As I blogged in my blog post in Fabruary, I was working towards a more fully-featured SIGTRAN stack in the Osmocom (C-language) universe.

The trigger for this is the support of 3GPP compliant AoIP (with a BSSAP/SCCP/M3UA/SCTP protocol stacking), but it is of much more general nature.

The code has finally matured in my development branch(es) and is now ready for mainline inclusion. It's a series of about 77 (!) patches, some of which already are the squashed results of many more incremental development steps.

The result is as follows:

• General SS7 core functions maintaining links, linksets and routes
• xUA functionality for the various User Adaptations (currently SUA and M3UA supported)
  • MTP User SAP according to ITU-T Q.701 (using osmo_prim)
  • management of application servers (AS)
  • management of application server processes (ASP)
  • ASP-SM and ASP-TM state machine for ASP, AS-State Machine (using osmo_fsm)
  • server (SG) and client (ASP) side implementation
  • validated against ETSI TS 102 381 (by means of Michael Tuexen's m3ua-testtool)
  • support for dynamic registration via RKM (routing key management)
  • osmo-stp binary that can be used as Signal Transfer Point, with the usual "Cisco-style" command-line interface that all Osmocom telecom software has.
• SCCP implementation, with strong focus on Connection Oriented SCCP (as that's what the A interface uses).
  • osmo_fsm based state machine for SCCP connection, both incoming and outgoing
  • SCCP User SAP according to ITU-T Q.711 (osmo_prim based)
  • Interfaces with underlying SS7 stack via MTP User SAP (osmo_prim based)
  • Support for SCCP Class 0 (unit data) and Class 2 (connection oriented)
  • All SCCP + SUA Address formats (Global Title, SSN, PC, IPv4 Address)
  • SCCP and SUA share one implementation, where SCCP messages are transcoded into SUA before processing, and re-encoded into SCCP after processing, as needed.

I have already done experimental OsmoMSC and OsmoHNB-GW over to libosmo-sigtran. They're now all just M3UA clients (ASPs) which connect to osmo-stp to exchange SCCP messages back and for the between them.

What's next on the agenda is to

• finish my incomplete hacks to introduce IPA/SCCPlite as an alternative to SUA and M3UA (for backwards compatibility)
• port over OsmoBSC to the SCCP User SAP of libosmo-sigtran
  • validate with SSCPlite lower layer against existing SCCPlite MSCs
• implement BSSAP / A-interface procedures in OsmoMSC, on top of the SCCP-User SAP.

If those steps are complete, we will have a single OsmoMSC that can talk both IuCS to the HNB-GW (or RNCs) for 3G/3.5G as well as AoIP towards OsmoBSC. We will then have fully SIGTRAN-enabled the full Osmocom stack, and are all on track to bury the OsmoNITB that was devoid of such interfaces.

If any reader is interested in interoperability testing with other implementations, either on M3UA or on SCCP or even on A or Iu interface level, please contact me by e-mail.