Gatekeeper Functionality

The concept of a zone is used to define the group of H.323 devices that an individual gatekeeper controls. That is, the devices that belong to the zone register with the controlling gatekeeper. Zones can cross subnets, and a gatekeeper can manage devices in one or more subnets. Up to 100 zones can be registered to an individual gatekeeper.

Prior to H.323 Version 2, only one gatekeeper could be active in a zone at a time. H.323 Version 2 introduced the concept of alternate gatekeepers. Implementing the alternate gatekeeper feature allows multiple gatekeepers to control one zone. This helps to provide redundancy and load sharing for large installations. The gateway must also support the alternate gatekeeper to use this feature.

Figure 16-1 is an example of two gatekeepers controlling devices in two separate zones. Notice that communication and signaling can cross between the zones.

Figure 16-1. Multiple Gatekeepers and Zones

The following are services for voice calls that the gatekeeper can provide:

• Address resolution H.323 IDs and E.164 telephone numbers can be resolved to an IP address for the destination gateway. This is similar to the way Domain Name Services (DNS) resolve fully qualified domain names to their associated IP addresses.
• Admission control Gateways or endpoints that are initiating a call request admission into the H.323 network. For example, a gateway can request to place a phone call, and the gateway then presents the called number to the gatekeeper. If the gatekeeper can resolve the number, security requirements are met, and bandwidth is available, the gatekeeper permits the call to be placed.

  The gatekeeper can track bandwidth that active calls use both within and between zones. You can configure the maximum bandwidth available. When you place a call, the bandwidth used is subtracted from the maximum. If bandwidth is insufficient to place the call, the call is rejected. When a call terminates, the bandwidth for that call is returned to the available pool.

• Bandwidth management Bandwidth is initially managed during admission control. If the bandwidth requirements change during a call, the gateway makes a bandwidth request of the gatekeeper. If the new bandwidth is available, the request is accepted; otherwise, it is rejected.
• Zone management The gatekeeper controls gateway registration within the controlled zone. Gateways make registration requests to the gatekeeper. The gatekeeper either allows the gateway to register or rejects the request.
• Call authorization This option allows a gatekeeper to control access to and from certain gateways or endpoints. You can do this using authentication, authorization, and accounting (AAA) services that Cisco IOS provides in conjunction with a RADIUS server.