In 1995, researchers wanted to solve the problem of how two computers could initiate communication in order to exchange audio and video media streams. H.323 and SIP (Session Initiation Protocol) were the two resultant solutions to that problem, but H.323 enjoyed the first commercial success. While both protocols allow users to do the same thing - to establish multimedia communication in single or in multimedia platforms – both protocols differ in design. H.323 is a binary protocol and SIP is an ASCII-based protocol.

When shopping for VoIP systems, you might notice that most new technology includes SIP rather than H.323. Ongoing debates over which system is better often leaves H.323 behind, but H.323 is superior in a number of ways. It often brings better interoperability with the PSTN and for video, and reliable out-of-band transport of DTMF (the tones heard when pressing a button on a telephone). One advantage SIP has over H.323 is its lack of complexity. SIP resembles the HTTP/SMTP protocols, which makes SIP easier for many individuals to use.

How it Works

An H.323 terminal is an endpoint in a LAN that participates in real-time two-way communications with another H.323 terminal, gateway, or multipoint control unit (MCU). H.323 endpoints are grouped together in zones, and each zone has one gatekeeper that manages all the endpoints in that zone. Each terminal must support audio communication, but they also can support audio with video, audio with data, or a combination of these capabilities.

H.323 can be referred to as “intelligent endpoint protocol,” which means that all the intelligence required to locate the remote endpoint and to establish media streams between the local and the remote device is an integral part of this protocol. “Device control protocols” are complementary to H.323, and those current protocols are H.248 and MGCP.

Basic Usage

To understand how H.323 is used, it helps to understand how the gateway works. In VoIP, the gateway usually is a device that offers an IP interface on one side and some sort of legacy telephone interface on the other side. The legacy telephone interface may be complex, such as an interface to a legacy Public Switched Telephone Network (PSTN) switch, or it may be a simple interface that allows the user to connect one or a few traditional telephones. Basically, the gateway converts media provided in one type of network to the format required for another type of network.

Originally, gateways were viewed as monolithic devices that had call control provided by H.323 (or SIP) and hardware required to control the PSTN interface. In 1998, the idea of splitting the gateway into two logical parts was proposed: one part, which contains the call control logic, is called the Media Gateway Controller (MGC) or call agent (CA), and the other part, which interfaces with the PSTN, is called the media gateway (MG). With this functional split, a new interface existed (going between the MGC and MG), driving the necessity to define MGCP and H.248.

The H.323 gateway can provide an interface between H.323 and a PSTN, but it also can provide an interface between H.320, V.70, H.324 and other speech terminals. H.323 uses CODECs to convert between circuit-switched and packet formats, and works with the gatekeeper through RAS protocols to route signals from voice and fax through the network.

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