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Subsystems and network elements in GSM

Let us now have a closer look at each of the individual network elements (BSC, BTS, and Transcoder.

Base Station Controller (BSC)

The BSC is the central network element of the BSS and it Controls the radio network. It has several important tasks, some of which are presented in the following:

Connection establishment between the MS and the NSS

Ali calls to and from the MS are connected through the group switch of the BSC (GSWB).

Mobility management

The BSC is responsible for initiating the vast majority of all handovers, and it makes the handover decision based on, among others, measurement reports sent by the MS during a cali.

Statistical raw data collection

Information from the Base Transceiver Stations, Transcoders, and BSC are collected in the BSC and forwarded via the DCN (Data Communications Network) to the NMS (Network Management Subsystem), where they are post-processed into statistical views, from which the network ąuality and status is obtained.

Air-and A-interface signalling support

In the A-interface, SS#7 (Common Channel Signalling System No. 7) is used as the signalling language, while the environment in the air interface allows the usage of a protocol adapted from ISDN standards, namely LAP-Dm (Link Access Protocol on the ISDN D Channel, modified version). Between the Base Transceiver Station and the BSC (Abis interface), a morę standardised LAP-D protocol is used. The BCSU (Base Station Controller Signalling Unit) in the BSC will therefore need to convert from LAP-D to SS#7 and vice versa in the uplink/downlink directions. The BSC also enables the virtual signalling connection needed between the MSC/VLR and the MS.

BTS and TC control

Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC maintains the BTSs. In other words, the BSC is capable of separating (barring) a BTS from the network and collecting alarm information. Transcoders are also maintained by the BSC, that is, the BSC collects alarms related to the transcoders.

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