Microsoft Lync Server 2010 : Planning for Voice Deployment - Media Bypass

Planning for Media Bypass is not too complicated because it only encompasses identifying which IP/PSTN gateways in the organization support the feature and then configuring the trunks appropriately to enable the support. The majority of the configuration typically involves defining the network topology correctly. Media bypass enables a Lync endpoint to communicate directly with an IP/PSTN gateway, bypassing the Mediation server role. Figure 1 displays how a user’s signaling traffic continues to flow through the server to the IP/PSTN gateway, but the actual audio stream is sent from the user directly to the IP/PSTN gateway.

Media Bypass is especially useful in branch office scenarios where no Mediation server is present, but a local IP/PSTN gateway exists. Without Media Bypass, calls are sent across a WAN link to a Mediation server at the central site, transcoded to G.711, and then sent back across the WAN link to the IP/PSTN gateway. With media bypass, endpoints can send the G.711 audio directly to the IP/PSTN gateway without traversing the WAN.

Bypass IDs

Media Bypass works by assigning a unique Bypass ID to each location, and each subnet associated with that location automatically inherits the same Bypass ID. When a Lync endpoint attempts a call, the subnet of the endpoint is examined. If the Bypass ID of the subnet matches the Bypass ID of the subnet where the IP/PSTN gateway resides, Media Bypass will be leveraged.

The same concept applies for inbound calls from the PSTN. When the IP/PSTN gateway receives a call and sees an endpoint with a matching Bypass ID, the audio flows directly to the Lync client. Using the network configuration example in Figure 2, the San Francisco, San Jose, and Sacramento sites share a Bypass ID because they are not bandwidth constrained. The Santa Cruz and Petaluma sites each receive a unique ID because they have WAN bandwidth limitations.

If a site does have a bandwidth policy profile assigned because WAN bandwidth is limited, a new Bypass ID is generated for the site. Users placing calls from within the bandwidth-constrained site are allowed to use Media Bypass to a local IP/PSTN gateway. When users attempt a call from a subnet with a different Bypass ID, Media Bypass is not allowed and the audio flows through a Mediation server. Figure 3 shows how a user can leverage Media Bypass in a local office, but be forced to send media through a Mediation server in the Santa Cruz office across a constrained WAN link. This is because the bypass ID of the user’s endpoint does not match the Bypass ID of the gateway in the Santa Cruz site.

Figure 3. Media Bypass Example

Enabling Media Bypass

After the network components are in place, an organization has two choices when enabling Media Bypass support:

• Always Bypass— This setting indicates endpoints should attempt to use Media Bypass with IP/PSTN gateways at all times. This configuration is useful for simple, small deployments where control over when Bypass use is not required. When a call is routed to a trunk with the Enable Media Bypass option selected, Lync endpoints bypass the Mediation server role. All sites and subnets are mapped to a single Bypass ID with this setting. This option cannot be selected if Call Admission Control is also used. This is because bandwidth policy profiles are not applied when bypassing a Mediation server. Media Bypass should only be used in scenarios where WAN bandwidth is not an issue, so it is assumed there is no need to check for available bandwidth.

• Use Site and Region Information— This setting enables Media Bypass based on scenarios defined in the network configuration. The actual values in the bandwidth policy profiles are not important with this option, but the profiles are used to determine when Bypass should be used. If there is any bandwidth policy profile assigned to a site or link, it will be assigned a unique Bypass ID. Subnets within the site have a matching Bypass ID so that only endpoints within the site are allowed to use Media Bypass with the IP/PSTN gateways in that location.

The final step in configuring Media Bypass is to ensure the IP/PSTN gateways used in each location supports Media Bypass. Also be sure to enable Media Bypass on the trunk configuration used with each IP/PSTN gateway.