Direct SIP

The easiest and generally most cost-effective way of integrating Lync Server 2013 with an existing PBX is if the PBX supports SIP trunking. Many IP PBXs support this functionality natively, and many other hybrid PBXs support SIP trunking through minor hardware or software upgrades. This type of integration requires no hardware because the link between systems is purely a logical trunk that exists between two IP endpoints.

In Direct SIP scenarios the Mediation Server role serves as the conversion point between the two systems. The signaling on both sides of a SIP trunk is SIP, but the Mediation role translates the media stream between G.711 on the PBX side and RTAudio on the Lync Server side, as shown in Figure 17.6.

Figure 17.6. Direct SIP integration.

Direct SIP integration allows for various end-user scenarios, which are discussed later in this chapter. Generally, specific extensions, or a range of extensions, will be configured to be “owned” by Lync Server instead of the PBX. These extensions will be then configured on the old PBX to route across the SIP trunk to let Lync handle the call. It is the PBX’s way of saying it is not responsible for those numbers, but it knows where they can be reached. This type of routing is similar to tie-lines or trunks that can exist between multiple legacy PBXs.

A very compelling new feature in Lync Server 2013 is the ability to route calls between two SIP trunks associated with Lync Mediation Servers with inter-trunk routing. Prior to Lync Server 2013, all calls had to either start or end with a Lync endpoint, but the voice routing can now be configured to send calls between two different trunks that are potentially attached to two different PBXs. This allows an organization to place the Lync servers “in front” of legacy systems, and handle the core routing logic within Lync, which can help simplify a migration.