H.323 Network Architecture

As is the case for most signaling systems, H.323 defines a specific network
architecture, which is depicted in Figure 8-3. This architecture involves
H.323 terminals, gateways, gatekeepers, and multipoint controller units
(MCUs). The overall objective of H.323 is to enable the exchange of media
streams between H.323 endpoints, where an H.323 endpoint is an H.323
terminal, a gateway, or an MCU.
An H.323 terminal is an endpoint that offers real-time communications
with other H.323 endpoints. It is typically an end-user communications
device. It supports at least one audio codec and may optionally support
other audio codecs and/or video codecs.
A gateway is an H.323 endpoint that provides translation services
between the H.323 network and another type of network, such as an Integrated
Services Digital Network (ISDN) or the Public Switched Telephone
Network (PSTN). One side of the gateway supports H.323 signaling and terminates
packet media according to the requirements of H.323. The other
side of the gateway interfaces to a circuit-switched network and supports the
transmission characteristics and signaling protocols of the circuit-switched
network. On the H.323 side, the gateway has the characteristics of an H.323
terminal. On the circuit-switched side, it has the characteristics of a node in
the circuit-switched network. A translation between the signaling protocols
and media formats of one side and those of the other side is performed internally
within the gateway. The translation is totally transparent to other
nodes in the circuit-switched network and in the H.323 network. Gateways
may also serve as a conduit for communications between H.323 terminals
that are not on the same network, where the communication between the
terminals needs to pass via an external network such as the PSTN.
A gatekeeper is an optional entity within an H.323 network. When present,
it controls a number of H.323 terminals, gateways, and multipoint controllers
(MCs). By control, we mean that it authorizes network access from
one or more endpoints and may choose to permit or deny any given call from
an endpoint within its control. It may offer bandwidth control services,
which, if used in conjunction with bandwidth and/or resource management
techniques, can help to ensure service quality. A gatekeeper also offers
address translation services, enabling the use of aliases within the network.
The set of terminals, gateways, and MCs controlled by a single gatekeeper
is known as a zone.A zone can span multiple networks or subnetworks and
it is not necessary that all entities within a zone be contiguous.
An MC is an H.323 endpoint that manages multipoint conferences
between three or more terminals and/or gateways. For such conferences, it
establishes the media that may be shared between entities by transmitting
a capability set to the various participants, and an MC may change the
capability set in the event that other endpoints join or leave the conference.
An MC may reside within a separate MCU or may be incorporated within
the same platform as a gateway, a gatekeeper, or an H.323 terminal.