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 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 busy hour traffic through those gateways
  
  
• the percentage of calls that are calls that bypass the Mediation Server
  
  

When planning, ensure that after you take into account the processing requirements for non-media bypass PSTN calls and for the A/V Conferencing Server (if it is collocated, and therefore running on Front End Servers in the same pool), there is enough processing to handle signaling interactions for the number of busy hour calls that need to be supported. (Allow at least 30% of CPU for this.) 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 the second, stand-alone pool.

If you deployed PSTN gateways, IP-PBXs, or 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 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
  
  

Note:
We also recommend collocating Mediation Servers with a Front End pool when an IP-PBX does not support media bypass, but the Front End pool that is hosting the Mediation Server can handle voice transcoding for calls to which media bypass does not apply,

You can use the Microsoft Lync Server 2010, Planning Tool to evaluate whether the Front End pool where you want to collocate the Mediation Server 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 with certified partners and has done some testing with Cisco IP-PBXs. Certification for SBCs is underway. Media bypass is supported only with products and versions listed on Unified Communications Open Interoperability Program – Lync Server at http://go.microsoft.com/fwlink/?LinkId=214406.

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 there 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 "Hello" for incoming calls from the IP-PBX to Lync Server 2010 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 "Hello".

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.