Gatekeeper Characteristics

Gatekeepers provide call-control services for H.323 endpoints, such as address translation and bandwidth management as defined within RAS (remote access server). Gatekeepers in H.323 networks are optional. Their purpose is to control the dialing procedures of all VoIP gateways in a VoIP environment. So if gatekeepers are present in a network, terminals and gateways must use their services. The H.323 standards both define mandatory services that the gatekeeper must provide and specify other optional functionality that it can provide.

An optional feature of a gatekeeper is call-signaling routing. Endpoints send call-signaling messages to the gatekeeper, which the gatekeeper routes to the destination endpoints. Alternately, endpoints can send call-signaling messages directly to the peer endpoints. This feature of the gatekeeper is valuable, as monitoring of the calls by the gatekeeper provides better control of the calls in the network. Routing calls through gatekeepers provides better performance in the network, as the gatekeeper can make routing decisions based on a variety of factors, for example, load balancing among gateways.

A gatekeeper is optional in a H.323 system. The services offered by a gatekeeper are defined by RAS and include address translation, admissions control, bandwidth control, and zone management (see Figure 1). A gatekeeper can also secure the whole VoIP service by restricting the registration of gateways, and largely simplifying the dialing plan setting of gateways. H.323 networks that do not have gatekeepers may not have these capabilities, but H.323 networks that contain IP–telephony gateways should also contain a gatekeeper to translate incoming E.164 telephone addresses into transport addresses. A gatekeeper is a logical component of H.323 but can be implemented as part of a gateway or MCU (multipoint control unit).

Figure 1. Gatekeeper Components

 

Mandatory Gatekeeper Functions

- Address Translation

Calls originating within an H.323 network may use an alias to address the destination terminal. Calls originating outside the H.323 network and received by a gateway may use an E.164 telephone number (e.g., 310-442-9222) to address the destination terminal. The gatekeeper translates this E.164 telephone number or the alias into the network address (e.g., 204.252.32:456 for an IP–based network) for the destination terminal. The destination endpoint can be reached using the network address on the H.323 network.

- Admission Control

The gatekeeper can control the admission of the endpoints into the H.323 network. It uses RAS messages, admission request (ARQ), confirm (ACF), and reject (ARJ) to achieve this. Admissions control may be a null function that admits all endpoints to the H.323 network.

- Bandwidth Control

The gatekeeper provides support for bandwidth control by using the RAS messages, bandwidth request (BRQ), confirm (BCF), and reject (BRJ). For instance, if a network manager has specified a threshold for the number of simultaneous connections on the H.323 network, the gatekeeper can refuse to make any more connections once the threshold is reached. The result is to limit the total allocated bandwidth to some fraction of the total available, leaving the remaining bandwidth for data applications. Bandwidth control may also be a null function that accepts all requests for bandwidth changes.

- Zone Management

The gatekeeper provides the above functions—address translation, admissions control, and bandwidth control—for terminals, gateways, and MCUs located within its zone of control.

 

Optional Gatekeeper Functions

- Call-Control Signaling

The gatekeeper can route call-signaling messages between H.323 endpoints. In a point-to-point conference, the gatekeeper may process H.225 call-signaling messages. Alternatively, the gatekeeper may allow the endpoints to send H.225 call-signaling messages directly to each other.

- Call Authorization

When an endpoint sends call-signaling messages to the gatekeeper, the gatekeeper may accept or reject the call, according to the H.225 specification. The reasons for rejection may include access-based or time-based restrictions, to and from particular terminals or gateways.

- Call Management

The gatekeeper may maintain information about all active H.323 calls so that it can control its zone by providing the maintained information to the bandwidth-management function or by rerouting the calls to different endpoints to achieve load balancing.