H.323 is important because it is designed for use in existing LANs allowing users to implement multimedia applications without having to overhaul their network infrastructure. In addition, H.323 allows devices or applications from any vendor to communicate with each other. Because of its power and ease of implementation, H.323 is the most widely used protocol for supplying multimedia content across packet-based networks. Though H.323 is made up of several different protocols, it has four major components:

1. Terminal
2. Gatekeeper
3. Gateway
4. Multipoint Control Unit (MCU)

Terminal
H.323 terminals are the endpoints on a network that provide real-time, full-duplex communication. A terminal can be any device that can implement H.323 communications, from a stand-alone IP phone to a computer running software with H.323 functionality. Terminals are the end points of H.323 communication and therefore must support call control signaling, RTP/RTCP protocols, audio codecs, and control channel signaling. Support for video communications is optional.

Gatekeeper
The H.323 gatekeeper is an optional entity in the H.323 component group. If a gatekeeper is present, then all H.323 devices must register with the gatekeeper before they can communicate with other H.323 devices using a UDP based signaling called RAS (Registration, Admission, Status), which is part of the H.225 specification. The gatekeeper is logically separate from the H.323 endpoints, allowing its functions to be co-located in the gateway or MCU (see below). The gatekeeper performs several functions:

• Address translation
• Admission control
• Bandwidth control
• Zone management
• Call control signaling (optional)
• Call authorization (optional)
• Bandwidth management (optional)
• Call management (optional)

A gatekeeper is very useful in environments that have a complicated network configuration, need some type of security, or have issues with bandwidth management. However, the functions of the gatekeeper are limited to H.323 transmissions. If the network is having bandwidth issues H.323 cannot fix them, but it can monitor and limit itself to keep from adding to the problem.

Gateway
The H.323 gateway provides many services, with the most significant being the translation between H.323 conferencing endpoints and other types of terminals. For example, this function allows callers using IP phones to communicate outside the H.323 environment with traditional public switched telephone network (PSTN) phones.

Multipoint Control Unit (MCU)
MCUs support centralized conferencing capabilities between three or more endpoints. The MCU handles negotiations between all terminals to determine common capabilities. The MCU function can be implemented in a stand-alone device or included in any other H.323 component. A MCU has two main parts: the multipoint controller (MC), which handles call control functions, and the multipoint processor (MP) that handles the processing of audio, video, and data transmissions.

H.323 is a monolithic protocol, which means it contains everything it needs to function. In some cases this is good. When developing a device or application, everything you need to make it work is included and well documented. On the other hand, monolithic protocols are not very flexible because you do not have the ability to modify them to the needs of the application – they are what they are until the specification is updated, which means they are slow to adapt to new technologies.