Bandwidth control with H.323 gatekeepers

In the lovely world of H.323, gatekeepers are responsible for a lot of tasks, including admission control. This means that the gatekeeper keeps track of all the gateways in its domain and they ask its permission before placing a call. The gatekeeper usually permits or denies a call based on the number of calls or the bandwidth required.

To configure the bandwidth control in most implementations, you can specify the total aggregate bandwidth in the zone, and the maximum bandwidth for any given session, but you can also specify the maximum bandwidth used between zones. This is tricky for a number of reasons.

First, you have to consider the difference between the codec's bandwidth and the actual bandwidth. Different vendors may treat this differently. Some endpoints send the codec bandwidth (without headers) in their Admission Request (ARQ) messages, while others send a more realistic value that includes the IP and other layer's headers. Obviously, these values vary fairly dramatically, so you want to pay attention to what your equipment does before you set your gatekeeper's maximum bandwidth values. Worse, this same behavior may change in different versions of the same vendor's software.

Next, you have to watch out to make sure your gatekeeper changes the bandwidth reservation when it receives a Bandwidth Request (BRQ) message. These are sent after the codec is negotiated. The new codec could have higher or lower bandwidth requirements and the

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gatekeeper should change its reservation appropriately. The problem is that in some circumstances, like calls between zones, the codec isn't negotiated (which happens during call setup) until after the ARQ message so that the gatekeeper doesn't really know how much bandwidth to reserve.

One last thing to keep an eye out for is that some gateways report unidirectional bandwidth, while others report bi-directional bandwidth. For instance, using G.729, one gateway might report 16kbps to the gatekeeper and another might report 32kbps. Obviously, this will make a big difference in the maximum setting you need to configure on your gatekeeper.

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


This was first published in September 2002

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