The A interface carries both user traffic and signalling.

The signalling protocols used on the A interface are called:

    BSSAP (Base Station System Application Part), subdivided into two parts:

                BSSMAP (Base Station System Management Application Part)

                DTAP (Direct Transfer Application Part)

    BSSOMAP (Base Station System Operation and Maintenance Application Part)

All signalling protocols on the A interface use SCCP over the SS7 MTP layers 1,2  and 3.

Signalling is carried over timeslot 16 of the MSC to BSC E1 link(s)

Traffic is carried over other 64kbps times slots of the E1 link(s)