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Gateway Characteristics
A gateway provides translation of protocols for call setup and release, conversion of media formats between different networks, and the transfer of information between H.323 and non–H.323 networks (see Figure 4). An application of the H.323 gateway is in IP telephony, where the H.323 gateway connects an IP network and SCN network (e.g., ISDN network).
Figure 4. Gateway Protocol Stack
On the H.323 side, a gateway runs H.245 control signaling for exchanging capabilities, H.225 call signaling for call setup and release, and H.225 registration, admissions, and status (RAS) for registration with the gatekeeper. On the SCN side, a gateway runs SCN–specific protocols (e.g., ISDN and SS7 protocols).
Terminals communicate with gateways using the H.245 control-signaling protocol and H.225 call-signaling protocol. The gateway translates these protocols in a transparent fashion to the respective counterparts on the non–H.323 network and vice versa. The gateway also performs call setup and clearing on both the H.323–network side and the non–H.323–network side. Translation between audio, video, and data formats may also be performed by the gateway. Audio and video translation may not be required if both terminal types find a common communications mode. For example, in the case of a gateway to H.320 terminals on the ISDN, both terminal types require G.711 audio and H.261 video, so a common mode always exists. The gateway has the characteristics of both an H.323 terminal on the H.323 network and the other terminal on the non–H.323 network it connects.
Gatekeepers are aware of which endpoints are gateways because this is indicated when the terminals and gateways register with the gatekeeper. A gateway may be able to support several simultaneous calls between the H.323 and non–H.323 networks. In addition, a gateway may connect an H.323 network to a non–H.323 network. A gateway is a logical component of H.323 and can be implemented as part of a gatekeeper or an MCU.
Gatekeeper Characteristics
Gatekeepers provide call-control services for H.323 endpoints, such as address translation and bandwidth management as defined within RAS. Gatekeepers in H.323 networks are optional. If they are present in a network, however, 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 an 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 5). 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.
Figure 5. 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. An H.323 zone is defined in Topic 3.
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.
