1 Global System for Mobiles GSM 2 TOPICS GSM CONCEPTS GSM SYSTEM ARCHITECTURE IDENTITIES USED IN GSM GSM CHANNELS GSM RADIO LINK MOBILITY MANAGEMENT

1

Global System for Mobiles

GSM

2

TOPICS

• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT

3

TOPICS


4

Telecom Basics

• Communication– Voice and Data– Analog and Digital– Circuit Switched and Packet Switched– Media - Copper Wire, Co-axial cable, Air, Optical

Fibre– Networks -PSTN, ISDN, PDN and Mobile

Networks

5

Background to GSM

• 1G : Advanced Mobile Phone Service (AMPS)• Analog, Circuit Switched, FDMA, FDD

• 2G : Digital Advanced Mobile Phone Service (D-AMPS)• Digital, Circuit Switched, FDMA, FDD

• 2G : Global System for Mobile (GSM)• Digital, Circuit Switched, FDMA and TDMA, FDD

• 2G : Code Division Multiple Access (CDMA)• Digital, Circuit Switched, CDMA, FDD

6

GSM History

7

1982: Groupe Spécial Mobile (GSM) created

1984: Description of GSM features

1985: List of recommendations settled

1987: Initial MoU (Memorandum of Understanding) aside the drafting of technical specifications was signed by network operators of 13 countries:

1988: Validation and trials, of the radio interface.

1991: First system trials are demonstrated at the Telecom 91 exhibition.

1992: Official commercial launch of GSM service in Europe. First Launch in Finland

1993: The GSM-MoU has 62 signatories in 39 countries worldwide.

1995: Specifications of GSM phase 2 are frozen.

1999: GSM MoU joins 3GPP (UMTS)GPRS Trials begins

2000: 480M GSM Network operators WorldwideFirst GPRS Networks roll out

End 2002: 792M GSM Net work Operators Worldwide

Development of the GSM Standard

12 SERIESOPERATION ANDMAINTENANCE

01 SERIESGENERAL 02 SERIES

SERVICE ASPECTS

03 SERIESNETWORK ASPECTS

04 SERIESMS-BSS INTERFACE AND

PROTOCOLS

05 SERIESPHYSICAL LAYER ON THE

RADIO PATH.

06 SERIESSPEECH CODINGSPECIFICATIONS

07 SERIESTERMINAL ADAPTERS

FOR MOBILE STATIONS

11 SERIESEQUIPMENT AND TYPE

APPROVAL SPECIFICATIONS

10 SERIESSERVICE INTERWORKING

09 SERIESNETWORK

INTERWORKING

08 SERIESBSS TO MSC INTERFACES

GSM Specifications

Increasing GSM Data Rates

Transmission Time

GPRS = General Packet Radio ServiceHSCSD = High Speed Circuit Switched DataEDGE = Enhanced Data rate for GSM EvolutionUMTS = Universal Mobile Telecommunication System

10 sec 1 min 10 min 1 hour0

UMTS

E/GPRS

ISDN

PSTN

GSM

webe-mail photo

web photoe-mail

web photo

video clipreportphoto

web photoe-mail

video clipreport

video clipreport

video clipreport

video clipreport

10

thro

ug

hp

ut

kbp

s

10 k

100 k64 k

1 M

2 M

1 k1998 1999 2000 2001 2002

Time frame

UMTS

GPRS

HSCSD

9.614.4

packet

GPRS = General Packet Radio ServiceHSCSD = High Speed Circuit Switched DataEDGE = Enhanced Data rate for GSM EvolutionUMTS = Universal Mobile Telecommunication System

EDGEcircuit

Wireless Data Technology Options

11

Circuit mode

Packet mode

A→FD→H

C→G

C→G

C→G

C→G

C→G

D→H

D→H

A→FD→H

D→H

A→F

A→F

A→F

A

B

C

D

E

F

G

H

Circuit-Switched or Packet-Switched

13

What are the types in GSM Network?

• GSM-900 (Channels 125 operating band 900Mhz carrier spacing 200khz spacing 45Mhz)

• GSM -1800 (Channels 374 spacing 95Mhz)

• GSM -1900(Used in USA)

14

Multiple Access Technique• Multiple Access – Achieved by dividing the available

radio frequency spectrum, so that multiple users can be given access at the same time.

