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
- 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