H.323 is a protocol for transmission of real-time audio, video and data information over packet switching-based networks. Such networks include IP-based (including the Internet), Internet packet exchange-based local area networks, enterprise networks and metropolitan and wide area networks. H.323 can also be applied to multipoint-multimedia communications. The technology provides a vast array of services which means it can be used in consumer, business and entertainment applications. H.323 is an essential element in ensuring that the compatibility of the mobile multimedia applications and services will be introduced with the implementation of third generation wireless technologies. The H.323 standard was specified within the ITU-T organization by Study Group 16. The original standard was realized in 1996 and further enhancements, up to Version 3, have been developed in the intervening years.

H.323 uses the procedures about logical channels signalling of H.245 standard, in which the content of each channel is defined when it is opened. These procedures are provided to fix the benefits of the caller and receiver, the call establishment, the exchange of information, call proceeding and how the call is codified and decodified. For example, when a telephone call is originated on Internet, both terminals must negotiate which one take the control, so that only one of them can send the special control messages. An important point is that the capabilities of the systems are due to determine, so that the information do not be transmitted if they cannot be managed by the receiver.

This standard defines a great number of characteristics and functions. Some are necessaries and other optional ones. But H.323 defines much more that functions. The standard defines also the following components:

· Terminal
· GateWay
· Gatekeeper
· MCU (Multipoint Control Unit)
· Multipoint Controller
· Multipoint Processor
· H.323 Proxy

1. Terminal

An H.323 terminal is an endpoint in the LAN that participates in real-time, two-way communications with another H.323 terminal, gateway, or multipoint control unit (MCU). A terminal must support audio communication and can also support audio with video, audio with data, or a combination of all three.

H.323 terminal options include support user equipment interfaces, audio and video CODECs, the T.120 data protocols, MCU capabilities and an H.225 component called RAS (Registration/Admission/Status) which is a protocol used to communicate with a Gatekeeper.

a. User equipment interfaces: The set of cameras, screens, microphones, speakers and data applications with their respective interfaces.

b. Audio Codec: Each terminal will have an audio codec, to codify and to decodify vocal signals (G.711), and they could transmit and receive a-law and μ-law. Optionally, a terminal, could codify and decodify vocal signals. The H.323 terminal can, also optionally, send more of one audio channel at the same time, for example, in order to transmit 2 different languages

c. Video Codec: In H.323 terminals is optional.

d. Data Channel: One or more data channels are optional. They could be in one direction or in both.

e. Reception Path latency: It includes the latency added to the packets to maintain the synchronization, and to consider the fluctuation in the packet arrivals. Usually it is not used in transmission but in the reception, to add the necessary latency, for example, to obtain the synchronization between the movement of the lips and the audio signal in a videoconference.

f. System Control Unit: Gives the terminal signaling. There are three different functions: H.245 Control function, H.225 call signalling function and RAS signaling function.

· H.245 Control function: The H.245 logical channel is used to take messages end-to-end of the H.323 protocol. It takes care to negotiate the capacities (bandwidth), to set up and set down the logical channels and to send the flow control messages. In each call, there will be any number of logical channels of each type (audio, video or data) but only one control logical channel, channel 0.

· H.225 Call signalling function: It uses one logical channel of signaling to send and receive establishment and end session messages between two H.323 end points. The channel is independent of the H.245 control channel. The H.245 logical channel opening and closing procedures are not used to establish the signaling channel. It is opened before the establishment of the H.245 control channel and any other logical channel. It can settle down terminal to terminal or terminal to gatekeeper.

· RAS Control function (Register, Admission, Status): It uses a logical channel of RAS signaling to carry out registry, admission, situation and bandwidth changes procedures between end points (terminal, gateway..) and gatekeeper. It is only used when a gatekeeper is present. The RAS signaling channel is independent of the call signaling channel, and of the H.245 control channel. The H.245 logical channel opening and closing procedures are not used to establish the RAS signaling channel. The RAS control function channel is opened before the establishment of the any other logical channel

g. H.225 Layer: It is in charge to give format to the video, audio, and transmitted data packets. In addition, it also takes care of the packet alignment, the sequential number and the error detection.

h. Packet network interface: It is specific of each implementation. This must provide the services described in the H.225 recommendation. That means that the reliable end-to-end service (for example, TCP) is mandatory for the H.245 control channel, the data channels and call signaling channels.

The non reliable end-to-end service (UDP, IPX) is mandatory for the audio channels, the video channels and the RAS channel These services can be duplex or simplex and unicast or multicast depending on the application, the terminals capacities and the network configuration.

2. Gateway
These devices provide many services, including the needed translation function between H.323 conferencing endpoints on the LAN and other ITU-compliant terminals on other ITU-compliant circuit-switched and packet-switched networks. These services include translation between transmission formats (for example H.225.0 to H.221) and between communications procedures (for example H.245 to H.242). In addition, the Gateway also does the translation between audio and video CODECs if this is needed as well as performs call setup and clearing.

3. Gatekeeper
Gatekeepers perform two important call control housekeeping chores which help preserve the integrity of the corporate data network. The first is address translation services between LAN aliases for terminals and gateways and IP or IPX addresses. The second Gatekeeper function is bandwidth management. For instance, if a LAN manager-specified threshold for the number of simultaneous conferences on the LAN has been reached, the Gatekeeper can refuse to make any more connections. The effect is to limit the total number of conferencing bits/s to some fraction of the total available; the remainder is left for email, file transfers, and other normal store-and-forward data functions. The collection of all Terminals, Gateways, and Multipoint Control Units managed by a single gatekeeper is known as an H.323 Zone.

A Gatekeeper make the following services if is present (it is not always necessary):

· Admission control: The gatekeeper can deny calls from a terminal because they do not have permission or they do not have permission at that moment of the day or some other criteria. Admissions Control may also be a null function which admits all requests.
· Bandwidth control: To control the number of H.323 terminals which can be simultaneously connected and to deny calls if the bandwidth is too low.
· Zone Management: The Gatekeeper provides the above functions for terminals, MCUs, and Gateways which have registered within its Zone of control.

4. MCU
An MCU is an endpoint on the LAN that provides the capability for three or more terminals and gateways to participate in a multipoint conference. It controls and mixes video, audio, and data from terminals to create a robust video conference. An MCU can also connect two terminals in a point-to-point conference that can later develop into a multipoint conference.

5. MULTIPOINT CONTROLLER
A multipoint controller is a component of H.323 that provides negotiation capacity with all the terminals to carry out different communication levels. Also, it can control conference resources such as video multicasting.

6. MULTIPOINT PROCESSOR
A multipoint processor is a hardware and specialized software component of H.323. It mixes, exchanges and processes audio, video and/or data flow for the participants of a multipoint conference, in such a way that the terminal processors are not used heavily. The multipoint processor can process an only average flow or multiple average flows depending on the supported conference.

7. PROXY H.323
An H.323 proxy server is a proxy specifically designed for the H.323 protocol that examines packets between two communicating applications. Proxies are able to determine the destination of a call and perform call-connection steps, if necessary.

H.323 proxies perform the following key functions:
· Voice terminals that do not support Resource Reservation Protocol (RSVP) can connect through remote access or local area networks (LANs) with relatively reliable quality of service (QoS) to the proxy. Pairs of proxies can then be employed to develop tunnels across the IP network.
· Proxies support routing H.323 traffic separately from ordinary data traffic using application-specific routing (ASR).
· Proxies are compatible with network address translation functions in gateways or gatekeepers, enabling H.323 to be deployed in networks using private address space.