Elements in a H.323 scenario

The ITU H.323 Recommendation describes the components of an H.323-compliant system. Components of an H.323 solution may include terminals, gateways, gatekeepers, multipoint controllers (MCs), multipoint processors (MPs), and multipoint control units (MCUs). Other components address deployment upon IP networks that have QoS mechanisms, and security elements such as proxies and firewalls. This section discusses the role of each element in an overall H.323 system as part of an end-to-end network.

H.323 Terminal

An H.323 terminal is an endpoint on the local area network which enables real-time, two-way communications with another H.323 terminal, gateway, or MCU. Any terminal may provide voice only, voice and data, voice and video, or voice, video and data. H.323 specifies the mode of operation for each type of terminal.

All H.323 terminals must support these features:

H.245 for negotiating channel usage and capabilities>
Q.931, a standard for call signaling and setup
Registration/Admission/Status (RAS), a protocol for communicating with gatekeepers
RTP/RTCP support, for sequencing audio and video packets transmission.

H.323 terminals may optionally support these features:

Video codecs
T.120 data conferencing protocols
MCU capabilities
Gateways

Gateway

A gateway is an optional element in the H.323 system, and is an endpoint on the LAN that provides communication between H.323 terminals or other gateways over the LAN and other ITU terminals on the WAN, using H.425 and Q.931 protocols. Gateways are not required when there is no connection to other networks.

Gateways are found where network administrators need to:

Establish links with analog PSTN terminals
Establish links with remote H.320-compliant terminals over ISDN
Establish links with remote H.324-compliant terminals over PSTN networks.

Proxy Gateway

A specialized gateway is a proxy gateway, which is a secured connection between H.323 sessions. It contains a proxy feature as part of the conference infrastructure that can provide QoS, traffic shaping, and security and policy management for H.323 traffic across secured connection.

Gatekeeper

Also an optional component, gatekeepers provide call control services to H.323 endpoints. One or more gatekeepers may be deployed, and are logically separate from the endpoints. While standards for gatekeeper-to-gatekeeper communications are not established, the physical implementation may coexist with a terminal, MCU, gateway, MC or other non-H.323 LAN devices.

Gatekeepers can provide these services:

Address translation—Performs alias address to transport address translation. Gatekeepers typically use a translation table updated using registration messages (as described later), although other table update methods are allowed.
Admissions control—Authorizes LAN access, based upon call authorization, bandwidth or other criteria set by the manufacturer.
Bandwidth control
Zone management—Provides above functions for registered terminals, MCUs, and gateways.

Multipoint Controller

A multipoint controller (MC) is a LAN-based H.323 entity that controls three or more terminals participating in multipoint conferences. It may also control a point-to-point conference which may develop into a multipoint conference. An MC provides capabilities negotiation with all terminals to establish common levels of communication. It may also control conference resources such as multicast video. An MC does not mix or switch audio, video and data.

Multipoint Processor

A multipoint processor (MP) is a LAN-based H.323 entity that centrally processes audio, video, and/or data streams in a multipoint conference. The MP mixes, switches, and performs other processing duties for streams controlled by an MC. It may process one or many media streams depending upon the type of conference it is supporting.

Multipoint Control Unit

A multipoint control unit (MCU) supports multipoint conferences between three or more endpoints. As defined by H.323, an MCU consists of a required MC and optional MP's. A typical MCU supporting centralized multipoint conferences consists of an MC unit, and an MP that supports audio, video, and data streams.

The H.323 recommendations allow either centralized or decentralized conference configurations, an MCU is required for the first, while the second can be administrated with multicast technologies.