• FDMA - Frequency Division Multiple Access– ( eg: GSM each Frequency channel is 200KHz)

• TDMA - Time Division Multiple Access– ( eg: GSM each frequency channel is divided into

8 timeslots)• CDMA - Code Division Multiple Access

– (eg: IS95- Each User data is coded with a unique code)

15

Duplex Technique

• Duplex - How the up link and Down link of a user is separated

• FDD - Frequency Division Duplex– (eg:In GSM the up link and down link of a user is

separated by 45MHz )

• TDD - Time Division Duplex – (the up link and down link of a user will be at the same

frequency but at different Time )

16

Frequency band Uplink 890 - 915 MHz Downlink 935 - 960MHzDuplex Frequency Spacing 45MHzCarrier separation 200KHzFrequency Channels 124Time Slots /Frame(Full Rate) 8Voice Coder Bit Rate 13KbpsModulation GMSKAir transmission rate 270.833333 KbpsAccess method FDMA/TDMASpeech Coder RPE-LTP-LPC

GSM System specifications

17

Uplink 890 MHz to 915 MHzDown Link 935 MHz to 960 MHz25 MHz divided into 125 channels of 200 KHz bandwidth

890.0 890.2 890.4 914.8 915.0

935.0 935.2 935.4 959.8 960.0

UP

DOWN

Access Techniques

18

Time Division Multiple AccessEach carrier frequency subdivided in time domain into 8 time slots Each mobile transmits data in a frequency, in its particular time slot - Burst period = 0.577 milli secs.8 time slots called a TDMA frame. Period is .577 * 8 = 4.616 milli secs

0 1 2 3 4 5 6 7

4.616 ms

0.577 ms

Access Techniques ...

19

20

GSM in comparison with other Standards• GSM gives mobility without any loss in Audio quality • Encryption techniques used gives high security in the

air Interface and also use of SIM.• Bit Interleaving for high efficiency in Transmission.• Variable Power (Power budgeting- extend battery life)• Minimum Interference.• Features-CCS7 Signaling

– SMS (Short Message Services)– Emergency Calls– CELL Broadcast

21

TOPICS


22

AuC

MS

MS

BTS

BTS

BTS

BSC

BSC

MSC

MSC

VLR

VLR

GMSC

HLR

PSTN

EIR

Um

Abis

Abis

A

A

OMC Server

Um

GSM - Network Structure

B

E

E

X.25

C

F

H

X.25

23

GSM Network

OMC

AUC

HLR

MSC

EIRVLR

BSC

BTS

MS

ExternalPSTN &PDN N/W

SS

BSS

SwitchingSystem

Base StationSystem

MS Mobile Station

BTS Base transceiver System

BSC Base Station Controller

MSC Mobile Switching Center

HLR Home Location Register

VLR Visitor Location Register

EIR Equipment Identity Register

AUC Authentication Center

OMC Operation And Maintenance Center

24

GSM Architecture

HLR

VLR

EIR

AUC

MSC

BSC

BSC

SMSC

PSTN

VMSC

MobileStation

GSM

Air interface

OMCR

TRAU

Base Station System

Network and switching subsystem

A interface SS7 / speech

SS7

X.25

BTSBTS

BTS

BTS

BTSBTS

Abis interface

A interface

OMCS

25

GSM utilizes two bands of 25 MHz. 890-915 MHz band is used for uplink while the 935-960 MHz is used for downlink.

The frequency bands are divided into 200 KHz wide channels called ARFCNs (Absolute Radio Frequency Channel Numbers) i.e. there are 125 ARFCNs out of which only 124 are used.

Each ARFCN supports 8 users with each user transmitting / receiving on a particular time slot (TS).

Fundamentals

124

123

…….

……

2

1

124

123

…….

……

2

1

0

1

2

3 4

5

6

7

0

1

2

3 4

5

6

7

Data burst = 156.25 bit periods = 576.9s

960 MHz

959.8MHz

200KHz

935 MHz

935.2 Mhz

915 MHz

200KHz

45 MHz

Downlink (TDMA frame) = 8 TS

Uplink (TDMA frame)

Delay

TS: Time slot

914.8 MHz

890.2 MHz

890 MHz

DOWNLINK

UPLINK

Therefore 1 TDMA frame = 156.25 x 8 = 1250 bits and has a duration of 576.92s x 8 = 4.615 msThe technology

26

Mobile Station (MS)

• Hand portable unit

• Contains Mobile Equipment(ME) and Subscriber Identity Module (SIM)

27

Mobile Equipment(ME)

• Frequency and Time Synchronization• Voice encoding and transmission• Voice encryption/decryption functions• Power measurements of adjacent cells• Display of short messages• International Mobile Equipment Identifier (IMEI)

