PDF of this tutorialThe mandatory functions of the gatekeeper per the International Telecommunications Union’s (ITU’s) specifications include the following:
- address translation
- admissions control
- bandwidth control
- network management
Address Translation
The gatekeeper provides address translation between alias and transport addresses upon an endpoint’s request for service. As users typically do not know the IP addresses of other terminals (or entities) they wish to call, the gatekeeper translates an alias address (H.323 identifier, URL, phone number, or e-mail address) to a transport address. There are several mechanisms that can be implemented to update the translation table, one of which includes using the registration, admission, and status (RAS) channel. Other mechanisms can be implemented to support endpoints that do not use the RAS channel for routing communications messages.
Admissions Control
The gatekeeper authorizes network access based on H.323 guidelines and other criteria using admissions request message (ARQ), admissions confirm message (ACF), and admissions reject message (ARJ). As a result of limited share resources, not all users will be able to access the network at the same time. Gatekeepers also protect the integrity of the network for all users according to specified policies. Gatekeepers will authorize network access along the policy guidelines that a network administrator has selected when setting up a gatekeeper and/or zone. Network access and special services (use of a gateway, for example) may be based on call authorization, bandwidth usage, or other criteria.
Bandwidth Control
The gatekeeper monitors and controls network bandwidth usage and ensures that audio and/or video traffic does not exceed maximum network load as defined by the network manager. Network managers have the ability to restrict usage for H.323 traffic to offer quality of service (QoS) to other more critical applications. The gatekeeper may also accept all requests for bandwidth changes where no policies are enforced.
Network Management
The gatekeeper uses a routed call-signaling model to route call signaling and control channels to the appropriate entities in the network. Furthermore, the gatekeeper can implement logic for granting/denying terminals, gateways, and MCUs access to the associated network assets such as bandwidth, gateways, MCU, directory services, etc. It performs this procedure by monitoring all concurrent calls in a zone and enforcing network management policies for any new calls (sessions) a user may initiate.
