Figure 2.1 Scope and components defined in H.323
H.323 defines a number of functional / logical components as shown in Figure 2.1:
- 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 P.14
[IP Telephony Cookbook] / Technological Background
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.
{ 2.2.1.2 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.
Audio
Conference
Gatekeeper
Data Applications
Video
Control