Call Admission Control Within a Cluster

Why do you need to configure call admission control for video calls? The answer to this question is quite simple. If you do not control the number of video and voice calls over your WAN links in a centralized deployment (shown in Figure 28-5), periodically you will see decreased video and voice throughout the whole system. With call admission control, you control the number of calls on the WAN link and ensure that all calls, video and audio-only, are processed throughout the network with acceptable quality.

Figure 28-5. Typical Centralized Call Control Environment

Cisco CallManager uses regions and locations to implement call admission control:

• Regions define the codec used and maximum video bandwidth allowed per call. This value is configurable for calls within each region and for calls between any pair of regions.
• Locations define the maximum bandwidth allowed for all calls to and from a location.

In Cisco CallManager, devices derive their region setting with the associated device pool configuration. You can assign locations on a per-device basis. If you assign locations at the device level, it overrides the setting inherited from the device pool.

Calls between devices at the same location do not need call admission control because the assumption is that these devices reside on the same LAN that has "unlimited" available bandwidth. However, for calls between devices at different locations, the assumption is that there is an IP WAN link in between that has limited available bandwidth.

Region Configuration

When you are configuring a region, you set two fields in Cisco CallManager Administration: the Audio Codec and the Video Call Bandwidth fields, as shown in Figure 28-6. Note that the audio setting specifies a codec type, whereas the video setting specifies the bandwidth that you want to allow. However, even though the notation is different, the Audio Codec and Video Call Bandwidth fields actually perform similar functions. The Audio Codec value defines the maximum bit rate allowed for audio-only calls and for the audio channel in video calls.

Figure 28-6. Cisco CallManager Region Configuration

For instance, if you set the Audio Codec value for a region to G.711, Cisco CallManager allocates 64 kbps as the maximum bandwidth allowed for the audio channel for that region. In this case, Cisco CallManager permits calls using G.711, G.728, or G.729. However, if you set the Audio Codec value to G.729, Cisco CallManager allocates only 8 kbps as the maximum bandwidth allowed for the audio channel; in addition, Cisco CallManager permits calls using only G.729, because G.711, G.722, and G.728 all require more than 8 kbps.

The Video Call Bandwidth value defines the maximum bit rate for the video call, that is, the bit rate of the voice and video channels. For instance, if you want to allow video calls at a speed of 384 kbps using G.711 audio, you would set the Video Call Bandwidth value to 384 kbps and the Audio Codec value to G.711. The bit rate that would be used by the video channel thus would be 320 kbps. If the Video Call Bandwidth value is set to None for the region, Cisco CallManager either terminates the call or allows the call to pass as an audio-only call, depending on whether the calling device has the Retry Video Call as Audio option enabled.

In summary, the Audio Codec field defines the maximum bit rate used for the audio channel of audio-only calls and for the audio channel of video calls, whereas the Video Call Bandwidth field defines the maximum bit rate allowed for video calls and includes the audio portion of the video call.

Note

Video endpoints typically support only G.711 and G.722, whereas audio-only endpoints typically support only G.711 and G.729. Because you cannot configure the audio codec for audio and video calls separately, often the only common audio codec for mixed environments (including third-party equipment) is G.711.

Location Configuration

When configuring locations, you also set two fields in Cisco CallManager Administration: the Audio Bandwidth and the Video Bandwidth values, shown in Figure 28-7. Unlike regions, however, audio bandwidth for locations applies only to audio-only calls, whereas video bandwidth again applies to the video call (that is, audio and video channels).

Figure 28-7. Cisco CallManager Location Configuration

The audio and video bandwidth are kept separate because if both types of calls shared a single allocation of bandwidth, it is very likely that audio calls would take all of the available bandwidth and leave no room for any video calls, or vice versa.

Both the Audio Bandwidth and the Video Bandwidth fields offer three options: None, Unlimited, or a field that accepts numeric values. However, the values entered in these fields use two different calculation models:

• For the Audio Bandwidth field, the value entered has to include the Layer 3 and 4 overhead required for the call. For instance, if you want to permit a single G.729 call to or from a location, you would enter the value 24 kbps. For a G.711 call, you would enter the value 80 kbps. The payload of a G.711 voice call is 64 kbps; the 80-kbps value is the 64-kbps payload plus 16-kbps RPT plus UDP.
• The value in the Video Bandwidth field, by contrast, is entered without any overhead. For instance, for a 128-kbps call, enter the value 128 kbps; for a 384-kbps call, enter the value 384 kbps. As with the values used in the Video Bandwidth field for regions, it is recommended that you always use increments of 56 kbps or 64 kbps for the Video Bandwidth field for locations.

Note

The value None in the Video Bandwidth field indicates that video calls are not allowed between this location and other locations. Video calls can, however, be placed within this location.

Call Admission Control Example

Figure 28-8 illustrates an example of audio and video bandwidth requirements for a company with a three-site network. The San Francisco location has a 1.544-Mbps T1 circuit connecting it to the San Jose main campus. The system administrator wants to allow four G.729 voice calls and one 384-kbps video call to or from that location.

Figure 28-8. Video Call Bandwidth Example

The Dallas location has two 1.544-Mbps T1 circuits connecting it to the San Jose main campus, and the administrator wants to allow eight G.711 voice calls and two 384-kbps video calls to or from that location.

For this example, the administrator might set the San Francisco and Dallas locations to the values in Table 28-5.

First, a video device in San Francisco calls a video device in San Jose. Call admission control allows exactly one 384-kbps video call between San Francisco and any other location. The video call is active.

Next, another video device from San Francisco tries to set up a video call to a video device in Dallas. Call admission control does not allow a second video call from San Francisco to any other location and denies the video call.

If the call fails because of insufficient location bandwidth, it will not be retried with lower-bit-rate codecs. In this scenario, with no further configuration of the Cisco CallManager, the call (both audio and video portions) will be rejected.

Retry Video Call as Audio

The Retry Video Call as Audio setting appears as a check box in the IP Phone configuration in Cisco CallManager Administration, as shown in Figure 28-9.

Figure 28-9. Configuring the Retry Video Call as Audio Setting

This setting is enabled by default on all device types and applies to these scenarios only:

• The region is configured not to allow video.
• The location is configured not to allow video, or the requested video speed exceeds the available video bandwidth for that location.
• The requested video speed exceeds the zone bandwidth limits of the gatekeeper.

When this option is activated (checked), if there is not enough bandwidth for a video call (for example, if the Cisco CallManager regions or locations do not allow video for that call), Cisco CallManager will retry the call as an audio-only call. When this option is deactivated (unchecked), Cisco CallManager will not retry the video call as audio only but will instead either reject the call or reroute the video call by whatever Automated Alternate Routing (AAR) path is configured.

The Retry Video Call as Audio option takes effect only on the terminating (called) device, allowing flexibility for the calling device to have different options (retry or AAR) for different destinations.

Note

The called device determines the result. In other words, when one device calls another and any of the discussed insufficient bandwidth conditions applies, Cisco CallManager looks at the destination device to see whether the Retry Video Call as Audio option is enabled.