1 Global System For Mobile Communication (GSM)

2 History  Europe cell tech fragmented in early 80’s  1982 GSM Study Group started  st Commercial System  Million Users, 22 Countries  Today 10 % of World Population  Today 709 Million users, 179 Countries  Today 71 % of entire digital wireless

3 Goals Full international roaming. Provision for national variations in charging and rates. Efficient interoperation with ISDN systems. Signal quality better than or equal to that of existing mobile systems Accommodation of non voice services Accommodation of portable terminals

4 Features: Multiple Access TDMA/FDM Multiple users share the same frequency channel sequentially Time slot sequence repeats FREQUENCY TIME TDMA User 3 User2 User 1

5 Cellular System SIR: 11 dB Reuse Factor: 3 Sectoring: 3 sectors/cell

6 Features: Service Telephone Data support packet switched protocol data rate from 300bps to 9.6kbps Other SMS (Short Message Service) limit 160 7bit ASCII characters

7 Frequency Characters Carrier Spacing: 200kHz Channels per carrier: 8 Modulation: 0.3 BT GMSK Data Rate: kbps MHz MHz GSM MHz MHz GSM MHz MHz GSM 1900 UplinkDownlink

8 Duplex Duplex: FDD Frequency spacing: 45MHz(GSM 900) 95MHz(GSM 1800) 80MHz(GSM 1900) Time slot spacing: 3 time slots

9 Architecture: Networking

10 Architecture: GSM Areas

11 Network Components The network system is divided into three major subsystems Base Station System (BSS) Switching System (SS) Operation Support System (OSS)

12 Base Station Subsystem (BSS)

13 Base Station Controller (BSC) Number of BSC varies Manages the allocation of radio resources for one or more BTSs, responsible for connection to MS on demand of MSC Controls handover between BTS and another BTS The link between the mobile station (MS) and the Mobile Switching Centre (MSC)

14 Base Transceiver Station (BTS) Handles the radio transceivers (transmitters and receivers) that define a cell Handles the radio-link protocols (air interface) with the Mobile Station (MS) SPP (Signal Processing Part) which includes the coding, encryption, modulation of the signal

15 Switching System (SS)

16 Mobile Switching Centre (MSC) Handles the switching of calls between external networks and the BSCs Controls handovers between BSC within the MSC area Decide which function that is going to be used (call, SMS etc) Coordinates VLR, HLR, OSS (OMC)

17 Home Location Register (HLR) Database that keeps information about all subscribers within it’s area Name, identification number, type of service, subscriber status, temporary roaming number for handovers Temporary information Permanent information Responsible for changing VLR-info if the subscriber changes VLR area

18 Visitor Location Register (VLR) Database that keeps information about all subscribers that temporary are within it’s area (MSC service area) Position updating if the MS moves to a different LA To connect up a call, the system now has no need to contact HLR, since VLR has all the necessary information Differs from HLR by the TMSI (Temporary Mobile Subscriber Identity) because of avoiding to send the IMSI (International Mobile Identification Number) via radio signals

19 Authentication Centre (AUC) Stores information regarding safety Encryption keys ki (subscriber authentication key) rand MSC computes SRES (Signal response) from ki and rand SRES = generated Identification parameter out of a randomly selected variable and the ki SRES now stored in HLR for use in case of call

20 Equipment Identity Register (EIR) EIR is an option available for any operator in GSM Contains information about stolen cellular, defect cellular that may not be used in the network etc Serial number, IMEI (international Mobile Equipment Identity) IMEI contains info about manufacturer, country of manufacturing and certificate

21 Operation Support System (OSS) Used mainly for supervision of GSM network Subscriber administration Configuration (connect cells, award identities to local areas (LAI) TRX administrator (specifies the TRX, channel administration) TMOS (Telecommunications Management and Operations Support) is a standard for implementation in a telecommunication network

22 Operation Support System (OSS) (continued) Errors from the network manually handled by an operator e.g. Defect circuit boards in a BTS Messages from a cellular that are registered in EIR that tries to call Checking the functionality of the systems components Statistics (current traffic between BTS-BSC etc) Keeps info about definitions of all cells Graphical tools for presenting images of the network

23 GSM Time Intervals

24 Traffic Multiframe

ms T3T3 DATA 57 bits F1F1 TRAIN 26 F1F1 DATA 57 bits T3T3 GUARD us GSM Traffic Time Slot T: Tail bits F: Flag Train: Equalizer Training Sequence

26 GSM Frames Super-frame Multi-frame Frame ms ms (Speech) s ms ms (Control) s Time Slot Hyper-frame A time slot contains bits  bit rate = * 8 / 4.615ms = 271 kbps

27 GSM Channels Traffic Channels (TCHs) Digitally encoded user speech or user data; Identical functions and formats on both the forward and reverse link. Control Channels (CCHs) Signaling and synchronizing commands between the base station and the mobile station Certain types of control channels are just for the forward or reverse link

