Overview of the GSM Interfaces

For the connection of the different nodes in the GSM network, different interfaces are defined in the GSM specifications.

· Air interface or Um-interface
The Air Interface is the interface between the BTS (Base Transceiver
Station) and the MS (Mobile Station). The air interface is required for
supporting:
- Universal use of any compatible mobile station in a GSM network
- A maximum spectral efficiency

· Abis-interface
The Abis-interface is the interface between the BSC (Base Station
Controller) and the BTS. The interface comprises traffic and control
channels. Functions implemented at the Abis-interface are:
- Voice-data traffic exchange
- Signaling exchange between the BSC and the BTS
- Transporting synchronization information from the BSC to the BTS

· A-interface
The A-interface is the interface between the BSC and the MSC.

· Proprietary M-interface
In the GSM network implementation of Lucent Technologies, the BSC
includes the TRAU (Transcoder/Rate Adapter Unit). The TRAU adapts the
transmission bit rate of the A-interface (64 kbit/s) to the Abis-interface (16
kbit/s). The interface between the physical BSC and the TRAU is known as
the M-interface.