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M:N trunk in Skype for Business Server

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Skype for Business Server Enterprise Voice supports M:N trunking between Mediation Server and components such as PSTN gateways, session border controllers, and IP-PBX.

Skype for Business Server supports greater flexibility in the definition of a trunk for call routing purposes from previous releases. A trunk is a logical association between a Mediation Server and listening port number with a gateway and a listening port number. This implies several things: A Mediation Server can have multiple trunks to the same gateway; a Mediation Server can have multiple trunks to different gateways; conversely a gateway can have multiple trunks to different Mediation Servers.

You must still create a root trunk whenever you use Topology Builder to adde a gateway to the topology. 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 Skype for Business Server, as described in Server requirements for Skype for Business Server 2015, then the estimate of how many active non-bypass 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 trunks associated with that route, but you do not specify which Mediation Servers are associated with that route. Instead, you use Topology Builder to associate trunks with Mediation Servers. In other words, routing determines which trunk to use for a call, and, subsequently, the Mediation Server associated with that trunk is sent the signaling for that call.

The Mediation Server can 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 12 (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:

  • PSTN gateway. A Skype for Business Server qualified gateway must implement DNS load balancing, which enables a qualified public switched telephone network (PSTN) gateway to act as a load balancer for one pool of Mediation Servers, and thereby to load-balance calls across the pool.

  • Session Border Controller. For a SIP trunk, the peer entity is a Session Border Controller (SBC) at an Internet telephony service provider. In the direction from the Mediation Server pool to the SBC, the SBC can receive connections from any Mediation Server in the pool. In the direction from the SBC to the pool, traffic can be sent to any Mediation Server in the pool. One method of achieving this is through DNS load balancing, if supported by the service provider and SBC. An alternative is to give the service provider the IP addresses of all Mediation Servers in the pool, and the service provider will provision these in their SBC as a separate SIP trunk for each Mediation Server. The service provider will then handle the load balancing for its own servers. Not all service providers or SBCs may support these capabilities. Furthermore, the service provider may charge extra for this capability. Typically, each SIP trunk to the SBC incurs a monthly fee.

  • IP-PBX. In the direction from the Mediation Server pool to the IP-PBX SIP termination, the IP-PBX can receive connections from any Mediation Server in the pool. In the direction from the IP-PBX to the pool, traffic can be sent to any Mediation Server in the pool. Because most IP-PBXs do not support DNS load balancing, we recommend that individual direct SIP connections be defined from the IP-PBX to each Mediation Server in the pool. The IP-PBX will then handle its own load balancing by distributing traffic over the trunk group. The assumption is that the trunk group has a consistent set of routing rules at the IP-PBX. Whether a particular IP-PBX supports this trunk group concept and how it intersects with the IP-PBX's own redundancy and clustering architecture needs to be determined before you can decide whether a Mediation Server cluster can interact correctly with an IP-PBX.

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. Therefore, 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 particular PSTN gateway, IP-PBX, or SIP trunk peer can route to multiple Mediation Servers or trunks. 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.