Valuable VoIP

H.323 History
One cornerstone of VoIP technology is the H.323 standard. H.323 defines realtime multimedia communications—audio, video, and data—over packet-based networks, such as IP-based networks. The ITU specified the H.323 standard and, in 1996, released the first version, which focused heavily on multimedia communications in LAN environments that don't provide a guaranteed Quality of Service (QoS).

While the ITU released this standard, researchers were experimenting with voice communications over the Internet. The use of proprietary methods for setting up calls, compressing voice traffic, and locating communicating parties resulted in incompatible products. Communities within research and industry quickly realized the need for a VoIP standard and adopted H.323 to achieve interoperability between VoIP products. New VoIP requirements, such as communication between a PC-based phone and a traditional phone, voice communication between two PC users by way of the Internet, quality of voice communication over the Internet, and call authentication and authorization, influenced the development of the second version of H.323. In 1998, the ITU released H.323 version 2, which accommodates these requirements. This version is not only applicable in LAN environments but also in WAN and metropolitan area network (MAN) environments. In 2000, the H.323 standard evolved to version 3, which incorporates features such as fax capabilities over packet networks, fast-call setup, and communication between gatekeepers.

H.323 is part of ITU's H.32x standard family. Other H.32x standards define multimedia communications over other network types (e.g., H.320 for ISDN, H.321 and H.310 for Broadband ISDN—B-ISDN,H.322 for LANs that provide guaranteed QoS, and H.324 for switched-circuit networks. PSTNs are switched-circuit networks).

H.323 Components
The H.323 standard comprises four components: terminals, gateways, gatekeepers, and Multipoint Control Units (MCUs). Together, these components can provide point-to-point and point-to-multipoint multimedia communications.

An H.323 terminal provides realtime multimedia communications with other terminals by supporting voice and optional video and data communications. The terminal can be a PC-based phone, a standalone device (e.g., an IP phone), or an application running on a PC. H.323 terminals are compatible with other H.32x terminals.

An H.323 gateway interconnects and enables communication between H.323 networks and non-H.323 networks, such as PSTNs. The gateway translates protocols for call setup and release and converts and transfers information between the two networks. (However, for communication between two H.323 terminals in an H.323 network, you don't need a gateway.) A popular H.323 gateway is the IP-PSTN gateway that connects an IP network and PSTN and lets an H.323 terminal talk to a traditional telephone in the PSTN.

An H.323 gatekeeper is the central point of an H.323 network and provides call control within the network. Its functions include address translation, admission control, bandwidth management, and tracking and reporting conversation times. A gatekeeper is optional, but when it's present, H.323 terminals and gateways (aka endpoints) must use it.

An H.323 MCU manages conferences between three or more H.323 terminals. When terminals participate in a conference, they must establish a connection to the MCU. The MCU ensures that all terminals in the conference have a common level of communication, controls conference resources (e.g., the specific terminal that multicasts video), and determines which audio or video coder-decoder (codec) to use between the terminals. In addition, the MCU can optionally provide centralized processing of conference media information streams.

Figure 3 depicts an H.323 network that contains the standard's four components and the interconnection with a switched-circuit network. The gateway, gatekeeper, and MCU are separate systems in Figure 3, but they're logical components, and vendors can implement all three components in one system. A collection of terminals, gateways, and MCUs managed by one gatekeeper in an H.323 network is an H.323 zone. Only one gatekeeper exists per zone, and you can have one or more zones across your IP network.

H.323 Protocols
The H.323 standard includes seven major protocols: audio codec; video codec; H.225.0 registration, admission, and status (RAS); H.255.0 call signaling (aka Q.931); H.245 control signaling; Real Time Protocol (RTP); and Real Time Control Protocol (RTCP). Audio codec encodes the voice signals from the microphone on the H.323 terminal into audio codes suitable for transmission on the H.323 network. It decodes the audio codes that the H.323 terminal receives from the H.323 network. An H.323 terminal supports one or more audio coding and decoding algorithms, including G.711, G.722, G.723.1, G.728, and G.729. (The ITU specified all these algorithms). H.323 requires vendors of H.323 terminal implementations to support G.711. For two terminals to understand each other, they must support at least one common audio codec algorithm.