The Global System for Mobile Communications (GSM)

Contents

OverviewGSM ServicesGSM ArchitectureRadio TransmissionConclusion

GSM Overview

History Group Special Mobile(GSM) formed to

study pan-European cellular system in 1982

First commercial service in Europe in 1991

Accepted as the digital standard in more than 70 countries

GSM Overview

GSM Features Hybrid air interface : FDMA/TDMA Standard based on ISDN and SS7 Standard in both air interface as well

as network interface Purely digital system

DCS1800 – Up-banded version of GSM900PCS1900 – North American version of GSM900

GSM Services

Bearer Services Give the subscriber the capacity required to t

ransmit appropriate signals between certain access points

Teleservices Provide the subscriber with necessary capabi

lities to communicate with other subscribersSupplementary Services

GSM Architecture

Mobile Station Mobile Equipment

Identified by International Mobile Equipment Identity(IMEI)

Subscriber Identity Module(SIM) Contains unique subscriber identity(IMSI) Removable smart card that free from

handsets

GSM Architecture

Base Station Subsystem Base Transceiver Station(BTS)

Contains the radio transceivers for a given cell

Base Station Controller(BSC) Manages radio resources, such as signal

processing, channel setup/teardown, handover

Responsible for one or more BTSs

GSM Architecture

Network Subsystem Mobile Switching Center(MSC)

Manages call routing and establishment, authentication, connection to fixed networks

Home Location Register(HLR) Stores subscription information and current

location of all subscribers in a given network Visitor Location Register(VLR)

Contains subscription information needed for call control, for all mobiles in area controlled by associated MSC

GSM Architecture

Equipment Identity Register(EIR) Contains the IMEI of all registered mobile

equipment, marked as black, gray, or white listed, depending on whether it is type approved or has been reported stolen

Authentication Center(AUC) Stores secret key held in SIM card

GSM Architecture

GSM Architecture

Mobile Application Part(MAP) Signaling between components of the NSS uses

the MAP which is built on top of SS7

MAP-B : MSC-VLR MAP-C : MSC-HLR MAP-D : HLR-VLR MAP-E : MSC-MSC MAP-F : MSC-EIR MAP-G : VLR-VLR MAP-H : AC-HLR MAP-I : User application protocol and HLR

GSM Architecture

Identification Codes IMSI(International Mobile Subscriber

Identity:15digits) Directory number assigned by operators to a

subscriber TMSI(Temporary Mobile Subscriber

Identity:32bits) Assigned by VLR to a subscriber Privacy and network security, avoiding

transmission of IMSI

GSM Architecture

MSISDN(Mobile Station ISDN no.) Used to reach a called party in a GSM network

PSTNGMSC

BTSBSC

MSCVLR

HLR1. MSISDN

2. MSISDN

3. IMSI 4. MSRN

5. MSRN

6. Route call

7. Route call8. Route call

GSM Architecture

IMEI(International Mobile Equipment Identity:15digits) Unique serial number assigned by

manufacturer to a terminal LAI(Location Area Identity:15digits)

Assigned by operating company to a BTS Mobile Country Code(MCC), Mobile

Network Code(MNC), Location Area Code(LAC)

GSM Architecture

Functional Planes

Operations, Administration,& Maintenance (OAM)

CommunicationManagement (CM)

Mobility Management(MM)

Radio ResourceManagement (RR)

Physical

Sending Entity

Receiving Entity

Channel

GSM Architecture

Radio Resources management(RR) Responsible for establishing and releasing a reliable

connection between the MS and the MSC(handling channel allocation and handover)

Internal Handover Channels(time slots) in the same cell Cells controlled by the same BSC

External Handover Cells under the control of different BSCs, but

belonging to the same MSC Cells under the control of different MSCs

GSM Architecture

Mobility Management(MM) Responsible for location management and

security(authentication and encryption) Authentication and encryption

Ki(Authentication Key) Secret key assigned by operating company

to a subscriber Used to compute the cipher key

Kc(Cipher Key) Computed by network and by MS Protect user information and network control

information

GSM Architecture Algorithm

A3 : Authentication algorithm A5 : Ciphering algorithm A8 : Ciphering key generating algorithm

