Cellular Network Infrastructure » cni-legacy » OpenBSC

= 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.

[wiki: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