GSM (A interface)

A interface between BSC and MSC consists of signaling and transmission channels.  TRAU placed at MSC compresses transmission data (coming from MSC), so we require fewer channels between BSC and MSC.

Signaling protocol stack is given in diagram below:



BSSAP (BSS Application Part) uses Connection Oriented services of SCCP. DTAP (Direct Transfer Application Part) forms a tunnel through which CM/MM messages are transferred to MSC. RR data is used by BSSMAP (BSS Mobile Application Part) for communication with MSC; part of RR data, in fact, is given to MSC.

BSSAP data received from SCCP has discriminator parameter to distinguish between DTAP and BSSMAP messages. Note that SCCP connection setup is done by BSSMAP. DTAP messages goes through BSSMAP established SCCP connection.

» BSC-MSC "A" interface - 8.06 (Use of SCCP/MTP), 8.08 (BSSAP Procedures) « 

Source:Internet