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.