One year ago I was starting the on the BSC NAT application from a Hotel room in Munich, I was flying from Taiwan to Munich and was happy to see the snow from my window. Shortly after this I began to implement something we called cellmgr_ng. The job was to take MTPLevel3 coming out of a library and put the SCCP payload into the IPA protocol and send it to a MSC. As part of MTP Level3 I had to implement the link bring up and such as well (once again without having too much time to read all the specs).

The problems were as most of the times not the ones one would expect. The system behind the E1/T1 link was sending wrong SCCP messages, when a SCCP connection was not closed fast enough it would send out its own release message but was confused about the source local and destination local addresses. As a result I had to add connection tracking to the codebase, I am also rewriting/removing Point Codes to avoid crashes and make the MSC happy.

Last month and this month I had the pleasure to resume the work on this. My knowledge of MTPL3 is at least a bit bigger now, we have done the libosmocore split so I can use more features without copying code around and there was new demand.

The first big thing is to support a LinkSet with multiple Links in it. This allowed me to do a lot of cleanups, some are still to do but now the abstraction is a lot better and I will do testing and force link failures and see how the system is behaving. This will also end with VTY commands to manage links via the telnet interface, more statistics collected and reported, being able to start writing PCAP files for the linksets from the VTY command.

The second big feature is the addition of M2UA to the codebase. The code does not support fail over scenario’s yet but this should be relatively easy to add inside the structure now.

The result is that we now have something that could be used as a MediaGateway and convert MTPL2/MTPL3/{SCCP,ISUP} into SCTP/M2UA/MTPL3{SCCP,ISUP} and convert the audio to and from RTP, the other part is a Signalling Transfer Point (STP) with some basic capabilities.