GSM

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WHAT IS GSM

• Global System for Mobile Communication• Developed by “Group Spe’ciale Mobile – An

Initiative of CEPT ( Conference of European Post & Telecomm )

• AIM : To replace the incompatible Analog System and provide International Roaming

• GSM was commercially launched in 1991• GSM is the most widely accepted and globally

implemented standard

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WHAT IS GSM (CONTD.)

• GSM makes use of TDMA technique for transmitting signals

• GSM is a 2G standard using digital cellular technology for transmitting voice and data

• GSM900 and GSM1800 is used in Europe and GSM1900 is used in US

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GSM Architecture

MS : MOBILE STATION

BSS : BASE STATION SUBSYSTEM

NSS : NETWORK SWITCHING SUBSYSTEM

OSS : OPERATION SUPPORT SUBSYSTEM

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GSM NETWORK AREAS• CELL :

– BASIC SERVICE AREA IDENTIFIED BY “CI”– ONE BTS PER CELL– CELL GLOBAL ID (CGI) = LAI + CI

• LOCATION AREA :– GROUP OF CELLS AS LOCATION AREA SERVED BY ONE OR MORE BSCs– IDENTIFIED BY “LAI”– LAI IS BROADCAST REGULARLY BY BTS

• MSC/VLR SERVICE AREA :– AREA COVERED BY MSC

• PLMN :– AREA COVERED BY A NETWORK OPERATOR HAVING ONE OR MORE MSCs– PUBLIC LAND MOBILE NETWORK

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GSM SUBSYSTEMS

• NSS : NETWORK SWITCHING SUBSYSTEM

• BSS : BASE STATION SUBSYSTEM

• OSS : OPERATION SUPPORT SUBSYSTEM

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NSS

• NETWORK SWITCHING SUBSYSTEM• CONSISTS OF MSC, AuC, VLR, HLR,EIR• PERFORMS SWITCHING OF CALLS BETWEEN

THE MOBILE AND OTHER SUBSCRIBERS• ALSO MANAGES SERVICES SUCH AS

AUTHENTICATION• CONNECTS THE MOBILE NETWORK TO OTHER

NETWORKS SUCH AS PSTN, ISDN, PLMN, ETC

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BSS

• BASE STATION SUBSYSTEM• CONSISTS OF BTS AND BSC• BSS MAY HAVE ONE OR MORE BASE STATIONS• IT USES ‘Abis’ INTERFACE BETWEEN BTS AND

BSC• A SEPARATE HIGH-SPEED LINE (T1 / E1) IS

CONNECTED FROM BSS TO THE MSC

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OSS

• OPERATION SUPPORT SUBSYSTEM• PROVIDES A NETWORK OVERVIEW• CONSISTS OF OPERATIONS AND MAINTENANCE CENTRE

(OMC)• OMC IS CONNECTED TO ALL EQUIPMENT IN NSS AND TO BSC

IN BSS• ITS FUNCTIONS ARE

– ADMINISTRATION– MAINTENANCE– NETWORK CONFIGURATION– SECURITY– COMMERCIAL OPERATIONS

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OMC

OPERATIONS AND MAINTENANCE CENTRE

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GSM ARRESSES AND INDENTIFIERS

• IMEI ( INTERNATIONAL MOBILE EQUIPMENT IDENTITY )– UNIQUELY IDENTIFIES A MOBILE STATION INTERNATIONALLY– ALLOCATED BY THE EQUIPMENT MANUFACTURER– STORED IN “EIR” BY THE NETWORK OPERATOR– INDICATES MANUFACTURER AND DATE OF MANUFACTURE– 19 DECIMAL PLACES

• IMSI ( INTERNATIONAL MOBILE SUBSCRIBER IDENTITY )– EACH REGISTERED USER IDENTIFIED BY IMSI– STORED IN SUBSCRIBER IDENTITY MODULE (SIM)– 15 DECIMAL PLACES CONTAINING COUNTRY CODE, NETWORK

