GSM Overview

Department of Electronic EngineeringDepartment of Electronic EngineeringELE 52PMC

Personal Mobile Communication

GSM Public Land Mobile Network GSM Public Land Mobile Network (PLMN)(PLMN)

Michael FeramezM.Feramez@latrobe.edu.auwww.ee.latrobe.edu.au/~mf

Lecture 4

2

Introduction

The European Telecommunication Standards Institute (ETSI) standardised the Global System for Mobile communications (GSM).

ETSI originally defined GSM as a European digital cellular telephony standard.

The GSM interfaces defined by ETSI lay the ground-work for a multivendor network approach to digital mobile communication.

GSM offers users good voice quality, call privacy, and network security.

3

GSM Statistics

Number of Countries/Areas with GSM System is 205 (Feb 2004)

GSM Total Subscribers - 1024.3 million (end Feb 2004)

SMS messages sent per month - 45.6 Billion SMS forecast for 2004 - 547.5 Billion GSM accounts for 72.5% of the World's digital market

and 72.0% of the World's wireless market

Ref.: [5] GSM Association

4

GSM Subscribers Growth

Blue bars = in-year net gains2001 is an estimate

5

GSM Subscribers by Region

6

Others ….

GSM/PSTN/ISDN

MSC

BTS

BSC

BTS PSTN

BTSISDN

SS#7MS

MS

MS

MS

MS

MS

7

Communication Path

OSI Protocol Reference Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

PhysicalBits

F A C Data Unit ( I Field ) FCS F

AP DataPH

Data UnitSH

Data UnitTH

Data UnitNH

AP DataAH

AP DataAP “X” AP “Y”

8

OSI of GSM

Transmission

RR

MM

CM

OAM

UserOperator

1. Radio transmission (Physical)1. Radio transmission (Physical)

2. Radio Resource Management2. Radio Resource Management

3. Mobility Management3. Mobility Management

4. Communication Management4. Communication Management

5. Operation Administration5. Operation Administration & Maintenance& Maintenance

9

System Elements

MS (Mobile Station)– MT (Mobile Terminal)

– TE (Terminal Equipment)

BS (Base Station)– BTS (Base Transceiver Station)

– BSC (Base Station Controller)

MSC (Mobile Switching Centre)

10

Maximum RF Power for MS

I 20

II 8

III 5

IV 2

V 0.8

Class Max. RF Power (W)

11

Functional Architecture

BSCMSC

HLR

VLR

MS

OAM

BTS

Base Station Subsystem(BSS)

Um

Air Interface

A-bis Interface

A Interface

MAP

MAP

12

Internal Environment

NSSBSSMS

OSS

Control flowUser data flow

OSS Operation SubsystemBSS Base Station SubsystemMS Mobile StationNSS Network & Switching Subsystem

13

External Environment

PSTNNSSBSS

OSS

Control flowUser data flow

OSS Operation SubsystemBSS Base Station SubsystemPSTN Public Switch Telephone NetworkNSS Network & Switching Subsystem

BSS

14

MTTE

MS

BTS

BTS

BTS

BSC

BSS

MTTE

MS

MTTE

MS

BTS

BTS

BTS

BSC

BSS

MTTE

MS

MSC

NMC

OMC

ADC

OMC

HLR

AUC

VLR

EIR

MSCA

Um

A: Interface between BSC and MSCA-bis: Interface between BTS and BSCUm: Air Interface between MS and BTS

A-bis

AUC: Authentication CentreEIR: Equipment Identity RegisterHLR: Home Location RegisterVLR: Visitor Location Register

15

Functions of MSC

Gateway to other networks (PSTN, ISDN, etc.), Paging of MS, MS location updating, and MS mobility management assisted by:

– Home Location Register (HLR), and

– Visitor Location Register (VLR).

