RTUComputer ScienceYr 2020 · Sem 82020

Q11Mobile Computing

Question

4 marks

Explain the architecture of GSM in detail.

Answer

GSM architecture consists of the Mobile Station (MS), Base Station Subsystem (BSS), and Network and Switching Subsystem (NSS).

The Global System for Mobile Communications (GSM) architecture comprises three main subsystems that together provide radio access, call switching, and subscriber management: the Mobile Station (MS), the Base Station Subsystem (BSS), and the Network and Switching Subsystem (NSS), tied together by an Operation and Support Subsystem (OSS).

1. Mobile Station (MS)

The MS is the equipment used directly by the subscriber. It consists of the Mobile Equipment (ME), the physical handset with its radio transceiver, and the Subscriber Identity Module (SIM), a removable smart card that stores the subscriber's identity (IMSI), authentication keys, and network-related information. Separating the SIM from the ME allows a subscriber to move their identity across different handsets.

2. Base Station Subsystem (BSS)

  • Base Transceiver Station (BTS): Houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile Station.
  • Base Station Controller (BSC): Manages radio resources for one or more BTSs, handling channel allocation, frequency hopping, and handovers between BTSs under its control.

3. Network and Switching Subsystem (NSS)

The NSS performs the switching of calls and mobility management functions and includes several key registers and switches.

  • Mobile Switching Center (MSC): The central component that performs call setup, routing, and switching, and interfaces with the fixed PSTN.
  • Home Location Register (HLR): A permanent database storing subscriber information such as service profile and current location.
  • Visitor Location Register (VLR): A temporary database holding information about subscribers currently located in the MSC's service area.
  • Authentication Center (AuC): Stores authentication and encryption keys used to verify subscriber identity and secure calls.
  • Equipment Identity Register (EIR): Maintains a list of valid, and blacklisted, mobile equipment based on the IMEI to prevent use of stolen handsets.

Together, these subsystems allow GSM to provide seamless voice and data services while supporting subscriber mobility across a network of interconnected cells.

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