Bandwidth Estimates

Table 28.2 defines the various bandwidth estimates for each protocol. Usually the typical bandwidth usage values can be used for planning purposes. Forward error correction (FEC) is enabled when Lync clients detect poor network connectivity and attempts to provide a more resilient voice connection to combat network jitter or latency.

Table 28.2 Bandwidth Codec Estimates

When a bandwidth policy limit is exceeded by a user, Call Admission Control kicks in with an attempt to reroute the call. First, the call attempts to be directed over the Internet. Instead of using the WAN link, the call traverses the Internet and Edge infrastructure. If that is not possible or fails, the call can reroute across the PSTN. Whether this is allowed depends on whether the voice policy assigned to the user allows this feature. If neither Internet nor PSTN rerouting is possible, the call will attempt to be sent directly to voicemail. Lastly, if voicemail is unavailable, the call will simply fail.

It is also important to note that Call Admission Control only applies to Lync Server endpoints traversing a WAN link. Other applications transmitting data on the same WAN link are not affected by Lync Call Admission Control policies. Organizations can define a bandwidth limit for Lync traffic and still see that WAN link become saturated due to other applications. In this scenario, it makes sense to enforce QoS policies on the WAN link to ensure Lync endpoints can always place calls.

The bandwidth override policy is enforced by the receiving endpoint and not the sender. When a call is placed, the receiving endpoint leverages its subnet information and checks whether the call will exceed the bandwidth policy limit. If not, the call will be allowed.

The only clients that actually respect Call Admission Control policies are Lync 2010 endpoints. Earlier clients, such as Office Communicator 2007 R2, are not able to perform a bandwidth check when a Lync client calls. However, media calls from Office Communicator 2007 R2 to a Lync endpoint enforce Call Admission Control policies.