16

Gateway MSC (GMSC) Key Role

GMSCUser

HLR

MSCVLRDirectory

NumberRoutingNumber

RoutingNumber

DirectoryNumber

GMSC handling of a domestic call

17

Radio S

ubsystem

RPE-LTPencoder

RPE-LTPdecoder

13 bit uniform to 8 bit A-Law

8 bit A-Law to 13 bit uniform

MS

C

PSTN/

ISDN

104 kbps

13 kbps

8000samples/s

13-bit uniformcode

Analoguespeech

8000 samples/s13-bit uniform

code

8000 samples/s8-bit A-Law

ITU-T G.711, G.712,G.713, G.714

RPE-LTPencoder

RPE-LTPdecoder

13 bit uniform to Analogue

O/P

Analogue to 13 bit uniform

I/P

System Description

18

Radio Access & Demod

GMSK

De-Interleaving

Forward Error

CodingInterleaving

Radio Access & ModGMSK

InterleavingForward

Error Coding

Error Correction

Radio Access & ModGMSK

Error Correction

De-Interleaving

Radio Access & Demod

GMSK

Dup Dup

Encoded speech456 bits/frame

Encoded speech456 bits/frame

Radio Subsystem

19

Digital Radio Link ProcessSpeechCoding

ChannelCoding

BitInterleaving

Encryption

Modulation

Burst Forming& Multiplexing

SpeechDecoding

ChannelDecoding

BitDeinterleaving

Decryption

DelayEqualisation

Demodulation

Demultiplexing

20

Transmission

Speech Coding– Regular Pulse Excitation (RPE)

– Linear Prediction Coding (LPC)

Data Service Modulation - GMSK GSM channels

– Physical channels

– Logical channels

Multiple-access scheme (TDMA/FDMA, FH)

21

Speech Coding

A 4-kHz analogue signal is first converted to 64 kbps and then to 13 kbps using RPE-LPC (Regular Pulse Excitation - Linear Prediction Coding).

RPE generates the impulse noise to simulate the nature of speech.

LPC compresses the speech waveform by using a filter with 8 transmitted coefficient.

There are two modes of voice transmission in GSM; continuous (normal) and discontinuous.

22

GSM Speech Processing - BS

8-bit A-Lawto 13-bituniform

Converter

RPE-LTPSpeechEncoder

13 x 8,000 = 104 kbps

DigitalSpeech Signal(64 kbps)

To ChannelEncoder

13 kbps

RPE-LPC: Regular Pulse Excitation - Linear Prediction CodingRPE-LTP: Regular Pulse Excitation - Long-Term Predictor

MSside

MSCside

23

GSM Speech Processing - MS

Low-PassFilter

RPE-LTPSpeechEncoder

To ChannelEncoder

A/DConverter

AnalogueSpeech Signal(4 kHz BW)

13 kbps

13 x 8,000 = 104 kbps

RPE-LPC: Regular Pulse Excitation - Linear Prediction CodingRPE-LTP: Regular Pulse Excitation - Long-Term Predictor

MS BS

24

0 1 2 3 4 5 6 7

260 260

456 bit456 bit

20 ms20 ms

D4

D1

D2

D3

D5

D6

D7

D8

D4

D1

D2

D3

D5

D6

D7

D8

Tail Bits3

57 bit data(first 20 ms)

Training Sequence26

57 bit data(second 20 ms)

Tail Bits3

Guard Period8.25

Normal Burst (NB)

1 1

57 bits each

Speech (13 kbps)

Speech coder

Channel encoder

TDMA frame (4.615 ms)

20 ms at 13 kbps

160 Samples (2,080 bits) 160 Samples (2,080 bits)

25

Data Services

The highest data rate is 9600 bps and has two different modes.

Transparent mode (T) employs FEC:– 2400 bps (intermediate rate is 3.6 kbps)

– 4800 bps (intermediate rate is 6 kbps)

– 9600 bps (intermediate rate is 12 kbps)

Non-Transparent mode (NT) employs ARQ.

26

Modulation

Gaussian Minimum-Shift Keying (GMSK) with BT = 0.3 normalised bandwidth Gaussian filter.

