H.323 Architecture Components

H.323 specifies components, protocols, and procedures for real-time point-to-point and multipoint multimedia communications over packet-based networks. It also sets interoperability guidelines for communication between H.323-enabled networks and the H.32X-based family of conferencing standards.

An H.323 implementation requires four logical entities or components. These are terminals, gateways, gatekeepers, and MCUs. A fifth component element, known as border elements, is optional. Terminals, gateways, and MCUs are collectively known as endpoints. Although an H.323 network can be configured with only terminals, the other components are essential to provide greater practical usefulness of the services.

Terminal

A terminal, or a client, is an endpoint where H.323 data streams and signaling originate and terminate. In an IP-PBX system, it may be an IP telephone; a non-IP communications device, such as an analog telephone with an IP adapter module; or a PC client softphone with an H.323 compliant stack. A terminal must support audio communication; video and data communication supports are optional.

Gateway

A gateway is an optional component in an H.323-enabled network. A gateway will be required if at least one of the terminals does not conform to H.323 standards but is designed for a different type of network. The gateway is usually located at the interface between the two networks. Through the provision of gateways in H.323, it is possible for H.323 terminals to interoperate with other H.32X-compliant conferencing terminals. For example, a PC client softphone station user can talk to a station user on an analog telephone. A gateway provides data format translation, control signaling translation, audio and video codec translations, and call setup and termination functionality on both sides of the network. Gateway equipment is composed of a Media Gateway Controller (MGC) and a Media Gateway (MG), which may co-exist or exist separately. The MGC handles call signaling and other nonmedia-related functions, and the MG handles the media. There are different types of gateways required to support H.310, H.320, H.321, H.322, or H.324 endpoints.

Gatekeeper

A gatekeeper is an optional component of an H.323-enabled network. Gatekeepers ensure reliable, commercially feasible communications and are used for admission control and address resolution. The gatekeeper may allow calls to be placed directly between endpoints or it may route the call signaling through itself to perform functions, such as followme/find-me, forward on busy, etc. A gatekeeper is often referred to as the controller of the H.323 enabled network, because it provides central management and control services. When a gatekeeper exists, all end- points (terminals, gateways, and MCUs) must be registered with it. Registered endpoints’ control messages are routed through the gatekeeper.

The gatekeeper and the endpoints it administers form a management zone. A gatekeeper provides several services to all endpoints in its zone.

Address translation. A gatekeeper maintains a database for translation between aliases, such as international phone numbers, and network addresses.

Admission and access control of endpoints. This control can be based on bandwidth availability, limitations on the number of simultaneous H.323 calls, or the registration privileges of endpoints.

Bandwidth management. Network administrators can manage bandwidth by specifying limitations on the number of simultaneous calls and by limiting authorization of specific terminals to place calls at specified times.

Routing capability. A gatekeeper can route all calls originating or terminating in its zone. This capability provides numerous advantages: accounting information of calls can be maintained for billing and security purposes; a gatekeeper can reroute a call to an appropriate gateway based on bandwidth availability; and rerouting can be used to develop advanced services such as mobile addressing, call forwarding, and voice mail diversion.

Multipoint Control Unit

The MCU is an optional component of an H.323-enabled network. It is responsible for managing multipoint conferences (two or more endpoints engaged in a conference). It consists of a mandatory Multipoint Controller (MC) that manages the call signaling and optional Multipoint Processors (MPs) to handle media mixing, switching, or other media processing. Although the MCU is a separate logical unit, it may be combined into a terminal, gateway, or gatekeeper.

The MC provides a centralized location for multipoint call set-up. Call and control signaling are routed through the MC so that endpoint capabilities can be determined and communication parameters negotiated. An MC may also be used in a point-to-point call, which can later be extended into a multipoint conference. The MC may also determine whether to unicast or multicast the audio and video streams depending on the capability of the underlying network and the topology of the multipoint conference.

The MCU is required in a centralized multipoint conference, where each terminal establishes a point-to-point connection with the MCU. The MCU determines the capabilities of each terminal and sends each a mixed media stream. In the decentralized model of multipoint conferencing, an MC ensures communication compatibility, but the media streams are multicast and the mixing is performed at each terminal.

Border Elements

Border elements are often colocated with a gatekeeper. They exchange addressing information and participate in call authorization between administrative domains. Border elements may aggregate address information to reduce the volume of routing information passed through the network and assist directly in call authorization/authentication between two administrative domains or via a clearinghouse.