Mediation Server is by default collocated on the Standard Edition server or Front End Server in a Front End pool at central sites. The number of public switched telephone network (PSTN) calls that can be handled and the number of machines required in the pool will depend on the following:

• The number of gateway peers that the Mediation Server pool controls

• The high-volume traffic periods through those gateways

• The percentage of calls that are calls whose media bypass the Mediation Server

When planning, be sure to take into account the media processing requirements for PSTN calls and A/V conferences that are not configured for media bypass, as well as the processing needed to handle signaling interactions for the number of busy-hour calls that need to be supported. If there is not enough CPU, then you must deploy a stand-alone pool of Mediation Servers; and PSTN gateways, IP-PBXs, and SBCs will need to be split into subsets that are controlled by the collocated Mediation Servers in one pool and the stand-alone Mediation Servers in one or more stand-alone pools.

If you deployed PSTN gateways, IP-PBXs, or Session Border Controllers (SBCs) that do not support the correct capabilities to interact with a pool of Mediation Servers, including the following, then they will need to be associated with a stand-alone pool consisting of a single Mediation Server:

• Perform network layer Domain Name System (DNS) load balancing across Mediation Servers in a pool (or otherwise route traffic uniformly to all Mediation Servers in a pool)

• Accept traffic from any Mediation Server in a pool

You can use the Microsoft Lync Server 2013, Planning Tool to evaluate whether collocating the Mediation Server with your Front End pool can handle the load. If your environment cannot meet these requirements, then you must deploy a stand-alone Mediation Server pool.

Mediation Servers at the central site can be used to route calls for IP-PBXs or PSTN gateways at branch sites. If you deploy SIP trunks, however, you must deploy a Mediation Server at the site where each trunk terminates. Having a Mediation Server at the central site route calls for an IP-PBX or PSTN gateway at a branch site does not require the use of media bypass. However, if you can enable media bypass, doing so will reduce media path latency and, consequently, result in improved media quality because the media path is no longer required to follow the signaling path. Media bypass will also decrease the processing load on the pool.

Note:
Media bypass will not interoperate with every PSTN gateway, IP-PBX, and SBC. Microsoft has tested a set of PSTN gateways and SBCs with certified partners and has done some testing with Cisco IP-PBXs. Media bypass is supported only with products and versions listed on Unified Communications Open Interoperability Program – Lync Server at http://go.microsoft.com/fwlink/p/?LinkId=268730.

If branch site resiliency is required, a Survivable Branch Appliance or combination of a Front End Server, a Mediation Server, and a gateway must be deployed at the branch site. (The assumption with branch site resiliency is that presence and conferencing are not resilient at the site.) For guidance on branch site planning for voice, see Planning for branch-site voice resiliency.

For interactions with an IP-PBX, if the IP-PBX does not correctly support early media interactions with multiple early dialogs and RFC 3960 interactions, there can be clipping of the first few words of the greeting for incoming calls from the IP-PBX to Lync endpoints. This behavior can be more severe if a Mediation Server at a central site is routing calls for an IP-PBX where the route terminates at a branch site, because more time is needed for signaling to complete. If you experience this behavior, deploying a Mediation Server at the branch site is the only way to reduce clipping of the first few words.

Finally, if your central site has a TDM PBX, or if your IP-PBX does not eliminate the need for a PSTN gateway, then you must deploy a gateway on the call route connecting Mediation Server and the PBX.

Note:

To improve the media performance of standalone Mediation Server, you should enable receive-side scaling (RSS) on the network adapters on these servers. RSS enables incoming packets to be handled in parallel by multiple processors on the server. For details, see "Receive-Side Scaling Enhancements in Windows Server" at http://go.microsoft.com/fwlink/p/?LinkId=268731. For details about how to enable RSS, see your network adapter documentation.