Transmission rate: (1/T) = 270 kbps Baseband: B = 270 x 0.3 = 81 kHz The GSM RF channel is 200 kHz which means the

bandwidth efficiency is 270 kbps / 200kHz = 1.35 b/s/Hz

27

GMSK Modulation

coscos

sinsin

GaussianFilter

GaussianFilter

dt dt

-sin t

cos t

cos [t + (t, n)]cos [(t, n)]

sin [(t, n)]

(t, n)

Phase pulse shaping

28

GSM Channels

Physical channels– Full rate

– Half rate

– One-eighth rate

Logical channels– Downlink common channels

– Uplink common channel

– Signalling channels

29

Physical Channels

Physical channel in a TDMA system is defined as a timeslot with a timeslot number TN in a sequence of TDMA frames.

GSM employs TDMA combined with FH and hence the physical channel is partitioned in both time and frequency.

FH is known to be very efficient in combating channel fading.

30

TCH (Traffic Channels)

TCH/F 22.8 kbps (Full Rate)– TCH/F9.6

– TCH/F4.8

– TECH/F2.4

TCH/H 11.4 kbps (Half Rate)– TCH/H4.8

– TCH/H2.4

TCH/8 (One-eighth Rate)

31

Downlink Common Channels

FCCH - Frequency Correction Channel SCH - Synchronisation Channel BCCH - Broadcast CCH PAGCH - Paging and Access Grant Channel CBCH - Call Broadcast Channel

32

0 1 2 3 4 5 6 7

1 TDMA Frame = 8 Timeslots

1 Timeslot = 156.25 Bit Duration

Tail Bits3

Encrypted Bits58

Training Sequence26

Encrypted Bits58

Tail Bits3

Guard Period8.25

Normal Burst (NB)

Tail Bits3

Fixed Bits142

Tail Bits3

Guard Period8.25

Frequency Correction Burst (FCB)

Tail Bits3

Encrypted Sync Bits39

Extended TrainingSequence 64

Encrypted Sync Bits39

Tail Bits3

Guard Period8.25

Synchronisation Burst (SB)

Tail Bits8

SynchronisationSequence 41

Encrypted Bits36

Tail Bits3

Guard Period68.25

Access Burst (AB)

33

Uplink Common Channel

RACH - Random Access Channel is used by MS to access a call.

RACH/F - Full rate, one timeslot every 8 BP. RACH/H - Half rate, using 23 timeslots in the 51 x 8

BP cycle. The frame is 8 BP (Burst Periods) of 4.615 ms duration.

34

Signalling Channels

CCCH - Common Control Channel ACCH - Associated CCH SACCH - Slow Associated CCH FACCH - Fast Associated CCH SDCCH - Stand-alone Dedicated CCH

35

Multiple-Access Scheme

A combination of FDMA and TDMA A total of 124 FDMA channels of 200 kHz bandwidth

each. The Uplink frequency band 935 - 960 MHz (25 MHz)

and Downlink 890 - 915 MHz (25 MHz). The duplex separation is 40 MHz. TDMA employs 8 timeslots forming a frame of 4.16

ms, 0.577 ms per timeslot.

36

TDMA/FDMA

F1 F’1F0

Cell Rx Cell Tx

F2 F’2

45 MHz

Amplitude

Frequency

37

GSM FDMA/TDMA

ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7

ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7

ch #1 ch #2 ch #3 ch #4 ch #5 ch #6 ch #8ch #7

Frequency 1

Frequency 2

Frequency 124

Fre

quen

cy D

omai

n

Time Domain

TS0 TS3 TS4 TS5 TS6 TS7TS1 TS2

38

Frequency Hopping

GSM employs a slow frequency hopping rate defined in bits per hop.

The regular rate is 217 hops/s, with 270 kbps transmission rate, the result is 1200 bits/hop.

It is worth mentioning that FH improves the propagation performance when considering the effect of frequency selective fading.

39

Radio Resource Management

Radio channels (resources) are allocated for call setup, handover, and release on a call basis.

Management involves three functions: location, handover, and roaming.

