The Base Station system Application Part (BSSAP) defines the messaging interface between a GSM Base Station (BS) and a Mobile service Switching Center (MSC). This interface uses the services provided by both connectionless and connection-oriented Signaling Connection Control Part (SCCP) for its transport layer. The IntelliNet BSSAP API allows for the quick development of the MSC-BSC interface.
 
The MTP and the SCCP are used to support signaling messages between the Mobile Services Switching Center (MSC) and the Base Station System (BSS). One user function of the SCCP, the BSS Application Part (BSSAP), is defined. In the case of point-to-point calls the BSSAP uses one signaling connection per active mobile station having one or more active transactions for the transfer of layer 3 messages. In the case of a voice group or broadcast call there is always one connection per cell involved in the call and one additional connection per BSS for the transmission of layer 3 messages.