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Global System For Mobile Communication (GSM)

Global System For Mobile Communication (GSM). History. Europe cell tech fragmented in early 80’s 1982 GSM Study Group started 1991 1 st Commercial System 1993 1 Million Users, 22 Countries Today 10 % of World Population Today 709 Million users, 179 Countries

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Global System For Mobile Communication (GSM)

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  1. Global System For Mobile Communication (GSM)

  2. History • Europe cell tech fragmented in early 80’s • 1982 GSM Study Group started • 1991 1st Commercial System • 1993 1 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. TIME User 3 User2 User 1 FREQUENCY TDMA Features: Multiple Access • TDMA/FDM • Multiple users share the same frequency channel sequentially • Time slot sequence repeats

  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. Uplink Downlink GSM 900 890-915 MHz 935-960 MHz GSM 1800 1710-1785 MHz 1805-1880 MHz GSM 1900 1850-1910 MHz 1930-1990 MHz Frequency Characters • Carrier Spacing: 200kHz • Channels per carrier: 8 • Modulation: 0.3 BT GMSK • Data Rate: 270.833kbps

  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

  25. GSM Traffic Time Slot • T: Tail bits • F: Flag • Train: Equalizer Training Sequence 4.615 ms 0 1 2 3 4 5 6 7 T 3 DATA 57 bits F 1 TRAIN 26 F 1 DATA 57 bits T 3 GUARD 8.25 577 us

  26. 6.12 s 6.12 s 0 0 1 1 2 2 48 23 24 49 50 25 120 ms (Speech) 235 ms (Control) 0 1 2 22 23 24 25 0 1 2 47 48 49 50 4.615 ms 4.615 ms 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 GSM Frames Hyper-frame 0 1 1022 1023 Super-frame Multi-frame Frame A time slot contains 156.25 bits  bit rate = 156.25 * 8 / 4.615ms = 271 kbps Time Slot

  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

  29. FWD Link Multiframe 41 S 0 F 49 C 2 B 4 B 20 F 6 C 39 C 8 C 12 C 40 F 42 C 1 S 3 B 5 B 21 S 13 C 50 I 9 C 7 C 10 F 11 S 22 C C: PCH/AGCH frame (CCCH) I: Idle frame F: FCCH frame (BCH) S: SCH frame (BCH) B: BCCH frame (BCH) 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

  30. FCCH 3 142 3 8.25 Tail bits All zeros Tail bits Guard period 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

  31. Contains Base Station Identity Code and current frame number in hyper frame Helps terminals synchronize their operations to a new base station SCH 3 39 64 39 3 8.25 Start bit Encrypted Data Training bits Encrypted Data Tail bit Guard period Synchronization Channel

  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. RACH 8 41 36 3 68.25 Start bit Synchronization Encrypted Data Tail bit Extended Guard period 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

  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 78 least important bits LPC-RPE Speech Coder 132 important bits Data field Of 4 time slots 50 essential bits Calculate 3 parity bits 53 bits Rate ½ Channel code Multiplex interleave 456 bits 378 bits 4 tail 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 217.6 hops / second

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