CODE AND MOBILE SUBSCRIBER IDENTIFICATION NUMBER(MSIN)

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GSM ARRESSES AND INDENTIFIERS

• MSISDN ( MOBILE SUBSCRIBER ISDN NUMBER ) – THE REAL TELEPHONE NUMBER OF A MOBILE STATION SIM

CARD– UPTO 16 DECIMAL PLACES– CONTAINS COUNTRY CODE, NATIONAL DESTINATION CODE AND

SUBSCRIBER NUMBER (10 DIGIT)• MSRN (MOBILE STATION ROAMING NUMBER)

– IT IS A TEMPORARY LOCATION DEPENDENT ISDN NUMBER– ASSIGNED BY VLR TO EACH MOBILE STATION IN IT’S AREA AND

ALSO STORED IN HLR– STRUCTURE SAME AS MSISDN– CALLS ARE ROUTED TO THE MOBILE USING MSRN

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GSM ARRESSES AND INDENTIFIERS

• TMSI (TEMPORARY MOBILE SUBSCRIBER IDENTITY)– VLR OF CURRENT CAN ASSIGN TMSI WHICH IS VALID ONLY AT

THAT LOCATION– USED TO HIDE THE IMSI (SUBSCRIBER IDENTITY)– STORED ONLY IN VLR AND NOT UPDATED IN HLR– VLR CHANGES THE TMSI PERIODICALLY AS A PART OF SECURITY

• LMSI (LOCAL MOBILE SUBSCRIBER IDENTITY)– ASSIGNED WHEN MOBILE STATION REGISTERS WITH VLR– IT IS AN ADDITIONAL KEY FOR SEARCHING TO ACCELERATE

DATABASE ACCESS– IT IS ALSO UPDATED IN HLR

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GSM OPERATION

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GSM TDMA / FDMA

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GSM FRAME HIERARCHY

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GSM AIR (RADIO) INTERFACE

• Um – THE GSM AIR (RADIO) INTERFACE PROVIDES THE PHYSICAL LINK BETWEEN MOBILE AND NETWORK

• AIR INTERFACE PARAMETERS :– FREQ. BAND MOBILE – BASE : 890 – 915 MHz– FREQ. BAND BASE – MOBILE : 935 – 960 MHz– 124 RADIO CARRIERS SPACED BY 200 KHz– TDMA STRUCTURE WITH 8 TIMESLOTS / CARRIER– GMSK MODULATION– SLOW FREQUENCY HOPPING AT 217 HOPS / SECOND– CHANNEL CODING WITH INTERLEAVING– DOWN LINK AND UP LINK POWER CONTROL– DISCONTINUOUS TRANSMISSION AND RECEPTION

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GSM PHYSICAL CHANNELS

• EACH 200 KHz CARRIER IS SUBDIVIDED INTO FRAMES THAT ARE REPEATED CONTINUOUSLY

• A FRAME IS SUBDIVIDED INTO 8 TIME SLOTS• EACH TIME SLOT REPRESENTS A PHYSICAL CHANNEL• EACH CHANNEL OCCUPIES THE 200 KHz CARRIER FOR

577 MuS EVERY 4.615 MS• DATA IS TRANSMITTED IN SMALL BURSTS INSIDE A

TIME SLOT (USER AND SIGNALLING DATA)

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GSM PHYSICAL CHANNELS (CONTD.)

