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Resource-Based CAC 601

In summary, a multizone gatekeeper network offers the following CAC attributes:

The WAN bandwidth at each connecting site can be protected, provided each site is also a zone. For small remote sites in an enterprise network, this often translates into a zone per gateway, which might not be a practical design.

The bandwidth within a site can be protected if necessary, but this is frequently of little value because there is only one gateway in the site (small remote offices, or a customer premises equipment [CPE] entry point to an SP-managed network service) or because a high-speed LAN is between the gateways (large sites and SP POPs).

Gatekeeper CAC is a method well suited to limit the number of calls between sites.

Gatekeeper CAC cannot protect the bandwidth on WAN segments not directly associated with the zones. For example, gatekeeper CAC cannot protect the backbone link marked with 20 calls in the simple enterprise topology shown in Figure 8-24, unless the slowest link approach is followed.

Zone-per-Gateway Design

Because the zone-per-gateway design offers the finest granularity of gatekeeper CAC, it is worth exploring a little further. In enterprise networks, this often makes sense from the following points of view:

Geographical considerations

CAC to protect the WAN access link into a site containing a single gateway

Dialing plans that coincide with sites, allowing a zone prefix to easily translate to the gateway serving a specific site

A gatekeeper is a logical concept, not a physical concept. Each gatekeeper therefore does not imply that there is a separate box in the network; it merely means that there is a separate local zone statement in the configuration.

With Cisco IOS Release 12.1(5)T and Release 12.2, a small remote site router has the capability to provide gateway, gatekeeper, and WAN edge functionality; however, a zone-per-gateway design complicates the scalability aspect that gatekeepers bring to H.323 networks. You should therefore carefully consider the advantages and limitations of such a design.

Gatekeeper in CallManager Networks

Of all the CAC mechanisms discussed in this chapter, gatekeeper zone bandwidth is the only method applicable to multi-site distributed CallManager clusters. In this scenario, the CallManager behaves like a VoIP gateway to the H.323 gatekeeper, as is shown in Figure 8-27.


Chapter 8: Call Admission Control and QoS Signaling



Figure 8-27 Gatekeeper in a CallManager Topology










Cluster A



Cluster B




IP Network




















Can I Make a Call? (ARQ)











Zone 1





Zone 2











Yes You Can (ACF) or





No You Cannot (ARJ)

Call Is Placed if Allowed


In this example, the CallManager in Zone 2 requests the WAN bandwidth to carry a voce conversation from the gatekeeper on behalf of the IP Phone x2111. The gatekeeper determines whether each zone has sufficient bandwidth to establish the call. If the gatekeeper allows the call to proceed, the CallManager in Zone 2 contacts the CallManager in Zone 1 to begin call setup. If the gatekeeper rejects the call, the CallManager in Zone 2 can reroute the call over the PSTN to reach the called party.

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

Resource-Based CAC 603

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

Host Site


Remote Site




Zone 1

Frame Relay

Zone 2










Z1-Z2: 144 kbps


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,

604 Chapter 8: Call Admission Control and QoS Signaling

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.

Zone Bandwidth Configuration

As of Cisco IOS Software Release 12.1(5)T and Release 12.2, the following types of zone bandwidth limitations can be configured on the gatekeeper:

The maximum bandwidth for all H.323 traffic between the local zone and a specified remote zone.

The maximum bandwidth allowed for a single session in the local zone. This configuration is typically used for video applications, not for voice.

The maximum bandwidth for all H.323 traffic allowed collectively to all remote zones.

Tables 8-16 and 8-17 list the gatekeeper command and options used to configure gatekeeper zone bandwidth.

Table 8-16 Gatekeeper Bandwidth Command


Mode and Function



bandwidth {interzone | total | session}

Specifies the gatekeeper zone bandwidth restrictions

{default | zone zone-name} max-






bandwidth remote max-bandwidth

Specifies the total bandwidth for H.323 traffic between


this gatekeeper and any other gatekeeper