28

SIM• Portable Smart Card with memory (ROM-6KB to

16KB-A3/A8 algorithm, RAM- 128KB TO 256KB, EEPROM- 3KB to 8KB )

• Static Information– International Mobile Subscriber Identity(IMSI)– Personal Identification Number (PIN)– Authentication Key (Ki)

• Dynamic Information– Temporary Mobile Subscriber Identity(TMSI)– Location Area Identity (LAI)– Phone memories, billing information– Ability to store Short Messages received

29

Base Transceiver Station (BTS)• Handles the radio interface to the mobile station.• Consists of one or more radio terminals for

transmission and reception• Each Radio terminal represents an RF Channel• TRX and MS communicates over Um interface• Received data transcoding• Voice encryption/decryption• Signal processing functions of the radio interface• Uplink Radio channel power measurements

30

Base Station Controller (BSC)• Provides all the control functions and physical links

between the MSC and BTS • External Interfaces

– ‘Abis’ interface towards the BTS– ‘A’ interface towards the MSC

• Monitors and controls several BTSs• Management of channels on the radio interface• Alarm Handling from the external interfaces• Performs inter-cell Handover• Switching from ‘Abis’ link to the ‘A’ link• Interface to OMC for BSS Management

31

Mobile Switching Center (MSC)

• Performs call switching • Interface of the cellular network to PSTN• Routes calls between PLMN and PSTN• Queries HLR when calls come from PSTN to mobile

user• Inter-BSC Handover• Paging• Billing

32

Home Location Register (HLR)• Stores user data of all Subscribers related to the

GMSC– International Mobile Subscriber Identity(IMSI)– Users telephone number (MS ISDN)– Subscription information and services– VLR address– Reference to Authentication center for key (Ki)

• Referred when call comes from public land network

33

Visitor Location Register (VLR)

• Database that contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR

• Identity of Mobile Subscriber• Copy of subscriber data from HLR• Generates and allocates a Temporary

Mobile Subscriber Identity(TMSI)• Location Area Code• Provides necessary data when mobile

originates call

34

Authentication Center (AuC)

• Stores Subscriber authentication data called Ki, a copy of which is also stored in in the SIM card

• Generates security related parameters to authorize a subscriber (SRES-Signed RESponse)

• Generates unique data pattern called Cipher key (Kc) for user data encryption

• Provides triplets - RAND, SRES & Kc, to the HLR on request.

35

EIR (Equipment Identity Register)

• EIR is a database that contains a list of all valid mobile station equipment within the network,

where each mobile station is identified by its International Mobile Equipment Identity(IMEI).

• EIR has three databases.,– White list - For all known,good IMEI’s– Black list - For all bad or stolen handsets– Grey list - For handsets/IMEI’s that are

on observation

37

GSM Entities and Signaling Architecture

38

GSM Protocols• CM - Connection Management• MM - Mobility Management• RR - Radio resource • LAPDm - LAPD for mobile• LAPD - Link Access Procedure for D channel• BTSM - BTS Management Part• BSSAP - BSS Application Part (BSC - MSC)• DTAP - Direct Transfer Application Part (MS - MSC)• MAP - Mobile Application Part• MTP - Message Transfer part of SS7• SCCP - Signalling Connection Control Part of SS7• TCAP - Transaction Capabilities Application Part• ISUP - ISDN User Part

39

Functional Plane of GSM

MS BTS BSC MSC/ HLR GMSC VLR

MS BTS BSC MSC/VLR HLR GMSC

CC

MM

RR

Trans

40

TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTIFIERS USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT

41

Subscriber Identity -MSISDN

• The MSISDN is a GSM directory number which uniquely identifies a mobile subscription in the Public Switched Telephone Network (PSTN).

• Calls will be routed from the PSTN and other networks based on the Mobile Subscribers’ MSISDN number.

• MSISDN= CC + NDC + SN– CC= Country Code (91)– NDC= National Destination Code(98370)– SN= Subscriber Number (12345)

42

International Mobile Subscriber Identity [IMSI]• Subscriber always identified within the GSM network

by the IMSI • This is used for all signaling in the PLMN stored in

SIM and HLR/VLR• The IMSI consists of three different parts

– MCC = Mobile Country Code(3 Digits)– MNC = Mobile Network Code(2 Digits)– MSIN = Mobile Station Identification Number(Upto 10 digits)

43

Temporary Mobile Subscriber Identity [TMSI]• The TMSI is used for the subscriber’s confidentiality.• It should be combined with the LAI to uniquely

identify a MS.• Since the TMSI has only local significance (that is,

within the MSC/VLR area), the structure may be chosen by each administration.