28 GSM Control Channels Control Channel (CCH) Broadcast CH (BCH) Broadcast CCH (BCCH) Frequency Correction CH (FCCH) Synchronization CH (SCH) Common CCH (CCCH) Paging CH (PCH) Random Access CH (RACH) Grant CH (Access AGCH) Dedicated CCH (DCCH) Stand Alone DCCH (SADCCH) Slow Assisted CCH (SACCH) Fast Associated CCH (FACCH)

29 Broadcast and Common Control Channels Synchronization and System operation info Multiplexed on Time Slot 0 (TS) and if needed it uses TS 2, 4 or 6 Occupies Control Multi-frame FWD Link Multiframe F: FCCH frame (BCH) S: SCH frame (BCH) B: BCCH frame (BCH) 0F0F 1S1S 2B2B 3B3B 4B4B 5B5B 6C6C 7C7C 8C8C 9C9C 11 S 12 C 13 C 20 F 21 S 22 C 49 C 50 I 39 C 40 F 41 S 42 C 10 F C: PCH/AGCH frame (CCCH) I: Idle frame

30 Frequency Correction Channel 148 0s transmitted in FCCH Used by terminal to adjust its frequency reference to match that of the base station Occupies time slot 0 in a frame of eight time slots FCCH Tail bits All zerosGuard period

31 Synchronization Channel Contains Base Station Identity Code and current frame number in hyper frame Helps terminals synchronize their operations to a new base station SCH Start bitTail bitEncrypted Data Training bitsGuard period 39

32 Paging and Access Grant Used to notify terminals of arriving calls and to direct a terminal to a stand alone dedicated control channel Every mobile is assigned to a particular paging group

33 Random Access Channel Used by mobiles to originate phone calls, initiate SMS, respond to paging massages and register their locations. Shared by all mobiles on contention basis RACH Start bitTail bitSynchronizationEncrypted DataExtended Guard period

34 Speech Coding GSM uses linear prediction coding with regular pulse excitation (LPC-RPE) Each block of 20 ms consists of 260 bits 36 bits carry information about eight linear prediction coefficients 188 bits carry excitation information 36 bits represent long term predictor Speech coding rate is (260 bits/block)/ (20 ms/block) = 13,000 bits/second

35 Channel Coding for Speech Signals LPC-RPE Speech Coder Calculate 3 parity bits Rate ½ Channel code Multiplex interleave 78 least important bits 132 important bits 50 essential bits 4 tail bits 378 bits 456 bits Data field Of 4 time slots 53 bits

36 Channel Coding The channel coding process generates a total of 456 bits every 20 ms 50 essential bits 3 error-detecting parity bits 132 important bits The speech transmission rate is (456 bits/block)/(20 ms/block) = 22,800 bits/second

37 Interleaving Interleaving takes error clusters and spreads them out over large intervals. 2 speech blocks, or 40 ms of speech (2x456=912 coded speech bits), distributed over 8 frames.

38 Multipath Equalization Time varying effect of radio channels Adaptive equalizer is an important component of every GSM receiver Extracts desired signal from multiple versions of the signal Uses 26-bit training sequence and inverse filter GSM specifies 8 different training sequences assigned to nearby cells which use the same carrier Enables terminals and base stations to confirm the received signal comes from the correct transmitter GSM can handle delay spreads up to 16us ~ 4-bit period

39 Power Control GSM specifies 5 classes of terminals by maximum transmitter power (20W, 8W, 5W, 2W, 0.8W) Power can adjust by steps of 2 dB to any of 16 power levels that range over 30 dB A full-rate transmitter is active for 1 time slot per frame Advantages: Minimizes co-channel interference Conserves power Power control should be handled carefully because terminals increasing their power can cause co-channel cells’ terminals to increase their power levels, which results in unstable condition.

40 GSM Innovations Mobile assisted handoffs (MAHO) Location-based mobility management Network interfaces in addition to air interface Made the subscriber mobile with the Subscriber Identity Module (SIM) Card

41 GSM Mobility management Location management In GSM a compromise between the two is achieved by requiring the mobile to register only when it changes a collection of cells called a location area. The mobile is then paged only in the cells in the location area it last registered from. This is a tradeoff between high number of registrations to high number of paging attempts.

42 GSM Mobility management Handoff – In GSM handoff is done using the assistance of the mobile. This is called as MAHO (mobile assisted handoff). Handoff process proceeds like this : The mobile detects that the bit error rate for the base station signal has increased beyond a threshold value The mobile then measures the signal strength of the surrounding cells and sends measurement reports to MSC MSC then decides the target cell for handoff. The handoff is hard - ‘break before make’ handoff

43 GSM Mobility Management Roaming Terminal Mobility – If same radio frequency is employed or if mobile has multiple modes, then terminal mobility is possible Subscriber Mobility – By swapping the SIM card to the appropriate GSM terminal, subscriber is freed from the terminal

44 Frequency Hopping Objective To avoid severe multipath problems Procedure Change the carrier frequency of a given user Characters Hopping frame by frame Maximum hops / second