H323 Architecture

H.323 is the ITU-T specification for audio and video communication across packet networks. It is actually a wrapper standard, encompassing several protocols, including H.225, H.245, and others. Each of these protocols has a specific role in the call setup process. All of them are binary protocols based on the ASN.1 standard, and all but one work on dynamic ports.

An H.323 network (see Figure 7-2) is usually made up of several endpoints known as terminals, a gateway, and possibly a gatekeeper, a backend service (BES), and a multipoint control unit (MCU).

The gateway serves as a bridge between the H.323 network and the outside world of non-H.323 devices. This includes both SIP and traditional PSTN networks. The gatekeeper is an optional, but widely used, component of a VoIP network. It is often one of the main components in H.323 architectures, providing address resolution and bandwidth control. If a gatekeeper is present, a backend service may also exist to maintain data about endpoints, including their permissions, services, and configuration. Finally, a multipoint control unit is another optional network element that facilitates multipoint conferencing and other communications between more than two endpoints.

H.323 backend service

H.323 backend service

H.323 terminal (handset)

H.323 terminal (softphone)

Multipoint control unit

Multipoint control unit

H.323 terminal (handset)

H.323 terminal (softphone)

1. H.225/H.245 signaling traffic and media stream with gatekeeper routing

2. Query to the BES and response

3. H.225/H.245 signaling traffic and media stream without gatekeeper routing

4. Communications to the outside world

5. Optional network element for multipoint communications and conferencing

Figure 7-2 H.323 architecture

Four different call models are defined in the H.323 standard:

• Gatekeeper routed call with gatekeeper routed H.245 signaling

• Gatekeeper routed call with direct H.245 signaling

• Direct routed call with gatekeeper

• Direct routed call without gatekeeper

Depending on the type of call, an H.323 VoIP session is initiated with an H.225 signal by either a TCP or a UDP connection. The address of the destination endpoint is obtained by negotiating with the gatekeeper through the Registration Admission Status (RAS) protocol. Then the Q.931 protocol (still within the realm of H.225 but based on fixed TCP port 1720) is used to establish the call itself and negotiate the addressing information for the H.245 signal. This setup next procedure is common throughout the H.323 progression where one protocol negotiates the configuration of the next protocol used. In this specific case, it is necessary because H.245 has no standard port assigned. While H.225 simply negotiates the establishment of a connection, H.245 defines the channels that will actually be used for media transfer, once again over TCP.

H.245 must establish several properties of the VoIP call, including the audio CoDecs that will be used and the logical channels for the transportation of media (namely RTP and RTCP ports). Overall, four connections must be established because the RTP/RTCP logical channels are only one direction. Each one-way pair must also be on adjacent ports. After H.245 has established all the properties of the VoIP call and the logical channels, the actual call can begin.

What was just described is a basic VoIP call setup process using the H.323 signaling standard. The H.323 suite has different protocols associated with more complex forms of communication, including:

• H.450.1, H.450.2, and H.450.3 (supplementary services)

• H.246 (interoperability with circuit-switched services)

H.323 also offers fast connect to set up a call using only one packet roundtrip. Finally, authentication may also be performed at each point in the process using symmetric keys or some preshared secret. Of course, the use of these extra protocols and/or security measures adds to the complexity of the H.323 call setup process, making interoperation with firewalls and NAT even more difficult.

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