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VoIP services need to be able to connect to traditional circuit-switched voice networks.

The H.323 standard components are: H.323 terminals that are endpoints on a LAN, gateways that interface between the LAN and switched circuit network, a gatekeeper that performs admission control functions and other chores, and the MCU (Multipoint Control Unit) that offers conferences between three or more endpoints.

H.323 terminals are LAN-based end points for voice transmission. Common examples are a computer running Microsoft NetMeeting and an Ethernet-enabled telephone. All H.323 terminals support real-time, 2-way communications with other H.323 entities.

The gateway serves as the interface between the H.323 and non-H.323 network. On one side, it connects to the traditional voice world, and on another side to packet-based devices.

The gatekeeper is not a required device in an H.323 network, but if present, it must perform a set of functions. Gatekeepers provide address translation (routing) for devices in their zone. This could be, for instance, the translation between internal and external numbering systems. Another important function for gatekeepers is providing admission control, specifying what devices can call what numbers.

MCU's allow for conferencing functions between multiple terminals. They contain two parts:

A multipoint controller (MC) that handles the signalling and control messages necessary to setup and manage conferences, and a multipoint processor (MP) that accepts streams from endpoints, replicates them and forwards them to the correct participating endpoints.

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