• The TMSI should not consist of more than four octets.

44

Mobile Station Roaming Number[MSRN]• HLR knows in what Service area the subscriber is

located. • In order to provide a temporary number to be used

for routing, the HLR requests the current MSC/VLR to allocate a Mobile Station Roaming Number(MSRN) to the called subscriber and to return it.

• At reception of the MSRN, HLR sends it to the MSC, which now can route the call to the VLR where the called subscriber is currently registered.

45

International Mobile Equipment Identity [IMEI]• The IMEI is used for equipment identification. An

IMEI • uniquely identifies a mobile station as a piece or

assembly of equipment.• IMEI = TAC + FAC + SNR + sp

– TAC= Type Approval Code (6 digits),determined by GSM body

– FAC= Final Assembly Code (2 digits), identifies themanufacturer

– SNR= Serial Number (6 digits), uniquely identifying all equipment within each TAC and FAC

– sp = Spare for future use (1 digit)

46

Location Area Identity• LAI identifies a location area which is a group of

cells..• It is transmitted in the BCCH. • When the MS moves into another LA (detected by

monitoring LAI transmitted on the BCCH) it must perform a LU.

• LAI = MCC + MNC + LAC – MCC= Mobile Country Code(3 digits), identifies the country– MNC= Mobile Network Code(1-2 digits), identifies the GSM-

PLMN– LAC= Location Area Code, identifies a location area within a

GSM PLMN network. The maximum length of LAC is 16 bits,enabling 65536 different location areas to be defined in one GSM PLMN.

47

TOPICS• GSM CONCEPTS• GSM SYSTEM ARCHITECTURE• IDENTITIES USED IN GSM• GSM CHANNELS• GSM RADIO LINK• MOBILITY MANAGEMENT• CALL MANAGEMENT• RADIO RESOURCE MANAGEMENT

48

Channels : differentiating between Physical and Logical channels

Physical channels : The combination of an ARFCN and a time slot defines a physical channel.

Logical channels : These are channels specified by GSM which are mapped on physical channels.

49

Physical channel:One timeslot of a TDMA-frame on one carrieris referred to as a physical channel.There are 8 physical channels per carrier inGSM,channel 0-7(timeslot 0-7)

Logical channel:A great variety of information must be transmitted between BTS and the MS,for e.g.user data and control signaling.Dependingon the kind of information transmitted we refer to different logical channels.These logicalchannels are mapped on physical channel.

Channel concept

50

Logical Channels on Air interface

LOGICALCHANNELS

COMMONCHANNELS

DEDICATED CHANNELS

BROADCAST CHANNELS

COMMONCONTROLCHANNELS

DEDICATED CONTROL CHANNELS

TRAFFIC CHANNELS

FCCH BCCHSCH SDCCH SACCH FACCH

PCH AGCHRACH TCH/F TCH/EFRTCH/H

51

Frequency plan and importance of BCCH

B3

B2

B9

B6 B4

B1

Sectored antennas

MS ( monitoring the broadcast radio B1 in ‘idle

mode’ )

F S B B B B …..

F S …..

…..

I

F0 F50F2 F3 F4 F5 F10 F11F1

• F,S,B exist in time slot 0 of each frame

B7

B8

B5

B10

B11

B12

BPL frequency plan:

Broadcast frequencies :

15 Broadcast channels = 48-62

15 Hopping channels = 32-46

52

Broadcast channels BCH

• Broadcast Channel-BCH – Alloted one ARFCN & is ON all the time in every cell.

Present in TS0 and other 7 TS used by TCH.

• Frequency correction channel-FCCH – To make sure this is the BCCH carrier.– Allow the MS to synchronize to the frequency.– Carries a 142 bit zero sequence and repeats once in every

10 frames on the BCH.

• Synchronization Channel-SCH– This is used by the MS to synchronize to the TDMA frame

structure within the particular cell.– Listening to the SCH the MS receives the TDMA frame

number and also the BSIC ( in the coded part- 39 bits).– Repeats once in every 10 frames.

53Back

1. The MS is monitoring the BCCH and has all the decoded information stored on the SIM ( including the LAC)

2. As soon as the mobile is on a TCH it sends the signal strength indication on the corresponding SACCH

3. The BSC monitors the signal strengths and on analysis sends a ‘handoff request’ on FACCH. The handoff process is completed on the FACCH.

