H323 Gatekeeper And Multiple Endpoint Server

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We have seen that SIP is a signaling protocol used for VoIP communication. Over the Internet are many more then one debates about H323 vs. SIP, that may lead to the conclusion that H323 is pretty much the same thing as SIP. Often H323 is associated only with the call signaling part of a VoIP transmission. Actually it’s a bit more complicated then this.

H323 is composed from an entire suite of protocols. The exact definition of H323 states that H323 is a recommendation published by the International Telecommunications Union (ITU) defining the internetworking of elements and protocols with the purpose of multimedia communications over an unreliable packet-based network.


Elements of an H323 system


A H323 Terminal can be any of a telephone, videophone, IVR device or other kind of endpoint inside a network, which can communicate with another endpoint or with a Multipoint Control Unit or with a Gateway using various types of media ranging from audio to video or other data.

Multipoint Control Units

Yate acting like a multiple endpoint server resembles such an element. A Multipoint Control Unit provides services needed by more endpoints to take part in a conference call. This is done through a Multipoint Controller that manages the call signaling. Optional Multipoint Processors can be present to handle the various media types exchange and processing during a conference.


The main purpose of the gateway is to make possible the protocol conversion between the endpoints that support H323 and the endpoints that use other protocols for VoIP communication including interfacing to PSTN. Typically a H323 gateway is formed from a Media Gateway for media handling and a Media Gateway Controller for call signaling and other functions.


Like the way it sounds a gatekeeper's use is actually to “guard” a network area. Yate acts like a H323 Gatekeeper providing basic admission control by permitting or denying communication between different endpoints in its zone of control. It also provides an address resolution service. A gatekeeper manages calls in the way that permits them to be placed directly between terminals or routes the signaling through it to perform different kinds of functions like follow me, find me, forward on busy and others. There are also gatekeepers that integrate proxy solutions.

H323 protocols

The protocols that H323 unites under its specifications vary from G.7xx codecs to various others descriptions. The most important protocols comprised under the H323 specifications are:


H225 protocol which in fact parts itself in three specifications more exactly RAS – Registration, Admission and Status, Call Signaling and Text Telephony. This has a direct relation with a gatekeeper – component which Yate is used as, more exactly specifies the modes by which an endpoint can register through the gatekeeper and be allowed to pass it to reach the network.


H245 is multimedia control protocol used between endpoints to establish the channels for media transmission.


RTP and RTPC are the protocols concerned with the media transport.


Q931 is the call signaling protocol for ISDN networks. The information included in the H225 data which also like stated is concerned with call signaling is used to complete the info carried by Q931 messages which is missing certain important things for a H323 transmission like the IP address.

See also

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