSS - MSC (Mobile Services Switching Centre)

- GMSC (Gateway Mobile Services Switching Centre)

- HLR (Home Location Register)

- VLR (Visitor Location Register)

- AuC- Authentication Centre

- EIR – Equipment Identity Register

OMS – OMC (Operation and Maintenance Centre)

2.5.1 Mobile Station

• MT (Mobile Termination) - equipment that provides all the

necessary functions for connections on the radio interface

• TE (Terminal equipment) - equipment directly in contact with

the user ( fax machine, desktop computer/ laptop / PDA); TE functionality

can be included in the MT

MT0

MT1

MT1

MT2

TE1

TATE2

TE2

• TA (Terminal adaptor)

-adaptation functions

4 wire ISDN

S interface

V24/RS 232 /emulated

serial interface

- MT0 – no TE connections

-MT1 allows connections of ISDN

terminals or non –ISDN (TE1/TE2)

-MT2 allows connections of non ISDN

terminals

• SIM card – subscriber identity module; microcontroller +

EEPROM/RAM memory, needed for operating a MS

- uniquely associated to a user/subscription and not to a mobile equipment

- contains static and non-static information such as:

➢ IMSI (International Mobile Subscriber Identity)

➢ secret authentication key (Ki)/ ciphering key (Kc)

➢ PIN/PUK codes

➢ TMSI (Temporary Mobile Subscriber Identity)

➢ Location information (LAI)

➢ List of forbidden PLMN’s/ list of licensed carriers (ARFCn)

➢ MSISDN (Mobile Station International ISDN number (optional)

➢ RAM like memory for storage of user directories/phone

numbers/SMS’s

- implements encryption (A8) and authentication (A3) algorithms

- the content of the SIM and its functionality

evolved during GSM’s standardization phases (phase 1, 2, 2+)

❑Base Transceiver Station (BTS)

• responsible for ensuring radio coverage on a given cell using a pair

or several pairs of duplex frequencies (typically up to 12 /cell in GSM)

• performs all signal processing tasks that an MS is performing except voice

coding and decoding

• implements multiplexing/demultiplexing functions for transmission over the

Abis interface

• provides measurement reports to BSC (for the uplink direction)

• a BTS is composed of

- several emitters/receivers (transceivers TRX (TRE)) operating each on

a pair of duplex frequencies

- a single or multiple omni directional or directional antennas

- a power unit (both AC supply and DC batteries)

- a local operation and maintenance unit

- duplexers (emission and reception paths on the same antenna) and

combiners (emission paths for multiple radio equipments on the same

antenna)

❑typical configuration with 4 TRX

combiners/duplexers

TRX1 TRX2 TRX3 TRX4

❑ includes an operation and maintenance unit

- controlled via BSC or locally using USB/serial connections

- handles the clock for the whole BTS

- clock usually derived from BSC; all BTSs are synchronized with the

later

BTSBTS

❑ typical connections to a BSC

- are done using a standard interface (the Abis interface)-

wired or wireless

- possible configurations

BSCBTS

BTS

BTS

BSCBTS

BTS

BTS

BSCBTS

BTS

BTS

star

loop (ring)

serial (chain)

❑Base Station Controller (BSC)

•“ intelligent” part of BSS

• connected to BTS via Abis and to MSC via the A interface

(or to TRAU via the Ater interface)

• manages radio channels in TRXs (allocation, release etc);

the channel allocation map is stored on a database

• manages the signaling channels of a MS: interprets them or relays

them if they are coming /going from NSS

• includes a digital switch network : switches channels from the Abis

interface onto channels on the A interface

• implements multiplexing/demultiplexing functions

• stores and allows software downloading to BTS

• often equipments inside BSS are provided by the same manufacturer -

> Abis interface is proprietary

• a BSC has an essential role for handovers

MSBTS1

BTS2

BSC

Measurement

report

(downlink)

Measurement

report (uplink)

Measurement

report

(downlink)

Channel

activation

Handover

command

Emission on

the new

channel

- in GSM the decision to perform a handover is always taken by

BSC using measurement reports

❑Transcoder and Rate Adaption Unit (TRAU)

•main functions: - voice transcoding from 13kbps to a 64kbps A law PCM - rate adaptation (8/16 kbit/s to 64 kbit/s)

- implements the GSM voice codec- involved in DTX mechanisms – insertion of comfort noise during

silent periods

MS BTS BSC MSCTRAU

13kbps 64kbps

MS BTS BSC MSCTRAU

13kbps 64kbps64kbps

MS BTS BSC MSCTRAU

13kbps 64kbps

Abis A

Abis A

Abis A

How to transport efficiently 13kbps channels using 64kbps channels?

•position inside BSS – left at the choice of the manufacturers/operators

Documents

2. GSM networks: services, architecture, functional