5.7 The Abis (BSC-BTS) Interface Protocols

A-bis Interface Protocol Stack

The A-bis interface interconnects BTSs to BSC within the Base Station Subsystem (BSS). The
physical medium at this interface can be either cable (copper – or more commonly optical
fibre) or microwave.

5.7.1 Abis INTERFACE - LAYER 1

The physical transmission structure is based on a 16kbps IDSN D-channel configuration.
Four D-channels are sub-multiplexed onto a single ISDN B-channel which can then
multiplexed onto a 2048kbps E1 link (32x 64kbps channels). This multiplexing option is
dependant upon the location of the TRAU (see description of TRAU above).

5.7.2 Abis INTERFACE - LAYER 2

The Abis interface data link layer protocol is based on the ISDN D-channel Link Access
Protocol (LAPD) which is itself based on the High-level Data Link Control protocol (HDLC)
as described above. LAPD performs similar functions to LAPDm but over the BTS-BSC Abis
interface.

5.7.3 Abis INTERFACE - LAYER 3

Layer 3 utilises the Base Transceiver Station Management (BTSM) protocol. Most RR
messages are passed transparently through the BTS to the BSC (and vice versa). However,
certain RR information must be interpreted by the BTS e.g. random access of the MS, start of
ciphering and paging. The BTSM contains functions for processing these messages and other
BTS-specific management procedures.