GSM

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

5. 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

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. 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 4.615 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 • 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 • 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)

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

31. 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

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

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