Zone Bandwidth Calculation

The gatekeeper does not have any knowledge of network topology and does not know how much bandwidth is available for calls. Nor does the gatekeeper know how much of the configured bandwidth on the links is currently used by other traffic. The gatekeeper takes a fixed amount of bandwidth, statically configured on the gatekeeper, and subtracts a certain amount of bandwidth for each call that is set up. Bandwidth is returned to the pool when a call is disconnected. If a request for a new call causes the remaining bandwidth to become less than zero, the call is denied. The gatekeeper does not use bandwidth reservation. Instead, the gatekeeper performs a static calculation to decide whether a new call should be allowed.

It is the responsibility of the gateways to inform the gatekeeper of how much bandwidth is required for a call. Video gateways therefore could request a different bandwidth for every call setup. One video session may require 256 kbps, whereas another requires 384 kbps. Voice gateways should consider codec, Layer 2 encapsulation, and compression features, such as compressed Real Time Protocol (cRTP), when requesting bandwidth from the gatekeeper. Sometimes these features are not known at the time of call setup, in which case a bandwidth change request can be issued to the gatekeeper after call setup to adjust the amount of bandwidth used by the call. As of Cisco IOS Software Release 12.2(2)XA, Cisco has implemented only the functionality of reporting any bandwidth changes when the reported codec changes.

Prior to Cisco IOS Software Release 12.2(2)XA on a Cisco H.323 Gateway, calls were always reported to require a bandwidth of 64 kbps. This is the unidirectional payload bandwidth for a Cisco G.711 codec. If the endpoints in the call chose to use a more efficient codec, this was not reported to the Cisco gatekeeper. In the Cisco IOS Software Release 12.2(2)XA version of the Cisco H.323 Gateway or a later version that conforms with H.323 version 3, the reported bandwidth is bidirectional. Initially, 128 kbps is specified. If the endpoints in the call select a more efficient codec, the Cisco gatekeeper is notified of the bandwidth change.

Figure 8-28 illustrates a network that uses a gatekeeper to limit bandwidth to 144 kbps between two zones. A connection is requested with an initial bandwidth of 128 kbps. After the call has been set up, the endpoints report the change in the bandwidth. Assuming the endpoints are using G.729, 16 kbps is subtracted from the configured maximum of 144 kbps. The 16 kbps represents a bidirectional G.729 payload stream to the gatekeeper.

Figure 8-28 Gatekeeper Bandwidth Calculation

Figure 8-28 Gatekeeper Bandwidth Calculation

In the event that the second call arrives before the endpoint requests the change in bandwidth for the first call, the Cisco gatekeeper rejects the second call, because the total requested bandwidth exceeds the 144 kpbs configured. As call 1 is being set up, for example, the gatekeeper records this call as a 128-kbps call and waits for a codec change before adjusting the recorded bandwidth. At this point, the gatekeeper has subtracted 128 kbps from the configured 144 kbps, leaving 16 kbps available. If call 2 requests admission before the codec change is reported in call 1, the gatekeeper does not have enough available bandwidth to allow this 128-kbps call to proceed.

Gatekeeper zone bandwidth remains an inexact science because the gateway may not have full knowledge of the bandwidth required by the call. The following examples describe common situations where the gateway will not have full knowledge of the bandwidth required per call:

• The gateway is attached to an Ethernet segment in a campus network where cRTP does not apply and where the Layer 2 headers are larger than they would be for Frame Relay or Multilink Point-to-Point Protocol (MLP) on the WAN legs.

• A different codec is used in the campus network from the WAN segments, leveraging codec transcoding functionality at the WAN edge.

• In the backbone of the network, ATM is used as the transport technology. In this case, cell padding should be taken into account for bandwidth calculations.

• cRTP may be used at the WAN edge router.

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