4. After the completion of call, the MS starts monitoring the BCCH again. On finding the LAC (stored on SIM) and that decoded from the BCCH to be different , the MS requests a ‘Location Update’ through SDCCH.

54

Broadcast channels BCH ...• BCH

– The last information the MS must receive in order to receive calls or make calls is some information concerning the cell. This is BCCH.

– This include the information of Max power allowed in the cell.

– List of channels in use in the cell.– BCCH carriers for the neighboring cells,Location Area

Identity etc.– BCCH occupies 4 frames (normal bursts) on BCH and

repeats once every Multiframe.– This is transmitted Downlink point to multipoint.

• Cell Broadcast Channel - CBCH– Used for the Transmission of generally accessible

information like Short Message Services(SMS)

55

What information does Broadcast Control channel

(BCCH) contain?

Serves as a Beacon for the Cell

Country Code (CC) and the Network Code (NC)

Location Area Identity (LAI)

List of neighboring cells which should be monitored by MS

List of frequencies used in the cell

Cell identity

Back

56

Common Control Channels CCCH• CCCH-

– Shares TS-0 with BCH on a Multiframe.

• Random access channel-RACH:– Used by Mobile Station for requesting for a channel. When

the mobile realizes it is paged it answers by requesting a signaling channel (SDCCH) on RACH. RACH is also used by the MS if it wants to originate a call.

– Initially MS doesn’t know the path delay (timing advance), hence uses a short burst (with a large guard period = 68.25 bits).

– MS sends normal burst only after getting the timing advance info on the SACCH.

– It is transmitted in Uplink point to point.

57

Common Control Channels CCCH ..• Access Grant Channel-AGCH

– On request for a signaling channel by MS the network assigns a signaling channel(SDCCH) through AGCH. AGCH is transmitted on the downlink point to point.

• Paging Channel-PCH– The information on this channel is a paging message

including the MS’s identity(IMSI/TMSI).This is transmitted on Downlink, point-to-multipoint.

58

Dedicated Control Channels-DCCH• Stand alone dedicated control channel(SDCCH)• AGCH assigns SDCCH as signaling channel on

request by MS.The MS is informed about which frequency(ARFCN) & timeslot to use for traffic.

• Used for location update, subscriber authentication, ciphering information, equipment validation and assignment of TCH.

• This is used both sides, up and Downlink point-point.

59

Dedicated Control Channels-DCCH• Slow associated control channel-SACCH

– Transmission of radio link signal measurement, power control etc.

– Average signal strengths(RXLev) and quality of service (RXQual) of the serving base station and of the neighboring cells is sent on SACCH (on uplink).

– Mobile receives information like what TX power it has to transmit and the timing advance. It is associated with TCH or SDCCH

• Fast associated control channel-FACCH– Used for Hand over commands and during call setup and

release. FACCH data is sent over TCH with stealing flag set

60

Traffic Channels-TCH

• TCH carries the voice data.• Two blocks of 57 bits contain voice data in the normal

burst.• One TCH is allocated for every active call.• Full rate traffic channel occupies one physical

channel(one TS on a carrier) and carries voice data at 13kbps

• Two half rate (6.5kbps) TCHs can share one physical channel.

61


62

GSM Radio Link

• Speech Coding -Done at Transcoder of BSC and MS– The Linear Predictive Coder uses RPE-LTP(Regular Pulse

Excitation- Long Term Prediction)– Converts 64kbps voice to 13kbps(260 bits every 20ms)

• Channel Coding - Done at BTS and MS– Uses Convolution Coding and CRC (Cyclic Redundancy

Check)– Converts 13 kbps to 22.8 kbps (456 bits per 20ms)

63

GSM Radio Link

• Bit Interleaving - Done at BTS and MS• Encryption - Done at BTS and MS

– EX OR data with cipher block, which is generated by applying A5 Algorithm to the Ciphering Key(Kc)

• Multiplexing - Done at BTS • Modulation - Done at BTS and MS

– GMSK(Gaussian filtered Minimum Shift Keying)– Phase change of +90 for 0 and -90 for 1

64

Speech Coding

BP A/D SPEECHENCODER

CHANNEL CODING

LP D/A SPEECHDECODER

CHANNEL DECODING

BAND PASS

300 Hz - 3.4 kHZ

Every 125 s value issampled from analog signal and quantised by 13 bit wordData rate = 13/125*10 -6

= 104 kbps

Every 20ms 160 samples takenData rate = 160 * 13/20ms = 104 kbps

Linear Predictive Coding & Regular Pulse Excitation Analysis

1. Generates 160 filter coeff2. These blocks sorted in 4 sequence 1,5,9,…37 / 2,6,10----38/ 3,7,11…39/8,12,16…403. Selects the sequence with most energy

So data rate = 104/4 = 26 kbps

Long term prediction analysis1. Previous sequences stored in memory2. Find out the correlation between the present seq. And previous sequences3. Select the highest correlation sequence4. Find a value representing the differencebetween the two sequences.

