Project

General

Profile

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
Add picture from clipboard (Maximum size: 48.8 MB)