Planning for Mediation server Role

Mediation server Role plays a crucial role in Lync/SfB system for establishing a call between a Lync/SfB and PSTN or any VoIP system.  There is a Microsoft TechNet article which discusses about the Capacity planning for Mediation server Role.
 [ MS articles

https://technet.microsoft.com/en-in/library/gg398399.aspx  

https://technet.microsoft.com/en-us/library/gg615015.aspx ]. 

However, after going through it i felt that the information provided in the TechNet article is not conclusive. So before we have the Design work shop for the SIP Trunk I went through some of Channel 9 videos regarding the Enterprise Voice features found that there are several important points to consider while planning for Mediation server Role.  So, we will discuss about  some scenarios where Media Bypass can be used and CANNOT be used.

Media Bypass:

One of the main question people have is, can we reduce the number of servers required for the Mediation server role if we have Media By pass enabled for a network.

What is Media ByPass?

Source: https://technet.microsoft.com/en-us/library/gg412740.aspx

Media bypass refers to removing the Mediation Server from the media path whenever possible for calls whose signaling traverses the Mediation Server.  Media bypass can improve voice quality by reducing latency, needless translation, possibility of packet loss, and the number of points of potential failure. Scalability can be improved, because elimination of media processing for bypassed calls reduces the load on the Mediation Server. This reduction in load complements the ability of the Mediation Server to control multiple gateways. Picture from the TechNet article about the Media bypass.

Media By pass is a feature which when enabled will allow the Agents to talk directly to the certified Gateways (bypassing the Mediation server). However, if Media By pass feature is not enabled or not configured for the network, then all the media traffic has to traverse Mediation server.

Media By pass is a great feature and it significantly reduces the load on the Mediation servers and improves the call quality. However, it is good to understand the certain scenarios where the Media Bypass is irrelevant. It is also important to consider the following scenarios (where though the Media By pass feature is enabled) but it will not work even if a Lync/SfB call is made from a Network where Media Bypass is enabled.

Features  / applications / devices  which does not work with Media Bypass:

•  Mobile devices does not use Media By pass. So if you have several thousands of Mobile devices in your infrastructure, then the all the Media traffic has to go through the Mediation servers.  Hence planning for Mediation servers needs to consider this situation too.

•  Response Group does not use Media By pass. So if you have a user calling a Response Group number then though the Calls are originated from a network segment where Media By pass is enabled, the Media Bypass feature will not be used and the media traffic will be going to and from the Mediation server.

•  If you have any trusted third party application like for Contact center solutions      then again Media bypass will not work.

•  CAC issues:As per TechNet artcile [source: https://technet.microsoft.com/en-us/library/gg413028(v  =ocs.15).aspx ]  If you have already optionally configured call admission control (CAC) note that the bandwidth reservation performed by call admission control does not apply to any calls for which media bypass is employed. 

• The Media Bypass feature does not work for the  Dial in Conferencing scenarios.

• The UCWA Applications and AV MCU does not support Media Bypass.

I hope  this post was intersting and please share your views. In case, if you feel that i have missed any other points to be considered while planning for Mediation server Role please share.