• THE REMAINING PART OF THE TIME SLOT IS “GUARD SPACE” TO AVOID OVERLAPPING OF BURSTS

• EACH PHYSICAL CHANNEL HAS A DATA RATE OF 33.8 KBPS AND EACH RADIO CARRIER TRANSMITS 270 KBPS OVER THE ‘Um’ INTERFACE

• THUS ONE PHYSICAL CHANNEL IS ONE BURST PERIOD ALLOCATED IN EACH TDM FRAME

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TYPES OF BURSTS IN GSM

1. NORMAL BURST2. SYNCHRONIZATION BURST3. FREQUENCY CORRECTION BURST4. ACCESS BURST5. DUMMY BURST– DUMMY BURST IS USED IF NO DATA IS AVAILABLE

FOR THE SLOT

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NORMAL BURST

T : TAIL BITSINFO : DATA BITSF : FLAG BITTRAINING : BITS TO SET RECEIVER PARAMETERSG : GUARD BITS

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SYNCHRONIZATION BURST

T : TAIL BITSINFO : DATA BITSG : GUARD BITS

LONG TRG SEQUENCE : SYNCHRONIZES MS WITH THE BTS IN TIME

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FREQUENCY CORRECTION BURST

T : TAIL BITSG : GUARD BITS

ALL BITS ARE SET TO ZERO.

ALLOWS MS TO CORRECT THE LOCAL OSCILLATOR TO AVOID INTERFERENCE WITH NEIGHBOURING CHANNELS

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ACCESS BURST

T : TAIL BITSG : GUARD BITS

USED FOR INITIAL CONNECTION SETUP BETWEEN MS AND BTS. IT HAS MUCH LARGER GUARD SPACE.

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GSM LOGICAL CHANNELS

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GSM LOGICAL CHANNELS

• ‘TCH’ (TRAFFIC CHANNEL) – USED TO TRANSMIT USER DATA ( VOICE & FAX )

• ‘CCH’ (CONTROL CHANNELS) ARE USED TO– CONTROL MEDIUM ACCESS– ALLOCATION OF TRAFFIC CHANNELS (MOBILITY MANAGEMENT)

• ‘BCCH’ : BROADCAST CONTROL CHANNEL – A BTS USES THIS TO SIGNAL INFO TO ALL MSs WITHIN THE CELL

• ‘CCCH’ : COMMON CONTROL CHANNEL – CONNECTION SETUP INFO I EXCHANGED BETWEEN MS AND BTS

• ‘DCCH’ : DEDICATED CONTROL CHANNEL – THESE ARE BIDIRECTIONAL AS AGAINST ALL OTHER CHANNELS WHICH ARE UNI-DIRECTIONAL

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GSM PROTOCOLS

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GSM PROTOCOLS

• RADIO– THE PHYSICAL LAYER– HANDLES RADIO SPECIFIC FUNCTIONS LIKE CREATING BURSTS,

MULTIPLEXING INTO TDMA FRAME, SYNCHRONIZATION WITH BTS, DETECTION OF IDLE CHANNELS, MEASUREMENT OF CHANNEL QUALITY, ENCRYPTION, DECRYPTION, CHANNEL CODING, ERROR DETECTION AND CORRECTION, VOICE ACTIVITY DETECTION, ETC

• LAPDm– THE DATA LINK LAYER– MODIFIED VERSION OF ISDN PROTOCOL– HANDLES SIGNALLING BETWEEN ENTITIES, RELIABLE DATA

TRANSFER, RESEQUENCING OF DATA FRAMES, FLOW CONTROL, SEGMENTATION AND REASSEMBLY OF DATA, ETC

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GSM PROTOCOLS (CONTD.)

• RR– NETWORK LAYER PROTOCOL– RADIO RESPONSE (RR) MANAGEMENT SUPPORTED BY BTS

MANAGEMENT (BTSM)– SETUP, MAINTENANCE AND RELEASE OF RADIO CHANNELS

• MM– NETWORK LAYER PROTOCOL– MOBILITY MANAGEMENT – REGISTRATION, AUTHENTICATION,

LOCATION UPDATING, PROVISION OF TMSI TO REPLACE IMSI, ETC

• CM– NETWORK LAYER PROTOCOL– CALL MANAGEMENT – CALL CONTROL (CC), SMS AND SUPPLEMENTARY

SERVICE (SS)

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GSM PROTOCOLS (CONTD.)

