H.323 is a Recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network

H. 323 provides standards for equipment, computers and services for multimedia communication across packet based networks and specifies transmission protocols for real-time video, audio and data details. H. 323 is widely used in IP based videoconferencing, Voice over Internet Protocol (VoIP) and Internet telephony.Users can communicate through the Internet and make use of a variety of products that are H.323 standard compatible.

The H.323 standard mainly depends on the Internet Engineering Task Force’s (IETF) Real-Time Control Protocol (RTCP) and Real-Time Protocol (RTP), with the utilization of various other protocols for data communication, call signaling and A/V communications.

H.323 Architecture: Components

Recommendation H.323 specifies components, protocols, and procedures for real-time point-to-point and multipoint multimedia communication 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. Figure 1 shows a general layout of a H.323 enabled inter-network with all the necessary components.

Figure 1. Layout of H.323-enabled inter-network

In a general H.323 implementation four logical entities or components are required. These are terminals, gateways, gatekeepers, and multipoint control units (MCU). Terminals, gateways, and MCUs are collectively known as endpoints. Even though an H.323-enabled network can be established with only terminals the other components are essential to provide greater practical usefulness of the services.

4.1 Terminal

A terminal, or a client, is an endpoint where H.323 data streams and signaling originate and terminate. It may be a multimedia PC with a H.323 compliant stack or a standalone device such as a USB (universal serial bus) IP telephone. A terminal must support audio communication; video and data communication support is optional.

4.2 Gateway

A gateway is an optional component in a H.323-enabled network. However, when communication is required between different networks a gateway is needed at the interface. Through the provision of gateways in H.323 it is possible for H.323 terminals to inter-operate with other H.32X compliant conferencing terminals. For example, it is possible for a H.323 terminal to set up a conference with terminals based on H.320 or H.324 through an appropriate gateway. A gateway provides data format translation, control signaling translation, audio and video codec translation, and call setup and termination functionality on both sides of the network. Depending on the type of network to which translation is required a gateway may support H.310, H.320, H.321, H.322, or H.324 endpoints.

4.3 Gatekeeper

A gatekeeper is a very useful, but optional, component of an H.323-enabled network. Gatekeepers are needed to ensure reliable, commercially feasible communications. A gatekeeper is often referred to as the brain of the H.323 enabled network because of the central management and control services it provides. When a gatekeeper exists all endpoints (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. These services include 

• 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. First, accounting information of calls can be maintained for billing and security purposes. Second, a gatekeeper can re-route a call to an appropriate gateway based on bandwidth availability. Third, re-routing can be used to develop advanced services such as mobile addressing, call forwarding, and voice mail diversion.

4.4 Multipoint Control Unit (MCU)

A multipoint control unit enables conferencing between three or more endpoints. It consists of a mandatory multipoint controller (MC) and zero or more multipoint processors (MP). Although the MCU is a separate logical unit it may be combined into a terminal, gateway, or gatekeeper. The MCU is an optional component of an H.323-enabled network.

The multipoint controller provides a centralized location for multipoint call setup. Call and control signaling are routed through the MC so that endpoints capabilities can be determined and communication parameters negotiated. A MC may also be used in a point-to-point call which can later be extended into a multipoint conference. Another useful job of the MC is to 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 multipoint processor handles the mixing, switching, and processing of the audio, video, and data streams among the conference endpoints.

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, a MC ensures communication compatibility but the media streams are multicast and the mixing is performed at each terminal.

Prerequisites for Configuring an H.323 Network

• Establish a working IP network. For information on IP configuration, see the references listed in the “Related Documents” section.
• Install the appropriate voice network module and voice-interface card for the Cisco router. For information on the module and card, see the Voice Network Module and Voice Interface Card Configuration Note that came with the voice network module.
• Configure your H.323 gateways, gatekeepers, and proxies. For information on VoIP configuration, see the resources in the “Related Documents” section.
• To ensure network security, configure a RADIUS authentication, authorization, and accounting (AAA) server. Configure the following information in your CiscoSecure AAA server:
  • In the /etc/raddb/clients file, ensure that the following information is provided:

#Client Name            Key
#-----------       -------------------
gk215.cisco.com         testing123

Where gk215.cisco.com is resolved to the IP address of the gatekeeper requesting authentication

• • In the /etc/raddb/users file, ensure that the following information is provided:

taeduk@cisco.com  Password = "thiswouldbethepassword"
User-Service-Type = Framed-User,
Login-Service = Telnet

Where taeduk@cisco.com is the h323-id of the gateway authenticating to gatekeeper gk215.cisco.com.

• Configure an NTP server for your network.
• For gatekeeper-management statistics, do the following:
  • Configure the Simple Network Management Protocol (SNMP) agent in global configuration mode.
  • Update the MIB data files on your management workstations so that the management application knows what the new objects are.

Why H.323 is Important

Trend: The explosive growth of the Internet and the almost universal deployment of corporate LANs have made packet-based networks ubiquitous. It is therefore natural for individuals and enterprises to use this resource for audio and video communication to offset some of the tariffs of public switched telephone networks (PSTN). Multimedia over packet-based networks (primarily IP networks) has grown rapidly in the last few years. Industry research put the growth at 37% annually and is expected to reach $39 billion by the year 2002 ( http://www.imtc.org/faq.htm). In a similar forecast, Probe Research estimates that by the year 2002 18.5% of all U.S. phone traffic will be carried over data networks. This rapid expansion and potential underlies the importance of an enabling and unifying standard such as H.323.

Standardization: In the 1995-1997 time period, several vendors developed products and services to cater for the emerging IP telephony market. These products and services, however, were based on proprietary protocols that prevented widespread interoperability. H.323 is a standard protocol that has been widely accepted. This will promote greater awareness, availability, and acceptability of multimedia conferencing over packet-based networks.

Internetworking: H.323 bridges multimedia communications between packet-based and switched-circuit networks SCN). Existing clients based on SCN conferencing standards like H.320 (ISDN), H.321 (ATM), and H.324 (PSTN) can inter-operate with H.323 clients. For example, it is possible to call from a H.323 client to a regular telephone on a PSTN. At the corporate level this internetworking capability allows enterprises to migrate voice and video from existing networks to their data network.

Integrated services: H.323 makes possible the development of additional services such as e-mail, voice mail, fax, call center functionality, and videoconferencing in an integrated environment. For example, an e-commerce business can provide a direct voice link from their web site to a sales representative to answer customers’ questions. A few services have been standardized in H.450.x (e.g. call transfer, call forwarding). Others will be added later.

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