Reduces data rate = 26 kbps/2 = 13 kbpsie 260 bits in 20ms

50 132 78

1A 1B 2

1A = Filter Coeff block ampl, LTP params1B = RPE pointers &pulses2 = RPE pulse & filterparams

50 3 132 4

3 crc bitsFour 0 bits for codec

378 coded bits

Conv coding rate = 1/2 delay = 4

78

456 bits in 20 ms = 22.8 kbps57 x 8 = 456

To modulator

65

Interleaving

57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57

57 57 57 57

57 57 57 57

57 57 57 57

57 57 57 5757 57 57 57

57 57 57 57 57 57 57 57

57 57 57 57

Encoded speech blocks - Diagonal Interleaving

Even bitsOdd bits

Tb 3

Coded Data 57

F 1

Training Sequence26

F 1

Coded Data 57

Tb 3

Gp8.25

Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3

57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57

57 57 57 57

57 57 57 57

57 57 57 57

57 57 57 5757 57 57 57

57 57 57 57

Encoded control channel blocks - Rectangular Interleaving

Even bitsOdd bits

Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3

66

Burst• The information format transmitted during one

timeslot in the TDMA frame is called a burst.• Different Types of Bursts

– Normal Burst– Random Access Burst– Frequency Correction Burst– Synchronization Burst

67

Normal Burst

T3

Coded Data57

S1

T. Seq.26

S1

Coded Data57

T3

GP8.25

Tail Bit(T) :Used as Guard TimeCoded Data :It is the Data part associated with the burstStealing Flag :This indicates whether the burst is carrying

Signaling data (FACCH) or user info (TCH).Training Seq. :This is a fixed bit sequence known both to

the BTS & the MS.This takes care of the signal deterioration.

156.25 bits 0.577 ms

68

T3

Training Sequence41

Coded Data36

T3

GP68.25

Random Access Burst

T3

Fixed Bit Sequence142

T3

GP8.25

T3

Coded Data39

Training Sequence64

CodedData 39

T3

GP8.25

Freq. Correc. Burst

Synchronization Burst

156.25 bits 0.577 ms

156.25 bits 0.577 ms

156.25 bits 0.577 ms

69

Transmission on the radio channels

• A timeslot has a duration of .577 m seconds (148 Bits)• 8 timeslots(8 x 0.577 = 4.62 ms) form a TDMA frame• If a mobile is assigned one TS it transmits only in this time

slot• and stays idle for the other 7 with its transmitter off, called

bursting• The start on the uplink is delayed from downlink by 3 TS

periods• One TS = duration of 156.25 bits, and its physical contents

is • called a burst

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

DownlinkBTS > MS

UplinkMS > BTS

Offset

70

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

MS1 near

MS2far

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

AtBTS

0 1 2 3 4 5 6 7MS1 near

0 1 2 3 4 5 6 7MS2far

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

AtBTS

Timing Advance

71

Frames Types On Um Interface• TDMA Frame

– 8 Time slots (Burst Period)– Length is 4.62 ms(8 * 0.577ms)

• 26-TDMA Multiframe– 26 TDMA Frames (24 TCH, SACCH, Idle)– 120 ms (26 * 4.62ms)

• 51-TDMA Multiframe– 26 TDMA Frames (FCCH, SCH, BCCH, SDCCH, CCCH)– 235.6 ms (51 * 4.62ms)

72

Frames Types On Um Interface• Super Frame

– 51* 26 TDMA Frames– 6.12 S

• Hyper Frame– 2048 * 51* 26 TDMA Frames– 3 Hours, 28 Minutes, 53 Secs and 760 ms

73


74

Mobility Management

• Mobility Management (MM)• Location updating- normal,periodic, IMSI attach• Paging• Security Management

– Preventing unauthorized users- authentication– Maintaining Privacy of users- ciphering

• Providing roaming facility• MM functionality mainly handled by MS, HLR,

MSC/VLR.

