To Collocate or not to Collocate – That is the Question
One of the new capabilities in Microsoft Lync 2010 is the ability to collocate your Mediation Server onto the Front End Server(s). There are few situations to be cognizant about in regards to collocation of the Mediation Server.
1. Amount of Media Bypass calls vs. non-Media Bypass calls. For information on what Media Bypass is, refer to my article on Media Bypass here.
2. Dedicated Audio/Video Server(s) or Collocated Audio/Video Servers.
3. Utilizing Direct SIP vs. SIP Trunking vs. Voice Gateway.
Amount of Media Bypass calls vs. non-Media Bypass calls
As stated in the General Information section, please refer to my article on Media Bypass here. In environments with at least one branch site, these branch sites may or may not be utilizing Media Bypass. If not utilizing Media Bypass, the clients in a site where Media Bypass is not enabled will be utilizing the Mediation Server(s) at the main site for both Signaling as well as Media. The Mediation will receive RTAudio media from these clients and transcode it to G.711 and send it the gateway. This takes processing power to do. In an environment where there is a lot of heavy voice users, this media transcoding will take a toll on the Front End Pool and possibly overload it causing a possible degradation of voice quality.
Dedicated Audio/Video Server(s) or Collocated Audio/Video Servers
This one is fairly simple. The existing guidance, which may change at RTM, is if you have over 10,000 users, deploy a dedicated Audio/Video Conferencing Pool. The Lync Server 2010 Planning Tool will assist in determining the amount of dedicated Audio/Video Conferencing Pool Servers you require. If both the A/V and Mediation Server are on the Front End Server(s), you should ensure that there is at least 30% CPU available for just the processing of calls. If 30% is unavailable, the Mediation Server(s) should be split into a separate Mediation Server Pool.
Utilizing Direct SIP vs. SIP Trunking vs. Voice Gateway
All Front End Server(s) in a Pool are created equally. The Topology Document’s view of a Voice Peer is applied consistently across all servers in a Pool. If the Mediation Servers are collocated, each Front End/Mediation Server will need to talk to the Mediation Server’s Peers in the same fashion. Internet Telephony Service Provider’s (ITSP) SIP Trunk Peers and IP-PBX Direct SIP Peers have certain recommendations in regards to collocation while IP Gateways have separate recommendations. Let’s take a look at these recommendations and why these recommendations exist.
SIP Trunks and Direct SIP
If you are utilizing SIP Trunks or using Direct SIP, you will need a trunk going to each Mediation Server. The Peer will provide load balancing mechanisms to ensure that all traffic to a Mediation Server is load balanced.
When taking SIP Trunks with an ITSP, some ITSPs will charge on a per trunk basis. So for each Mediation Server used and connecting to a Session Border Controller (SBC) when utilizing a SIP Trunk, you will most likely be spending more money. If you have 10 Front End Servers with a SIP Trunk going to each, the costs can be high. Instead, it would make more sense to have dedicated Mediation Servers where only possibly 3 are required and now you are only paying for 3 trunks, you have dedicated processing for Mediation Traffic and you reduce hardware utilization on your Front End Server(s). The same issue occurs with Direct SIP to an on-site PBX. You don’t have the cost issue, but you may have the requirement to do application layer load balancing which is another reason why you want a Sip Trunk defined from each Mediation Server to each SBC/Direct SIP PBX.
This is all different when utilizing Certified IP Gateways. Certified IP Gateways should support DNS Load Balancing to the Mediation Pool. The Certified IP Gateways can also receive traffic from any Mediation Server. Keep in mind, if not using DNS Load Balancing, you’ll still have to set up a SIP Trunk to each Mediation Server.
Standard Edition Front End Server with Collocated Mediation Server
This all becomes a moot point if you only have one Front End Server anyways since you don’t have to worry about any type of load balancing or extra costs associated with more than one SIP Trunks to an ISTP.