One of my paid contracts required me to start moving directly into the GSM core network, the SS7 domain. While so far I had a fairly good understanding of SCCP, TCAP and MAP (which are required for GSM/3G roaming interfaces), I've now had to look at the actual telephony signalling side of things.
Even in GSM networks, the gateway or inter-working MSC will translate all the GSM TS 04.08 Call Control messages into ISUP, the ISDN User Part, originally designed for call control signalling between ISDN networks.
As apparently always in SS7, there are many, many different standardized and proprietary protocol stackings that can be seen in the wild. I'm now working on a stack that looks like IP->SCTP->M2UA->MTP3->{ISUP, SCCP->TCAP->MAP}.
Luckily, for now there is no need to do any fully-fledged implementation of it, simple message parsing and encoding routines are sufficient for the task at hand.
It's been about time that I'm closing those last gaps in the knowledge of GSM core network protocols. The only part that I'm still missing so far is CAMEL. I know roughly how it works, but I've never played with it or implemented any part of it.