75

Network Attachment

· Cell Identification· MS scans complete GSM frequency band for

highest power· Tunes to highest powered frequency and looks

for FCCH. Synchronizes in frequency domain· Get training sequence from SCH which follows

FCCH. Synchronizes in time domain.· Accesses BCCH for network id, location area and

frequencies of the neighboring cells. · Stores a list of 30 BCCH channels

76

Network Attachment…..· PLMN Selection

· Get the operator information from SIM.

· Cell Selection· Selected cell should be a cell of the selected

PLMN· Signal strength should be above the threshold.· Cell should not be barred

· Location Update· Register with the network by means of location

updation procedures.

77

MS BTS BSC (G)MSC VLR HLR

Action

Channel Request (RACH)

Channel Assignment (AGCH)

Authentication Request (SDCCH)

Authentication Response (SDCCH)

Comparison of Authentication params

Accept LUP and allocTMSI (SDCCH)

Ack of LUP and TMSI (SDCCH)

Entry of new area and identity into VLR and HLR

Channel Release (SDCCH)

MS Location Update (registration)

Location Update Request (SDCCH)TMSI + old LAI

78

Security - Authentication

Authentication center provides RAND to MobileAuC generates SRES using Ki of subscriber and RANDMobile generates SRES using Ki and RANDMobile transmits SRES to BTSBTS compares received SRES with one generated by AuC

MSKi RAND

A3

SRES

RAND

SRES

SRES

Auth Result

AuCBTSMS

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Security - Ciphering

Data sent on air interface ciphered for security A5 and A8 algorithms used to cipher dataCiphering Key is never transmitted on air

MSKi RAND

A8

Kc

MS NetworkUm interface

A5 A5

Kc Kc

Data DataCiphered

Data

80


81

Communication Management (CM)• Setup of calls between users on request• Routing function i.e. Choice of transmission

segments linking users• Point to Point Short message services

82

Mobile Originated Call

• Request for Service• Authentication• Ciphering• Equipment Validation• Call Setup• Handovers• Call Release

83

MS

MS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

GMSC

HLR

PSTN

EIR

AuC

Req for dedicated channel for signaling (RACH)

Give SDCCH

Allocates SDCCH using the AGCH

Sends call set-up request including dialled digits on SDCCH

Call set-up forwarded to BSC

Call set-up forwarded to MSC

Authentication request(SDCCH)

Authentication response(SDCCH)

Authentication response(SDCCH)

Assigns TCH Req

Activate TCHTCH assigned

Assn complete

Release SDCCH

SDCCH released

ACM

Ring tone over FACCH

Ring alert

Called Sub answers

Connect message

Ring tone ceases over FACCHSpeech path enabled

Mobile Originated Call

84

Mobile Terminated Call

• Paging• Authentication• Ciphering• Equipment Validation• Call Setup• Handovers• Call Release

85

Mobile Terminated CallMS

MS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

GMSC

HLR

PSTN

EIR

AuC

Land to Mobile call(MSISDN)

Query forVLR info

Reply(MSRN)

Routeto MSC

Query VLR for LAC andTMSIPaging

the area(+TMSI)

TMSI Pagedon PCH

Paging

Ch. REQover RACHAllocate SDCCHover AGCHPage RESP on SDCCH ( TMSI + LAI)

Page RES Page RES

Authentication and Ciphering procedure done as seen in Location Updation

Assign. REQ

Assignment CMD(=TCH) on SDCCH

* MS tunes* Assgn CMP* Phone rings

Connect traffic Ch.to trunkfrees SDCCHAssgn CMP

Network Alerting

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87

Radio Resource Management

• Establish maintain and release stable connections between MS and MSC

• Manage Limited Radio and Terrestrial resources• Handover process is the sole responsibility of the RR

Layer• Functions of RR layer are performed by MS and BSC

and partly by MSC

88

Radio Resource Management

• Power Control• Hand over Control• Discontinuous Transmission• Frequency Hopping

89

BTS commands MS at differentdistances to use different power levelsso that the power arriving at the BTS’s Rx isapproximately the same for each TS

- Reduce interference- Longer battery life

Power Control

90

HandoverMeans to continue a call even a mobile crosses the border of one cell to anotherProcedure which made the mobile station really roamHandover causes

RxLev (Signal strength , uplink or downlink)RxQual (BER on data)O & M interventionTiming AdvanceTraffic or Load balancing

91

Handover Types

• Internal Handover (Intra-BSS)– Within same base station - intra cell– Between different base stations - inter cell

