GSM Mobile Network – mobile network of 2nd generation
Architecture of GSM Network

The whole basic structure of the GSM system is shown in the figure below. The basic structure of the GSM system can be divided into three fundamental parts: Base Station Subsystem, Networks Switching Subsystem, and Operation Support Subsystem.

## Base Station Subsystem (BSS)

The MSs communicate with BSs. Several BSs are assigned to a BSC, whose main task is to allocate and release the free radio channels for communication with MSs and ensure the correct handover. Operation of the system requires that each MS, which is in operation, provides the information to the system on its location, it means about the cell in which it is located. MS monitors also signals from nearest base stations and always selects the optimum BTS, through which the connection is established.

## Network Switching Subsystem (NSS)

This subsystem includes in particular the MSC exchange, which is represented by a common type of telephone exchange that is supplemented by additional features resulting from the mobility of the MSs. These additional features are stored in various databases comprising:

- HLR (Home Location Register) – keeps track of all participants in the area. Authentication (identification) of subscriber is provided by AuC. Each participant of the network is stored only in a single HLR.
- VLR (Visitor Location Register) – temporarily stores the latest information on position of the MSs in the range of the MSC. The VLR always requires and obtains data from the HLR and if the MS leaves the visited area, the MS's related data are always deleted from the VLR.
- EIR (Equipment Identity Register) - stores information about MSs (for example, list of authorized stations or stolen stations, etc.)

## Operation Support Subsystem (OSS)

The OSS is responsible for the operation of BSS and NSS. It contains mainly supervisory block, ADC (Administrative Centre), addressing administrative tasks (e.g., report participation fees, billing, etc.), followed by a NMC (Network Management Centre) block providing overall management of information flow in the network, and operational and service block, OMC (Operation and Maintenance Centre), addressing the role of maintenance and operation of the network.

Architecture of GSM system