H.323 Control and Signaling Mechanisms & H.225.0 RAS

The flow of information in an H.323-enabled network consists of a mix of audio, video, data, and control packets. Control information is essential for call set-up and tear-down, capability exchange and negotiation, and administrative purposes. H.323 uses three control protocols: H.245 media control, H.225/Q.931 call signaling, and H.225.0 registration, admission, and status (RAS). The Q.931 protocol was originally developed for ISDN control signaling, and is currently used for inter-PBX networks implementing Qsig standards (see Networking chapter).

Figure 1 shows the H.323 protocol stacks for control and signaling processes.

Figure 1: H.323 system: protocol architecture.

The diagram follows the ISO OSI seven-layer model. H.323-specific protocols are above the transport layer (Layer 4). Real-Time Transport Protocol (RTP)/RTCP, RAS, H.225, and H.245 span across the fifth and sixth layers. Q.931, sometimes included as part of the H.225 protocol set, is at the terminal applications layer (Layer 7). Data communications are supported by multiple T.120 protocols and use TCP/IP trans- mission protocols, which are different from the H.323 protocols that support real-time audio and video communications requirements

H.225.0 RAS

H.225.0 RAS messages define communications between endpoints and a gatekeeper. H.225.0 RAS is only required when a gatekeeper exists. Unlike H.225.0 call signaling and H.245, H.225.0 RAS uses unreliable transport for delivery. RTP is used to guarantee delivery transport.

Gatekeeper Discovery

Gatekeeper discovery is used by endpoints to find their gatekeeper. An endpoint needing to find the transport address of its gatekeeper(s) will multicast a gatekeeper request (GRQ) message. One or more gatekeepers may reply with a GCF message containing the gatekeeper transport address.

Endpoint Registration

Once a gatekeeper exists, all endpoints must be registered with it. This is necessary because gatekeepers need to know the aliases and transport addresses of all endpoints in its zone to route calls.

Endpoint Location

Gatekeepers use this message to locate endpoints with a specific transport address. This process is required, for example, when the gatekeeper updates its alias transport address database.

Other Communications

A gatekeeper performs many other management and control duties such as admission control, status determination, and bandwidth management, which are all handled through H.225.0 RAS messages.

Figure 2 shows RAS. The terminal sends a request to the gatekeeper for registration and admission. The endpoints acknowledge and confirm the requests. When the call is completed, the terminal notifies the gatekeeper of the call status and receives confirmation that the request to disconnect has been received.

Figure 2: RAS.