Three link protocols are used for RR functions:– Radio Link Protocol (LAPDm)

– Link Access protocol (LAPD)

– Message Transfer Protocol (MTP)

40

Mobile-Originating Call

Mobile StationMobile Station Base StationBase Station

Random Access

Immediate Assignment

Exchange of Call Setup Information

Traffic Channel Assignment

Data Flow

41

Mobile-Assisted Handoff (MAHO)

MAHO algorithm is carried out within the MS. MS scans for another RF carrier under direction from a

BS. BS requests MS to measure the signal strength of a

specific RF carrier. Upon request, MS forwards the result to BS. BS initiates handover on the basis of:

– Signal strength at MS from the candidate BTS– Signal strength at the candidate BTS from MS

42

Mobility Management

Location updates, handovers, and roaming Cell selection Authentication:

– PIN checked locally by SIM– GSM network RAND and SRES

Encryption User identity protection - Security management, SIM

(MS side) and AUC (network side).

43

Cell Selection

C1 = A - max(B, 0)C1 = A - max(B, 0)A = received level average - p1A = received level average - p1B = p2 - maximum RF power of the MSB = p2 - maximum RF power of the MSp1 = a value between -110 and -48 dBmp1 = a value between -110 and -48 dBmp2 = a value between 13 and 43 dBmp2 = a value between 13 and 43 dBmMS maximum power = 29 to 43 dBmMS maximum power = 29 to 43 dBm

MS chooses the best cell depending on:MS chooses the best cell depending on:1. The level of the signal received by MS,1. The level of the signal received by MS,2. The maximum transmission power of MS2. The maximum transmission power of MS3. Two parameters p1 and p2 specified by the cell3. Two parameters p1 and p2 specified by the cell

Both values of p1 and p2 are broadcast from the cell.Both values of p1 and p2 are broadcast from the cell.

44

Authentication

Authentication protects the network against unauthorised access.

Phase One - PIN is checked locally by the SIM. Phase Two - A 128-bit RAND is sent from the network

to MS.– RAND mixes with MS’ secret parameter Ki in A3 process

algorithm producing a 32-bit SRES.

– SRES (signed result) is sent from MS to network for verification.

45

Authentication Computation

Ki RAND

A3

SRES

Ki RAND

A3

SRES

RAND

Equal?

MS Network

46

User Identity Protection

SIM (MS side) and AUC (network side) are the repositories of the subscriber’s key Ki.

The key Ki never transmits over the air. Both sides perform A3 and A8 computations.

47

Communication Management

CM layer provides telecommunication services (speech, fax, and data) via RR and MM layers.

The management functions of CM layer are:– Call Control

– Service Management

– Short Message Service

48

0 1 11 241312SACCH 0 1 2 49 50

0 1 2 7

TB3 58 Encrypted bits 56 bits Training Seq. 58 Encrypted bits

TB3

GP8.25

0 1 2 49 50

24 250 1

0 1 2 3 2046 2047

1 hyperframe = 2048 superframes=2,715,648 TDMA frames (3 hours, 28 minutes, ..)

1 superframe = 1326 TDMA frames (6.12 s)

1 timeslot = (156.25 bits duration ~ 0.577 ms

1 TDMA frame = 8 timeslots (4.615 ms)

1 multiframe = 26 TDMA frames (120 ms) 1 multiframe = 51 TDMA frames (235 ms)

e.g. TCH/FS

e.g. BCCH

Idle/SACCH

49

Objectives of GSM PLMN

To provide the subscriber with a wide range of services and facilities, both voice and nonvoice, that are compatible with those offered by existing networks (e.g. PSTN, ISDN)

To introduce a mobile RS that is compatible with ISDN To provide certain services and facilities exclusive to

mobile situations To give compatibility of access to the GSM network for

a mobile subscriber in a country that operates the GSM system

50

Objectives of GSM PLMN - cont.

