Trunks

Trunks are just a logical representation between Lync and some form of PSTN or PBX connectivity. The end of a trunk might be a media gateway, another PBX, or an ITSP providing a SIP trunk. A Lync trunk consists of an IP/PSTN gateway name or IP, a listening port on the IP/PSTN gateway, a Mediation Server, and a listening port on the Mediation Server. Routes include a list of trunks that can be used to place a call after a route is selected.

Trunk configuration in Lync is used to apply additional logic to a call after a voice route and trunk have been selected. At this point the server knows which trunk the call will be sent to, but trunks can all have different settings. This allows Lync to accommodate many gateway vendors and PBX integrations with different support for features like media bypass, SIP refer messages, and the Real-Time Control Protocol (RTCP).

The other main use for trunk configuration settings is to provide manipulation for the called and calling party numbers. While numbers and routing decisions within Lync are based on E.164, the majority of PBXs and telephony providers don’t support the format. The trunk configuration allows Lync to manipulate the numbers into a format that will be supported on the other end of the trunk immediately before delivering the call.

Each trunk configuration contains an ordered list of trunk translation rules that evaluated in a top-to-bottom order. These trunk translation rules are very similar to dial plan rules in that they are composed of a pattern to match and a translation rule. This allows Lync administrators to search for specific patterns on numbers and manipulate the format before sending the call to a PBX or gateway. If there are no trunk translation rules, or if the number dialed does not match any patterns, then the call is placed in the E.164 format.

An example of a trunk translation rule might be to prepend a leading external-access prefix used by the PBX such as a 9 to PSTN calls. Another example would be to prepend the 011 string to any international calls.

New to Lync Server 2013 is the capability to manipulate the calling party (caller’s) number. Lync Server 2010 allowed manipulation of the called party (dialed) number, but any source-number manipulation had to be done at the PBX or gateway. The addition of the calling party manipulation per trunk is a great addition that should make life much easier on Lync administrators.