• PCM– PULSE CODE MODULATION SYSTEM

• SS7– SIGNALLING SYSTEM 7

• BSSAP– BASE STATION SUBSYSTEM (BSS) APPLICATION

PART

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GSM SPECIFICATIONS

• MODULATION– GAUSSIAN MINIMUM SHIFT KEYING ( GMSK )

• ACCESS METHOD– COMBINATION OF TDMA / FDMA– FREQUENCY OF TOTAL 25 MHz BANDWIDTH DIVIDED INTO 124

CARRIER FREQUENCIES OF 200 KHz BANDWIDTH– EACH OF THESE CARRIER FREQUENCIES IS THEN DIVIDED IN TIME

INTO 8 TIME SLOTS– ONE SLOT IS USED FOR TRANSMISSION AND ONE FOR RECEPTION– THE TWO TIME SLOTS ARE ALSO SEPARATED IN TIME. SO RECEIVE

AND TRANSMIT IS NOT AT THE SAME TIME– EACH BASE STATION IS ASSIGNED A SET OF CARRIER FREQUENCIES

FROM THE TOTAL CARRIER FREQUENCIES

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GSM SPECIFICATIONS (CONTD.)

• TRANSMISSION RATE– TRANSMISSION RATE OF TIME SLOT : 22.8 KBPS– OVER THE AIR THE BIT RATE : 270 KBPS

• FREQUENCY BAND– GSM900

• UPLINK : 890 TO 915 MHz• DOWNLINK : 935 TO 960 MHz

– GSM1800/DCS1800• UPLINK : 1710 TO 1785 MHz• DOWNLINK : 1805 TO 1880 MHz

– GSM1900/PCS1900 (IN US)• UPLINK : 1850 TO 1910 MHz• DOWNLINK : 1930 TO 1990 MHz

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GSM SPECIFICATIONS (CONTD.)

• CHANNEL SPACING– SEPARATION BETWEEN ADJACENT CARRIER

FREQUENCIES : 200 KHz• SPEECH CODING– LINEAR PREDICTIVE CODING (LPC)– SPEECH IS ENCODED AT 13 KBPS

• DUPLEX DISTANCE– IT IS THE DISTANCE BETWEEN UPLINK AND DOWNLINK

FREQUENCIES– A CHANNEL HAS TWO FREQUENCIES 80 MHz APART

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GSM SPECIFICATIONS (CONTD.)

• DUPLEX TECHNIQUE– FREQUENCY DIVISION DUPLEXING (FDD)– ACCESS MODE ALSO CALLED WCDMA

• FRAME DURATION– 4.615 MS

• SPEECH CHANNELS PER RF CHANNEL– 8 CHANNELS

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GSM SERVICES

• GSM HAS DEFINED THREE CATEGORIES OF SERVICES– BEARER SERVICES ( DATA SERVICES )– TELE SERVICES ( TELEPHONY )– SUPPLEMENTARY SERVICES

• GSM VALUE ADDED SERVICES

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BEARER SERVICES ( DATA SERVICES )

• CONNECTION ORIENTED, CIRCUIT OR PACKET SWITCHED

• PERMITS SYNCHRONOUS OR ASYNCHRONOUS DATA TRANSMISSION UPTO 9.6 KBPS

• FOR INTERNETWORKING WITH PSTN, PSDN AND PACKET SWITCHED PUBLIC DATA NETWORKS

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TELE SERVICES ( TELEPHONY )

• GSM MAINLY FOCUSES ON VOICE-ORIENTED TELE SERVICES

• EMERGENCY NUMBER SERVICE• VOICE MESSAGING SERVICE• GROUP 3 FAX SERVICE ( FAX DATA IS

TRANSMITTED AS DIGITAL DATA OVER THE TELEPHONE NETWORK

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SUPPLEMENTARY SERVICES

• MAY VARY FROM OPERATOR TO OPERATOR• TYPICAL SERVICES ARE :– USER IDENTIFICATION– CALL REDIRECTION OR FORWARDING– MULTI-PARTY COMMUNICATION– CLOSED USE GROUP (CUG)– CALL WAITING– CALL BARRING, ETC.

