With Lync Server 2010, the Mediation Server role is integrated in the Front End Server and Standard Edition server. Optionally, it can also be deployed as a stand-alone server. A couple of factors introduced in Lync Server 2010 make it highly scalable to be integrated with the Front End Server. A single Mediation Server role can route outbound calls to multiple media gateways instead of a single media gateway as in the case of Office Communications Server.

Another enhancement made to the Mediation Server is media bypass. With supported media gateways, Lync clients and phones can directly route media traffic to the media gateway without routing through the Mediation Server. The signaling traffic (SIP) still continues to route through the Mediation Server role, but the audio no longer has to. This substantially optimizes audio traffic and eliminates the problem of hairpinning call paths.

The Mediation Server is a server role necessary to bridge the PSTN traffic to and from the media gateway to the Lync Server network. Because some existing media gateways do not support the SIP protocol over Transport Layer Security (TLS) and optimized media codecs used by Lync Server 2010, the Mediation Server is needed to translate the RTAudio and RTVideo codecs to the G.711 and G.723 codecs that are commonly used by media gateways. In addition to performing codec translation, the Mediation Server performs reverse number lookups (RNLs) to resolve phone numbers from incoming calls that arrive from the media gateway to the corresponding SIP Uniform Resource Identifier (URI). After phone numbers are resolved into SIP URIs, the Mediation Server routes the call to the user’s home server.