Topic Last Modified: 2011-03-24

New for the Mediation Server in Lync Server 2010 is the ability for a single Mediation Server to control multiple gateways. In previous releases, there was a 1:1 ratio of Mediation Servers to gateways. A given gateway can be assigned to multiple call routes and associated with one Mediation Server or a pool of Mediation Servers. Multiple gateways can be associated with either a single Mediation Server or a pool of Mediation Servers. In this release, the number of gateways that a given Mediation Server can handle depends on the processing capacity of the server during peak busy hours. If you deploy a Mediation Server on hardware that exceeds the minimum hardware requirements for Lync Server 2010, as described in “Supported Hardware” in the Supportability documentation, then the estimate of how many active calls a stand-alone Mediation Server can handle is approximately 1000 calls. When deployed on hardware meeting these specifications, the Mediation Server is expected to perform transcoding, but still route calls for multiple gateways even if the gateways do not support media bypass.

When defining a call route, you specify the gateways associated with that route, but you do not specify which Mediation Servers are associated with that route. Instead, you use Topology Builder to associate gateways, and therefore routes, with Mediation Servers. In other words, routing determines which gateway to use for a call and the Mediation Server associated with that gateway handles the call.

Also new for Lync Server 2010 is the ability for a Mediation Server to be deployed as a pool; this pool can be collocated with a Front End pool, or it can be deployed as a stand-alone pool. When a Mediation Server is collocated with a Front End pool, the pool size can be at most 10 (the limit of the Registrar pool size). Taken together, these new capabilities increase the reliability and deployment flexibility for Mediation Servers, but they require associated capabilities in the following peer entities:

A Mediation Server pool must have a uniform view of the peer gateway with which it interacts. This means that all members of the pool access the same definition of the peer gateway from the configuration store and are equally likely to interact with it for outgoing calls. Thus, there is no way to segment the pool so that some Mediation Servers communicate with only certain gateway peers for outgoing calls. If such segmentation is necessary, a separate pool of Mediation Servers must be used. This would be the case, for example, if the associated capabilities in PSTN gateways, SIP trunks, or IP-PBXs to interact with a pool as detailed earlier in this topic are not present.

A Lync Server 2010 deployment assumes that a particular PSTN gateway, IP-PBX, or SIP trunk peer depends on only a single Mediation Server pool; calls are routed to that pool by the Lync Server 2010 Front End pool so that they can get to that gateway peer.

For SIP trunks, IP-PBXs, and PSTN gateways where a separate pool of Mediation Servers must be used, the following scheme can be used to achieve redundancy:

The number of gateways that a particular pool of Mediation Servers can control depends on the number of calls that use media bypass. If a large number of calls use media bypass, a Mediation Server in the pool can handle many more calls, because only signaling layer processing is necessary.