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GSM VALUE ADDED SERVICES

• THESE SERVICES ARE PROVIDED BY MEANS OF PERIPHERAL UNITS ATTACHED TO THE CELLULAR NETWORK

• TYPICAL VALUE ADDED SERVICES ARE :– MESSAGING SERVICE– INFORMATION SERVICE– PRIVATE INTERCONNECT

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MESSAGING SERVICE

• IN ADDITION TO THE VOICE MESSAGING SERVICE, GSM NETWORKS INCORPORATE “SHORT MESSAGE SERVICE” (SMS)

• SMS IS A METHOD OF COMMUNICATION THAT USES TEXT MESSAGING BETWEEN CELL PHONES OR FROM PC OR HANDHELD DEVICES TO CELL PHONES

• MAXIMUM SIZE OF TEXT IS 160 CHARACTERS• THE CONTROL CHANNEL PROVIDES A PATHWAY FOR SMS

MESSAGES.• THE SMS FLOWS THROUGH SMSC – TO THE TOWER – THEN

TO THE CELL PHONE AS A LITTLE PACKET OF DATA ON THE CONTROL CHANNEL

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SMS PATH

MSC MSCMSC

CENTRALIZED SMSC

MS MS

CORE NETWORK

SMS CENTRE

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SMS

• SMS IS A “STORE-AND-FORWARD” SERVICE• DESTINATION CELL PHONE NEED NOT BE ‘ON’ OR ‘IN

RANGE’ TO SEND AN SMS• THE MESSAGE IS STORED IN THE ‘SMSC’ – THE

CENTRALIZED SMS CENTRE TILL THE DESTINATION CELL PHONE TURNS ON OR MOVES INTO THE RANGE AND THEN IT IS DELIVERED TO IT

• THUS SMS IS CHARACTERIZED BY LOW BANDWIDTH MESSAGE TRANSFER AND IS HIGHLY EFFICIENT SERVICE

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INFORMATION SERVICE

• PROVIDES USEFUL INFORMATION TO THE MOBILE USER

• EXAMPLES :– TRAVEL INFORMATION– WEATHER INFORMATION– AREA INFORMATION BASED ON CURRENT

LOCATION OF THE MOBILE USER

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PRIVATE INTERCONNECT

• DIRECT CONNECTIONS CAN BE MADE AVAILABLE BETWEEN THE CELLULAR NETWORK AND THE CORPORATE PRIVATE NETWORK, THUS BYPASSING THE PSTN

• CALL CHARGES FOR SUCH DIRECT CONNECTIONS ARE SUBSTANTIALLY LOWER THAN THE PSTN CALL CHARGES AND HENCE ARE BENEFICIAL TO THE CORPORATES

• AN ADDITIONAL BENEFIT IS THAT THE PRIVATE INTERCONNECT ALLOWS CALLS TO BE DELIVERED DIRECT TO THE EXTENSIONS REMOVING THE NEED FOR SWITCHBOARD OPERATOR

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GSM SECURITY

• GSM OFFERS SEVARAL SECURITY SERVICES USING CONFIDENTIAL INFORMATION STORED IN THE AuC AND THE ‘SIM’

• SIM STORES PERSONAL, SECRET DATA AND IS PROTECTED WITH A PIN TO PREVENT UNAUTHORIZED USE

• SECURITY SERVICES OFFERRED ARE :– ACCESS CONTROL AND AUTHENTICATION– CONFIDENTIALITY– ANONYMITY

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ACCESS CONTROL AND AUTHENTICATION

• USER NEEDS A SECRET ‘PIN’ TO ACCESS THE SIM

• ALGORITHM USED IS ‘A3’

