GSM and CDMA standards; GSM architecture

GSM and CDMA standards

• GSM (Global System for Mobile communications): 2G digital cellular standard from ETSI (Europe, 1991). Uses FDMA/TDMA with 200 kHz carriers, 8 time slots per carrier, GMSK modulation, FDD (890–915 / 935–960 MHz in GSM-900), RPE-LTP 13 kbps speech, SIM-based subscriber identity, hard handoff. It became the most widely used 2G system (in Nepal, used by Nepal Telecom and Ncell).
• CDMA (IS-95 / cdmaOne): 2G standard from Qualcomm/TIA (USA, 1993). Uses DS-CDMA: all users share a 1.25 MHz carrier at the same time and are separated by codes (64 Walsh codes on forward link, long PN codes on reverse link), chip rate 1.2288 Mcps. Features frequency reuse 1, soft handoff, fast power control and RAKE receivers. It evolved to CDMA2000.

Architecture of GSM

       Um          Abis          A
 [MS]~~~~~~[BTS]-------[BSC]-------[MSC]----[GMSC]--PSTN/
 ME+SIM     [BTS]------'  (TRAU)      |  |      ISDN
                                      |  +--[VLR]
 |<---- BSS ---------------->|        +--[HLR]--[AuC]
                                      +--[EIR]
                      |<-------- NSS -------->|
                [OMC] (OSS) monitors BSS & NSS

• MS = Mobile Equipment (IMEI) + SIM (IMSI, Ki).
• BSS: BTS – radio transceivers of a cell (Um interface); BSC – controls many BTSs (Abis), channel allocation, power control, handoff; TRAU – speech transcoding.
• NSS (via A interface): MSC – call switching, handoff, billing; GMSC – link to PSTN; HLR – permanent subscriber database; VLR – temporary data of visiting users; AuC – authentication and ciphering keys; EIR – IMEI check (stolen phones).
• OSS: OMC – operation, maintenance and network management.