CAC and Media Bypass in Lync Server 2010

Within a Lync Server 2010 deployment there are a number of features which have been introduced to meet the needs of an Enterprise class VoIP solution; namely Call Admission Control, Media Bypass and E911. This post will investigate the first two of these features.

Each of these features is based on the networking configuration of Lync Server 2010. Before configuring any of the features the network configuration has to be defined. Lync views the network as a number of Network Regions, each with one or more Network Sites connected by Network Links.

  • Network regions consist of multiple network sites. A network region represents a network hub or backbone. Network regions are interconnected through a wide area network (WAN) link. A network region is a collection of sites inter-connected by high speed links. Essentially, the sites within a region don't need to have CAC applied because the available bandwidth is abundant. Each region has a central Lync Site that manages the bandwidth. A single Lync site can be associated with multiple Network regions.
    A note on terminology: Microsoft documentation gives examples of network regions, such as North America, Europe and Asia-Pac. However, if the enterprise has a branch location with restricted bandwidth to the HQ or Data Centre, then the branch site would be a region.
  • Network sites identify locations within a network region. A network site represents a physical location that belongs to an organization such as a branch office or a regional office.

  • Network links refer to the WAN link that connects two network regions. Such WAN links have limited bandwidth capacity compared to LAN links; therefore, call admission control is enforced on such network links.

  • Network route refers to the network regions traversed between two given endpoint regions.

This information is probably already available with the enterprise's Network support team.

Call Admission Control (CAC) is defined by a call admission policy which defines the audio and video bandwidth allocated over a network route.

Call admission control policy defines the following information:

  • Maximum total bandwidth to allocate for audio
  • Maximum total bandwidth to allocate for video
  • Maximum bandwidth that can be allocated for a single audio call (session)
  • Maximum bandwidth that can be allocated for a single video call (session)
Call admission control is enforced at the client level by the receiving client and only if that client is Lync 2010 (not Office Communicator).

Each network site is comprised of one or more subnets. The subnets specified in the Lync configuration must be the subnet as configured on the client, super-netting or subnet aggregation is not supported. Also, the external IP address of the AV Edge server(s) must be added with a 32 bit subnet mask.

How Media Bypass Works

When you enable media bypass, a unique bypass ID is automatically generated for a network region and for all network sites without bandwidth constraints within that region. Sites with bandwidth constraints within the region and sites connected to the region over WAN links with bandwidth constraints are each assigned their own unique bypass IDs.

When a user makes a call to the PSTN, the Mediation Server compares the bypass ID of the client subnet with the bypass ID of the gateway subnet. If the two bypass IDs match, media bypass is used for the call. If the bypass IDs do not match, media for the call must flow through the Mediation Server.

When a user receives a call from the PSTN, the user's client compares its bypass ID to that of the PSTN gateway. If the two bypass IDs match, media flows directly from the gateway to the client, bypassing the Mediation Server.

At the Global level, media bypass configuration has two options: Always Bypass or Use Site and Region Information. Always Bypass is used where there is no need for CAC. Always Bypass assigns all networks the same bypass ID.

  • The system automatically assigns a single unique bypass ID to each region.
  • Any site connected to a region over a WAN link without bandwidth constraints inherits the same bypass ID as the region.
  • A site associated with the region over a WAN link with constrained bandwidth is assigned a different bypass ID from that of the region.
  • Subnets associated with each site inherit the bypass ID for that site.
Media bypass and CAC work together to manage bandwidth control for call media. Media bypass facilitates media flow over well-connected links; CAC manages traffic on links with bandwidth constraints. If you enable CAC, you cannot select Always Bypass, and vice-versa, because the two configurations are mutually exclusive.

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