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CONFIDENTIALITY

• ALL USER RELATED DATA IS ENCRYPTED• ALGORITHM ‘A5’ AND CIPHER KEY ‘Ki’ IS USED

TO ENCRYPT THE DATA• ALGORITHM ‘A8’ IS USED TO GENERATE THE

CIPHER KEY

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ANONYMITY

• ALL DATA IS ENCRYPTED BEFORE TRANSMISSION

• USER IDENTIFIERS ARE NOT SENT OVER THE AIR INTERFACE

• GSM TRANSMITS A TEMPORARY IDENTIFIER (TMSI) NEWLY ASSIGNED BY THE VLR AFTER EACH LOCATION UPDATE

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AUTHENTICATION

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AUTHENTICATION

• SUTHENTICATION BASED ON SIM• SIM STORES

– INDIVIDUAL AUTHENTICATION KEY– USER IDENTIFICATION (INSI)– ALGORITHM ‘A3’

• AUTHENTICATION USES A “CHALLENGE-RESPONSE” METHOD– A RANDOM NO. ‘RAND’ IS GENERATED AS CHALLENGE– MS ANSWERS WITH ‘SRES’ – THE SIGNED RESPONSE

• AuC GENERATES ‘RAND’, ‘SERS’ AND CIPHER KEY ‘Ki’ FOR EACH IMSI AND FORWARDS IT TO HLR

• FOR AUTHENTICATION, VLR SENDS THE ‘RAND’ TO THE SIM

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AUTHENTICATION (CONTD.)

• BOTH NETWORK AND MOBILE PERFORM THE SAME OPERATION WITH ‘RAND’ AND ‘Ki’ USING THE ALGORITHM ‘A3’

• MOBILE SENDS BACK THE GENERATED ‘SRES’• VLR COMPARES THE ‘SRES’ VALUES

COMPUTED BY NETWORK AND THE MOBILE• IF THEY ARE SAME THEN THE SUBSCRIBER IS

ACCEPTED ELSE THE SUBSCRIBER IS REJECTED

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ENCRYPTION

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ENCRYPTION

• TO ENSURE PRIVACY, ALL MESSAGES CONTAINING USER RELATED INFORMATION ARE ENCRYPTED IN GSM

• THE CIPHER KEY ‘Kc’ IS GENERATED USING THE INDIVIDUAL KEY ‘Ki’ AND A RANDOM VALUE BY APPLYING THE ALGORITHM ‘A8’

• THE CIPHER KEY ‘Kc’ IS NOT TRANSMITTED OVER THE AIR INTERFACE

• MOBILE AND BTS THEN ENCRYPT AND DECRYPT DATA USING THE ALGORITHM ‘A5’ AND THE CIPHER KEY ‘Kc’

• ‘Kc’ IS 64 BIT KEY – WHICH IS NOT VERY STRONG BUT PROVIDES GOOD PROTECTION AGAINST SIMPLE EAVESDROPPING

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GSM SECURITY

THUS GSM SECURITY FEATURES AS FOLLOWS : • On air interface, GSM uses encryption and TMSI

instead of IMSI.• SIM is provided 4-8 digit PIN to validate the

ownership of SIM• 3 algorithms are specified :

- A3 algorithm for authentication- A5 algorithm for encryption- A8 algorithm for key generation

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LOCATION UPDATING

• MOBILE SUBSCRIBER MOVES FROM PLACE TO PLACE

• THE MOBILE NETWORK KEEPS TRACK OF THE MOBILE LOCATION BY UPDATING THE LOCATION INFORMATION IN THE REGISTERS

• THERE ARE THREE TYPES OF LOCATION UPDATING1. NORMAL2. IMSI ATTACH3. PERIODIC REGISTRATION

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LOCATION UPDATING - NORMAL

• FROM TIME TO TIME, MS LISTENS TO THE “LAI” (LOCATION AREA IDENTITY) BROADCAST BY THE BTS AND COMPARES IT WITH THE “LAI” STORED IN THE SIM CARD

