The federation needs an external trusted certificate, to allow access from everywhere. What can we do if we would like to federate only one company, and we need to cut the project cost? The solution is the self-signed certificate. How it works? Let me explain.
The Microsoft Office Communications Server (OCS) is able to provide for calls to and from the public switched telephone network (PSTN). There are many solutions existing with the telephony integration and one of them is the co-existing scenario. The co-existing scenario consists a Private Branch Exchange (PBX), and the OCS at the same time. It means that a user reachable on PBX phone or OCS or both of them.
I built my environment with Asterisk PBX which connects to the OCS system via Direct SIP. When the Microsoft Unified Communications arrived with Exchange UM, the problem was in the transport layer. The Asterisk used UDP, and the Microsoft used TCP for SIP. I needed a little trick to connect Exchange with Asterisk – converted SIP UDP to SIP TCP. Asterisk has supported TCP SIP since 1.6, so the problem no longer exists. The OCS also uses TCP SIP (and TLS of course). In my previous post you could read about the Asterisk PBX installation on Ubuntu Server, and I enabled the TCP SIP in the configuration file.