• External Handover (Inter-BSS)– Within same MSC -intra MSC– Between different MSCs - inter-MSC

92

Handover Types

BSC

BSC

BSC

BSC

MSC

MSC

GMSC

C-1 C-2

C-3

C-4

93

Periodic Measurement Reports (SACCH)

Periodic Measurement Reports

HO requiredActivate TCH(facch)with HoRef#

if 1. Check for HO passed2. Channel avail in new BTS

Acknowledges and alloctes TCH (facch)

HO cmd with HoRef#Receives new BTS data(FACCH)

MS tunes into new frequency and TS and sends HO message to new BTS (facch)

Periodic Measurement Reports (SACCH)

HO performed

Release TCH

Cell 1

Cell 2

BSC

BTS 1

BTS 2

Intra BSC handover

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Discontinuous Transmission

• Discontinuous Transmission(DTX) allows the radio transmitter to be switched off most of the time during speech pauses.

• A Silence Indicator Block is transmitted at 500bps, which generates a comfort noise

• Down Link interference is decreased.• Up link battery is saved

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Frequency Hopping• Frequency Hopping permits the dynamic

switching of radio links from one carrier frequency to another.

• Base Band Hopping– At the BTS each the timeslot is shifted to

another transceiver, which is transmitting at the hop frequency. User will be connected to different Transceivers depending on hop sequence.

• Synthesis Hopping– At the BTS transceiver changes the

frequencies used. The user will be connected to only one transceiver.

• Decreases the probability of interference• Suppresses the effect of Rayleigh fading

96

Wireless Data

98 99 2000 2001

GSMDATA

GSMDATA

HSCSDHSCSD

GPRSGPRS

EDGEEDGE

UMTSUMTS

SIMToolkit

SIMToolkit

WAPWAP

Data Application

Time

Circuit Switched technology

Packet Switched technology

Technology for Applications

SMS Data: 160 -numeric charactersUser Data Rate : 9.6kbpsOne time slot over the air interface

High Speed Circuit Switched DataUser Data Rate:14.5kbpsUse multiple timeslots (max=8), hence max rate = 115.2kbps.Needs a duplexor in MS for simultaneous Tx and Rx

Add-on to GSM network : PCU; Packet Segmentation/re-assembly and scheduling• Radio channel access control and management• Transmission error detection and retransmission.• Power controlSGSN: GPRS mobility• Encryption• Charging GGSN : Interface to the PDN, Internet

Max user data rate : 21.4 kbpsDynamic rate adaptation to suit the radio conditions at that time ( 9.05 kbps, 13.4 kbps, 15.6 kbps 21.4 kbps)

W@P Gateway W@P ServiceW@P

Fone

InternetMobile Network

Surf the Internet while on the move

W@P Gateway :• Adaptation of the information to the mobile• Compression of the data• Buffering of the information

Enhanced Data rate for GSM Evolution• EDGE is an enhancement of GPRS and CSD technologies.• Based on the current GSM technology - same TDMA frame

structure, same bandwidth (200 kHz).• Uses 8-PSK modulation instead of GMSK.• Requires good propagation conditions. • Allows upto 48 kbps (EGPRS) and upto 28.8 kbps (ECSD) on

every radio channel• EDGE helps GSM-Only operators to compete with UMTS.

Universal Mobile Telecommunication Standards • Innovative Service Architecture : VHE Concept - providing the us the same look and feel of its personalized services independent of network and terminal.• Global Convergence : Fixed/Mobile, Telecom/Datacom,

public/private• Mobile Multimedia driven market.• Wideband bearers - 2GHz band ( 5 MHz per carrier), -max. 2Mbps

97

References• Wireless and Personal Communication Systems.

Vijay.K.Garg and Wilkes • Overview of the GSM System and Protocol

Architecture, IEEE Comm. Magazine, Moe Rahnema.

• The GSM System for Mobile Communications- Michel Mouly & Marie-Bernadette Pautet

• Overview of the GSM Comm- John Scourias.

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99

Location Updates

Location Updates can be classified into two:

Periodic Location Updates:

This occurs as per the timer set by the network operator. If the MS does not perform this update the MSC marks the MS as ‘Detached’ on the VLR.

Location Update on a handover:

This occurs if during a handover the MS is moved into a new Location Area Code (LAC).

Documents

1 Global System for Mobiles GSM 2 TOPICS GSM CONCEPTS GSM SYSTEM ARCHITECTURE IDENTITIES USED IN GSM GSM CHANNELS GSM RADIO LINK MOBILITY MANAGEMENT