3GPP compliange A over IP interface¶

In more recent versions of the A interface specification (used to be GSM TS 08.08, now is 3GPP TS 48.008),
the A protocol is officially specified over IP as an alternative bearer. This is a great alternative
to the slightly hackish existing implementations using SCCPLite (SCCP in an IPA multiplex) and MGCP for
mapping of CICs to RTP port numbers.

osmo-bsc does not yet support this standardized version of A-over-IP.

Differences from classic A interface¶

The major differences from classic A interface (over E1 or SCCPLite+RTCP) are:

• BSSAP ASSIGNMENT COMMAND includes
• no CIC!
• MSC-side IP/port
• list of MSC preferred speech codecs
• call identifier (random 4bytes of MSC)
• BSSAP ASSIGNMENT COMPLETE includes
• no CIC!
• BSC-side IP/port
• chosen speech codec
• BSSAP RESET IP RESOURCE
• replacement for (RESET RESOURCE with CIC)
• MSC sends list of call identifiers which are to be reset

Required changes to osmo-bsc for AoIP support¶

Architecturally, the bsc-mgcp and any media related handling can be completely replaced by

• ASSIGNMENT COMMAND handling
• checking if AoIP transport address IEI is present and distinguishing old A and AoIP based on that
• storing call identifier IE in connection
• sending IPA_CRCX to the BTS
• saving BTS side IP/Port from IPA_CRCX_ACK
• sending IPA MDCX to the BTS, using MSC-side IP/port
• selecting codec based on MSC preferred speech codec list IE
• ASSIGNMENT COMPLETE handling
• including IP/port from IPA_CRCX_ACK
• including selected/chosen speech codec
• RESET IP RESOURCE handling
• iterate over list of call identifiers in msg
• delete/reset any connection (and lchan, ...) for given call identifier