The use and general aspects of the A-bis interface are given in TS 48.051
This TS gives the principle basis for the rest of the specifications specifying the interface between the Base Station Controller, BSC, and the Base Transceiver Station, BTS, with its transceivers, TRX. These components together form the Base Station System, BSS. (The interface between MSC and the BSS is specified in TS 48.001
and TS 48.020
The intention with this interface is to get a unified way of connecting remotely located BTSs/TRXs to a BSC allowing for the interconnection of BSCs and BTSs/TRXs from different manufacturers.
In order to keep the BTS as simple as possible, BTS contains only those functions which have to recide close to the radio interface.