Another level of security :IMEI query to the EIR White-listed : allow to connect to the

network Grey-listed : under observation from the

network Black-listed : not allow to connect to the

network

GSM Architecture

Location management

GSM Architecture

Communication Management(CM) Responsible for call establishment and

routing Call routing Supplementary services management Short message services management

Radio Transmission

Frequency Allocation GSM900 radio frequencies

890-915MHz(uplink) and 935-960MHz(downlink) 125 carriers(ARFCN) of 200KHz bandwidth

GSM1800 radio frequencies 1710-1785MHz(uplink) and 1805-1880MHz(Down) 375 carriers of 200KHz bandwidth

PCS1900 radio frequencies 1850-1910MHz(uplink) and 1930-1990MHz(down) 300 carriers of 200KHz bandwidth

Radio Transmission

Frame and Time Slot

9

8

51

26

Radio Transmission

FLAG : 1bit Indicates whether user information or

control information TAIL bits : 3bit

All set to “0”

Radio Transmission

GUARD(Guard time) : 8.25bits(30.5sec) Reduce interference to signals in nearly cells using

the same carrier TRAIN(Training sequence) : 26bits

Used to train the adaptive equalizer, which compensates for effects of multipath propagation

Used to estimate the characteristics of the time-varying radio channel

8 different sequences with low mutual cross-correlation

Radio Transmission

Logical Channels

Radio Transmission

Broadcast channels FCCH(Frequency correction channel)

Used to allow MS to accurately tune to BS

SCH(Synchronization channel) Used to provide TDMA frame-oriented

synchronization data to MS

BCCH(Broadcast control channel) Transmit the information that MS need to

setup a call

Radio Transmission

Common control channels PCH(Paging channel)

Used to notify MS of arriving calls

AGCH(Access grant channel) Used to assign MS to a stand-alone dedicated

control channel

RACH(Random access channel) Used to send messages to originate calls,

transmit short messages, respond to paging messages

Radio Transmission

Dedicated control channels SDCCH(Stand-alone dedicated control channel)

Used for system signaling during idle periods and call setup before allocating TCH

FACCH(Fast associated control channel) Call establish progress, authentication, handover

SACCH(Slow associated control channel) Used for non-urgent procedures

BTS MS

Physical ChannelsFCCH

Training&slot informationSCH

System parametersBCCH

Channel requestRACH

Channel assignmentAGCH

Location update requestSDCCH

Authentication requestSDCCH

Authentication responseSDCCH

Ciphered mode commandSDCCH

ACK ciphered mode commandSDCCH

Location update confirmSDCCH

ACK of the updateSDCCH

1. Lock onto highest power level & best freq. (of a FCCH)

2. Synchronization data

3. LAC,MNC, other frequencies, etc4. Request & grant of a stand-alone dedi

cated control channel5. Move to SDCCH & make a location upd

ate request6. Prove who you claim to be

7. This is who I am

8. We will cipher traffic

9. I agree

10. Update VLR(s) & HLR as necessary

BTS MS

Page requestPCH

Channel requestRACH

Immediate assignmentAGCH

Paging responseSDCCH

Authentication requestSDCCH

Authentication responseSDCCH

Ciphering mode commandSDCCH

Ciphering mode completeSDCCH

TMSI reallocation commandSDCCH

TMSI reallocation completeSDCCH

Continue

1. Page the mobile station

2. Request & grant of a stand-alone dedicated control channel

3. Move to SDCCH & answer the page

4. Prove who you claim to be

5. This is who I am

6. We will cipher traffic

7. I agree

8. Set up temporary ID

Assignment commandSDCCH

Assignment completeFACCH

ALERTING messageFACCH

CONNECT messageFACCH

CONNECT ACK messageFACCH

ConversationTCH

ConversationTCH

SETUP messageSDCCH

SETUP ACK messageSDCCH

Continued

9. You have a call

10. I will accept it

11. Assign a traffic channel

12. (ringing)

13. Mobile off-hook, send CONNECT & ACK

14. Conversation ensues

Conclusion

Very complex standardFirst serious attempt to fulfill the requirements for a universal personal communication systemFoundation for DCS1800 and PCS1900Defines both the air interface and the network interface