To provide facilities for automatic roaming, locating, and updating of mobile subscribers

To provide services to a wide range of mobile stations, including vehicle mounted stations, portable stations, and handheld stations

To provide for the efficient use of the frequency spectrum

To allow for low-cost infrastructure, terminal, and service cost

51

GSM PLMN Services

A telecommunication service supported by the GSM PLMN is defined as a group of communication capabilities that the service provider offers to the subscribers

GSM PLMN offers 3 basic telecommunication services:– bearer services

– teleservices, and

– supplementary services

Please refer to Reference [1] for a list of services

52

Bearer Services Support

Data services x x

Alternate speech/data x x

Speech followed by data x x

Clear 3.1 kHz audio x x

Unrestricted digital information (UDI) x x

Packet Assembler/Disassembler (PAD) x

3.1 kHz external to PLMN x

Others x

Service GSM ISDN

53

Teleservices Support

Circuit speech (telephony) x x

Emergency call x x

Short message point-to-point x x

Short message cell broadcast x x

Alternate speech/facsimile group 3 x x

Automatic facsimile group 3 x

Voice-band modem (3.1 kHz audio) x x

Messaging teleservices x

Service GSM ISDN

Paging teleservices x

Others x

54

Access methodAccess method TDMA/FDMATDMA/FDMA

Frequency band (MS to BS)Frequency band (MS to BS) 890 - 915 MHz890 - 915 MHz

Frequency band (BS to MS)Frequency band (BS to MS) 935 - 960 MHz935 - 960 MHz

Channel bandwidthChannel bandwidth 200 kHz200 kHz

ModulationModulation GMSKGMSK

Bit rateBit rate 270.833 kbps270.833 kbps

FilterFilter BT = 0.3 (Gaussian)BT = 0.3 (Gaussian)

Voice channel codingVoice channel coding RPE-LPC Convolutional 13 kbpsRPE-LPC Convolutional 13 kbps

GSM Specification

Frequency HoppingFrequency Hopping Slow hopping (217 hops/s)Slow hopping (217 hops/s)

55

Cont.

Handoff methodHandoff method MAHOMAHO

Adaptive equalisationAdaptive equalisation yes (up to 16 yes (up to 16 s time dispersion)s time dispersion)

Users per channelUsers per channel 88

MS power levelMS power level 0.8, 2, 5, 8, 20 Watts0.8, 2, 5, 8, 20 Watts

Number of channelsNumber of channels 124124

Frame IntervalFrame Interval 8 timeslots = 4.615 ms8 timeslots = 4.615 ms

InterleavingInterleaving 40 ms40 ms

Associated control channelAssociated control channel Extra frameExtra frame

TimeslotTimeslot 0.577 ms0.577 ms

56

MSMS Mobile Station Mobile StationTETE Terminal Equipment Terminal EquipmentMTMT Mobile Terminal Mobile TerminalBSBS Base Station Base StationBTSBTS Base Transceiver Station Base Transceiver StationBSCBSC Base Station Controller Base Station ControllerMSCMSC Mobile Switching Centre Mobile Switching CentreHLRHLR Home Location Register Home Location RegisterVLRVLR Visitor Location Register Visitor Location RegisterNMCNMC Network Management Centre Network Management CentreOMCOMC Operation and maintenance Centre Operation and maintenance CentreADCADC Administration Centre Administration CentreAUCAUC Authentication Centre Authentication CentreEIREIR Equipment Identifier Register Equipment Identifier Register

57

MAHO Mobile Assisted HandoffMAHO Mobile Assisted HandoffGMSCGMSC Gateway MSC Gateway MSCRPERPE Regular Pulse Excitation Regular Pulse ExcitationLPCLPC Linear Prediction Coding Linear Prediction CodingBPBP Burst Period Burst PeriodRSRS Radio Service Radio Service

58

References

[1] Garg, Vijay K and Wilkes Joseph E, “Wireless and Personal Communication Systems”. Prentice Hall PTR

[2] Parsons J D and Gardiner J G, Mobile Communication Systems, Blackie USA Halsted Press

[3] Lee, William C. Y., Mobile Cellular Telecommunications Systems, McGraw-Hill, Inc.

[4] Gibson, D. Jerry, The Communications Handbook, 1997 CRC Press, Inc.

[5] GSM World from GSM Association: www.gsmworld.com