1. GSM

2. 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

3. 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

4. BSS : BASE STATION SUBSYSTEM
GSM Architecture
MS : MOBILE STATION
BSS : BASE STATION SUBSYSTEM
NSS : NETWORK SWITCHING SUBSYSTEM
OSS : OPERATION SUPPORT SUBSYSTEM

5. GSM NETWORK AREAS CELL : LOCATION AREA : MSC/VLR SERVICE AREA : PLMN :
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

6. GSM SUBSYSTEMS NSS : NETWORK SWITCHING SUBSYSTEM
BSS : BASE STATION SUBSYSTEM
OSS : OPERATION SUPPORT SUBSYSTEM

7. 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

8. 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

9. 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

10. OPERATIONS AND MAINTENANCE CENTRE
OMC
OPERATIONS AND MAINTENANCE CENTRE

11. 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)

12. 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

13. 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

14. GSM OPERATION

15. GSM TDMA / FDMA

16. GSM FRAME HIERARCHY

17. 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

18. 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 MS
DATA IS TRANSMITTED IN SMALL BURSTS INSIDE A TIME SLOT (USER AND SIGNALLING DATA)

19. 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

20. TYPES OF BURSTS IN GSM NORMAL BURST SYNCHRONIZATION BURST
FREQUENCY CORRECTION BURST
ACCESS BURST
DUMMY BURST
DUMMY BURST IS USED IF NO DATA IS AVAILABLE FOR THE SLOT

21. NORMAL BURST T : TAIL BITS INFO : DATA BITS F : FLAG BIT
TRAINING : BITS TO SET RECEIVER PARAMETERS
G : GUARD BITS

22. SYNCHRONIZATION BURST
T : TAIL BITS
INFO : DATA BITS
G : GUARD BITS
LONG TRG SEQUENCE : SYNCHRONIZES MS WITH THE BTS IN TIME

23. FREQUENCY CORRECTION BURST
T : TAIL BITS
G : GUARD BITS
ALL BITS ARE SET TO ZERO.
ALLOWS MS TO CORRECT THE LOCAL OSCILLATOR TO AVOID INTERFERENCE WITH NEIGHBOURING CHANNELS

24. ACCESS BURST
T : TAIL BITS
G : GUARD BITS
USED FOR INITIAL CONNECTION SETUP BETWEEN MS AND BTS. IT HAS MUCH LARGER GUARD SPACE.

25. GSM LOGICAL CHANNELS

26. 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

27. GSM PROTOCOLS

28. GSM PROTOCOLS RADIO LAPDm 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

29. GSM PROTOCOLS (CONTD.) RR MM CM NETWORK LAYER PROTOCOL
RADIO RESPONSE (RR) MANAGEMENT SUPPORTED BY BTS MANAGEMENT (BTSM)
SETUP, MAINTENANCE AND RELEASE OF RADIO CHANNELS MM
MOBILITY MANAGEMENT – REGISTRATION, AUTHENTICATION, LOCATION UPDATING, PROVISION OF TMSI TO REPLACE IMSI, ETC CM
CALL MANAGEMENT – CALL CONTROL (CC), SMS AND SUPPLEMENTARY SERVICE (SS)

30. GSM PROTOCOLS (CONTD.) PCM SS7 BSSAP PULSE CODE MODULATION SYSTEM
SIGNALLING SYSTEM 7
BSSAP
BASE STATION SUBSYSTEM (BSS) APPLICATION PART

31. GSM SPECIFICATIONS MODULATION ACCESS METHOD
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

32. 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

33. 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

34. 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

35. GSM SERVICES GSM HAS DEFINED THREE CATEGORIES OF SERVICES
BEARER SERVICES ( DATA SERVICES )
TELE SERVICES ( TELEPHONY )
SUPPLEMENTARY SERVICES
GSM VALUE ADDED SERVICES

36. 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

37. 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

38. 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.

39. 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

40. 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

41. SMS PATH
MSC
CENTRALIZED SMSC MS
CORE NETWORK
SMS CENTRE

42. 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

43. INFORMATION SERVICE PROVIDES USEFUL INFORMATION TO THE MOBILE USER
EXAMPLES :
TRAVEL INFORMATION
WEATHER INFORMATION
AREA INFORMATION BASED ON CURRENT LOCATION OF THE MOBILE USER

44. 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

45. 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

46. ACCESS CONTROL AND AUTHENTICATION
USER NEEDS A SECRET ‘PIN’ TO ACCESS THE SIM
ALGORITHM USED IS ‘A3’

47. 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

48. 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

49. AUTHENTICATION

50. 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

51. 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

52. ENCRYPTION

53. 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

54. 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

55. 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 UPDATING
NORMAL
IMSI ATTACH
PERIODIC REGISTRATION

56. 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

57. 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

58. 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

59. 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

60. CALL ROUTING – OUGOING CALL
MS sends dialled number to BSS
BSS sends dialled number to MSC
3,4 MSC checks VLR if MS is allowed the requested service.If so,MSC asks BSS to allocate resources for call.
MSC routes the call to GMSC
GMSC routes the call to local exchange of called user
7, 8,
9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS

61. CALL ROUTING – INCOMING CALL
Calling a GSM subscribers
Forwarding call to GSMC
Signal Setup to HLR
5. Request MSRN from VLR
Forward responsible MSC to GMSC
Forward Call to current MSC
9. Get current status of MS
11. Paging of MS
13. MS answers
15. Security checks
17. Set up connection

62. HANDOVERS Between 1 and 2 – Inter BTS / Intra BSC Between 1 and 3 –
Inter BSC/ Intra MSC
Between 1 and 4 –
Inter MSC