GSM system architecture

GSM (Global System for Mobile communications) is a 2G digital cellular system using TDMA/FDMA with 200 kHz carriers (8 time slots each), GMSK modulation and FDD (890–915 MHz uplink, 935–960 MHz downlink for GSM-900). Its architecture has four parts: Mobile Station (MS), Base Station Subsystem (BSS), Network and Switching Subsystem (NSS) and Operation Support Subsystem (OSS).

       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

1. Mobile Station (MS)

• Mobile Equipment (ME): the handset hardware – radio, speech codec, display; identified by the IMEI.
• SIM card: holds the IMSI, secret key Ki, A3/A8 algorithms, PIN and user data. Any ME works with any SIM, so the subscription is portable.

2. Base Station Subsystem (BSS)

• BTS (Base Transceiver Station): radio transceivers and antennas for one cell; does modulation/demodulation, channel coding, interleaving, encryption, frequency hopping and measurements over the Um (air) interface.
• BSC (Base Station Controller): controls many BTSs over the Abis interface; allocates radio channels, sets transmit power, manages frequency hopping and handles handoffs between its BTSs.
• TRAU (Transcoder and Rate Adaptation Unit): converts 13 kbps GSM speech to 64 kbps PCM for the MSC.

3. Network and Switching Subsystem (NSS)

Connected to the BSC by the A interface.

• MSC (Mobile Switching Centre) – the central switch. Sets up, routes and releases calls; handles handoff between BSCs/MSCs, billing records, and mobility management. Works with HLR/VLR to locate users.
• GMSC (Gateway MSC) – the MSC that connects the GSM network to PSTN/ISDN/other networks; for incoming calls it asks the HLR where the subscriber is.
• HLR (Home Location Register) – permanent database of every subscriber of the operator: IMSI, MSISDN, subscribed services, and the current VLR/MSC area of the user.
• VLR (Visitor Location Register) – temporary database attached to an MSC; holds data of subscribers currently roaming in its area (copied from HLR), TMSI and location area. Reduces queries to the HLR.
• AuC (Authentication Centre) – stores each subscriber's secret key Ki and runs the A3/A8 algorithms to generate authentication triplets (RAND, SRES, Kc) for authentication and ciphering.
• EIR (Equipment Identity Register) – list of IMEI numbers: white (allowed), grey (monitored) and black (stolen/blocked) handsets.

4. Operation Support Subsystem (OSS)

• OMC (Operation and Maintenance Centre): monitors and configures all network elements; handles fault, performance, configuration, security and billing management, and subscriber administration.

Interfaces

Call flow example (mobile terminated): PSTN → GMSC asks HLR → HLR asks current VLR for roaming number → GMSC routes to the serving MSC → MSC pages the MS via BSC/BTS → authentication with AuC data → channel assignment → call connected.