• IF THE TWO VALUES DIFFER, IT MEANS THAT MS HAS ENTERED A NEW LOCATION AREA AND MS MUST PERFORM A LOCATION UPDATE TYPE NORMAL

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LOCATION UPDATING - NORMAL• WHEN MS FINDS A DIFFERENT “LAI”, IT SENDS A CHANNEL REQUEST• BTS FORWARDS THE REQUEST TO BSC.• BSC ALLOCATES “SDCCH” IF AVAILABLE AND TELLS BTS TO ACTIVATE IT• MS IS TOLD TO TUNE TO SDCCH• MS SENDS LOCATION UPDATE REQUEST• AUTHENTICATION PARAMETER IS SENT TO MS• MS RESPONDS• IF AUTHENTICATION IS SUCCESSFUL, THE VLR IS UPDATED AND THE

OLD HLR/VLR ARE ALSO UPDATED• MS RECEIVES ACCEPTANCE• BTS IS TOLD TO RELEASE THE SDCCH• MS IS TOLD TO RELEASE THE SDCCH

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LOCATION UPDATING – IMSI ATTACH

• IMSI ATTACH IS A PROCEDURE USED BY MS TO NOTIFY THE NETWORK THAT IT HAS POWERED ON

• MS REQUESTS FOR AN SDCCH• NETWORK RECEIVES THE IMSI ATTACH REQUEST

FROM THE MS• MSC SENDS IT TO THE VLR TO RESET THE IMSI

DETACHED FLAG• MS RECEIVES THE IMSI ATTACH

ACKNOWLEDGEMENT MESSAGE

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LOCATION UPDATING – PERIODIC REGISTRATION

• PERIODIC REGISTRATION PROCEDURE IS USED TO AVOID UNNECESSARY PAGING OF MS IN CASE MSC NEVER RECEIVES THE IMSI DETACH REQUEST FROM MS

• MS LISTENS ON THE ‘BCCH’ TO CHECK IF PERIODIC REGISTRATION IS USED IN THE CELL

• IF YES, MS IS TOLD BY BTS HOW OFTEN IT MUST REGISTER– THE VALUE CAN RANGE FROM 0 – 255 DECI HOURS ( A UNIT OF 6 MINUTES)– IF THE VALUE IS ‘0’, PERIODIC REGISTRATION IS NOT USED IN THAT CELL– IF IT IS 10, MS MUST REGISTER EVERY HOUR

• WHEN THE TIMER IN MS EXPIRES, MS PERFORMS THE LOCATION UPDATE – TYPE PERIODIC REGISTRATION

• IF MS DOES NOT REGISTER WITHIN THE TIME INTERVAL PLUS A GUARD TIME, THEN MSC FLAGS THE MS AS DETACHED

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CALL ROUTING – OUGOING CALL1. MS sends dialled number to BSS 2. BSS sends dialled number to

MSC3,4 MSC checks VLR if MS is

allowed the requested service.If so,MSC asks BSS to allocate resources for call.

5 MSC routes the call to GMSC6 GMSC routes the call to local

exchange of called user7, 8,9,10 Answer back(ring back) tone

is routed from called user to MS via GMSC,MSC,BSS

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CALL ROUTING – INCOMING CALL1. Calling a GSM subscribers2. Forwarding call to GSMC3. Signal Setup to HLR4. 5. Request MSRN from

VLR6. Forward responsible MSC

to GMSC7. Forward Call to current

MSC8. 9. Get current status of

MS10.11. Paging of MS12.13. MS answers14.15. Security checks16.17. Set up connection

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HANDOVERS

• Between 1 and 2 – Inter BTS / Intra BSC

• Between 1 and 3 – Inter BSC/ Intra MSC• Between 1 and 4 – Inter MSC