5. The H.323 signaling protocol

The three commonly used signaling protocols in VoIP are:

• H.323
• Session Initiation Protocol(SIP)
• Media Gateway Control Protocol(MGCP)

The H.323 Signaling protocol

H.323 is an ITU-T specification for transmitting audio, video and data across an IP network . It is implemented at layer 5 of the OSI reference model. It provides ideal telephone communication, support simultaneous voice and data transmission and can also transmit binary message that are encoded using basic encoding rules. It also relies o 2 additional signaling protocols, the H.225 and H.245 for call setup and management.

H.323 components include the following:

• Terminals
• Gateway
• Gate keeper
• Multipoint control Units.

Terminals

• It is an end point device such as IP telephone, softphone or a dedicated conferencing device.
• It is designed to support voice, data traffic and IP telephony.
• It mus H.245 for channel and negotiation, RAS(Registration Admission stations)
• Q931 is used for signalling and setup. It also provide setup and support RTP and RTCP on the stream of the media
• Terminals must support audio with video and data transmission.

Gateway

• It perform call setup and tear down of the session between two parties
• It also help to translate audio and video signals.
• Besides that, it provide RAS for registration .
• It reflects the characteristics of a switched circuit network(SCN) endpoint and H.323 endpoint.
• Provides an interface between IP telephony system and PSTN.
• Can be implemented on a gate keeper , a MCU or on a voice enabled router and switch.

Gatekeeper

• Provides pre-call and call level control services to H.323 endpoints.
• It is separated from other network elements in H.323 environment.
• If more than one gatekeeper is used, an intercommunication is accomplished in an unspecified manner.
• Manage all the registered terminals, gateways and MCUs in a single H.323 zone.
• Services such as addressing, authorization of H.323 components , bandwidth management , accounting and billing can be configured on the gatekeeper.
• It can be implemented on a dedicated server.

Multipoint Control Units(MCU)

• Support conferences between 3 or more endpoints in a multi point conference.
• Consists of a Multipoint controller(MC) and an optional Multipoint Processor(MP)
• The MC is responsible for H.245 functions while the optional MP handles the actual mixing of media streams to avoid bandwidth contention.
• It act as a bridge between centralized and decentralized enviornment.
• It receives audio, video and data and distribute them to endpoint participating in a multipoint conferences.
• Both MC and MP can resides on to a dedicated components , a gateway, terminals or gatekeeper. But it is recommended that MC be utilized on a gateway.

The 5 steps of signaling in H.323 is as followed:
1. Call setup
2. Initial communication capability
3. Audio/video communication establishment
4. Communication
5. Call termination.

At step 1, when user dial the number of the other party, the first set of signal that are exchange is the TCP connection. TCP-SYN, TCP-SYN-ACK and then the TCP-ACK are generated between two user. At this point the H.225.0 SETUP ON TCP signal informs the party that the connection can be set up on TCP. Next, the users can now request a certain bandwidth from it associated gateway server of the called party.

The requested bandwidth from the gateway granted the bandwidth if there are sufficient bandwidth, id not the call has to find another gatekeeper to register. This phase is handle by H.225 and H.245 protocols.

At step2, all the end point's communication are available over TCP. This phase is necessary as the type of communication service requested depends on the communication capabilities at both ends.

At Step3. it implements the establishment of a logical channel, which H.323 is unidirectional, so a logical channel must be establish in either direction in order to have a full duplex communication.But at the same time more band width is needed

Step 4 handles the communication between two user. This phases is control by using RTP over UDP. This step can be any kind of media flow depending on the size and type of the channel establish in step 4.

At step 5 , is the termination by either user and the termination is usually done by gatekeeper.

The following shows the signalling process of H.323:

H.323 Protocol suite

The H.323 protocol suite is based on several protocols. The protocol family support admission, call setup, status ,tear-down , media streams and message in H.323 systems. These protocols are supported by both reliable and unreliable packet delivery mechanism over data networks.

Although most of the H.323 implement TCP as the transport protocols for signalling, H.323 version 2 does not implement TCP but instead use UDP as the transport protocol.

The following shows the H.323 protocol suite

Fig 5 H.323 Protocol suite

This protocol suit can be divided into three main areas of control:

• Registration, Admission , and Status(RAS) Signalling
• Call control Signalling
• Media control and Transport