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