Protocols: H323

H.323 has continuously evolved towards becoming a technically sound and functionally rich protocol platform for IP Telephony applications.
New features help facilitate more scalable Gateway and MCU solutions to meet the growing market requirements.
H.323 has been the undisputed leader in voice, video, and data conferencing on packet networks, and Version 4 endeavours to keep H.323 ahead of the competition.
The scope of H.323 encompasses multimedia communication in IP-based networks, with significant consideration given to gatewaying to circuit-switched networks (in particular to ISDN-based video telephony and to PSTN/ISDN/GSM for voice communication).

H.323 defines a number of functional / logical components:

- Terminal
Terminals are H.323-capable endpoints, which may be implemented in software on workstations or as stand-alone devices (such as telephones).
They are assigned to one or more aliases (e.g. a user's name/URI) and/or telephone number(s);

- Gateway
Gateways interconnect H.323 entities (such as endpoints, MCUs, or other gateways) to other network/protocol environments (such as the telephone network).
They are also assigned one or more aliases and/or telephone number(s).
The H.323 Series of Recommendations provides detailed specifications for interfacing H.323 to H.320, ISDN/PSTN, and ATM-based networks.
Recent work also addresses control and media gateway specifications for telephony trunking networks such as SS7/ISUP;

- Gatekeeper
The gatekeeper is the core management entity in an H.323 environment.
It is, among other things, responsible for access control, address resolution and H.323 network (load) management and provides the central hook to implement any kind of utilisation / access policies.
An H.323 environment is subdivided into zones (which may, but need not be congruent with the underlying network topology); each zone is controlled by one primary gatekeeper (with optional backup gatekeepers).
Gatekeepers may also provide added value, e.g., act as a conferencing bridge or offer supplementary call services.
An H.323 Gatekeeper can also be equipped with the proxy feature. Such a feature enables the routing through the gatekeeper of the RTP traffic (audio and video) and the T.120 traffic (data), so no traffic is directly exchanged between endpoints.
(It could be considered a kind of IP-to-IP gateway that can be used for security and QoS purposes);

- Multipoint Controller (MC)
A Multipoint Controller is a logical entity that interconnects the call signalling and conference control channels of two or more H.323 entities in a star topology. MCs coordinate the (control aspects of) media exchange between all entities involved in a conference.
They also provide the endpoints with participant lists, exercise floor control, etc. MCs may be embedded in any H.323 entity (terminals, gateways gatekeepers) or implemented as stand-alone entities.
They can be cascaded to allow conferences spanning multiple MCs;

- Multipoint Processor (MP)
For multipoint conferences with H.323, an optional Multipoint Processor may be used that receives media streams from the individual endpoints, combines them through some mixing/switching technique, and transmits the resulting media streams back to the endpoints;

- Multipoint Control Unit (MCU)
In the H.323 world, an MCU is simply a combination of an MC and an MP in a single device.
The term originates in the ISDN videoconferencing world where MCUs were needed to create multipoint conferences out of a set of point-to-point connections.


Signalling protocols
H.323 resides on top of the basic Internet Protocols (IP, IP Multicast,TCP, and UDP) in a similar way as the IETF protocols discussed in the next subsection, and can make use of integrated and differentiated services along with resource reservation protocols.

Components communicate using three control protocols:

Audio
Video
Gatekeeper Data Applications Conference
Control
T.120 Relaiable MC H.245 H.225.0 RAS RTP/RTCP RSVP
TCP + RFC 1006 UDP
IP / IP Multicast
Intergrated / Differentiated Services Forwarding

Optimisations: H.225.0 supports the following enhancements:

- Multiple Calls
To prevent using a dedicated TCP connection for each call, gateways can be built to handle multiple calls on each connection.

- Maintain Connection
Similar to Multiple Calls, this enhancement will reduce the need to open new TCP connections.
After the last call has ended, the endpoint may decide to maintain the TCP connection to provide a better call setup time for the next call.

- H.245 Conference Control-
The conference control channel is used to establish and control two-party calls (as well as multiparty conferences).
Its functionality includes determining possible modes for media exchange (e.g., select media encoding formats that both parties understand) and configuring
actual media streams (including exchanging transport addresses to send media streams to and receive them from).

An H.323 endpoint usually registers with a gatekeeper that provides basic services like address resolution for calling the other endpoints.
H.323 defines and utilises several address types.The one most commonly used and derived from the PSTN world is the dialled digit address, which is defined as a number dialled by the endpoint.
Unlike SIP addresses, an H.323 address can only be registered by one endpoint (per zone), so a call to that address only resolves to a single endpoint.To call multiple destinations simultaneously in H.323 requires a gatekeeper that actively maps a single address to multiple different addresses and tries to contact them in sequence.

Communication phases

In a H.323, communication may be identified in five different phases:
- Call set up;
- Initial communication and capability exchange;
- Establishment of audiovisual communication;
- Call services;
- Call termination.

Additional (call) services

- Conferencing
H.323 inherently supports multipoint tightly-coupled conferencing.

- Broadcast conferencing;

Security

- Authentication
Authentication can be achieved by using a shared secret (password) or digital signatures.
- Message Integrity
Integrity is achieved by generating password-based checks on the message;
Privacy Mechanisms are provided to setup encryption on the media streams.

In summary, it can be said that the H.323 family of standards provides a mature basis for commercial products in the field of IP Telephony.While the details of the protocol are often dominated by their legacy from various earlier ITU protocols, there is an active effort to profile and simplify the protocol to reduce the complexity.