H.323 Discovery and Registration | Cisco VOIP

The five stages of an H.323 call and details of each of these connections are listed.

  1. Discovery and registration

  2. Call setup

  3. Call-signaling flows

  4. Media stream and media control flows

  5. Call termination

A lot happens within each of these stages; from the time the call is requested to the time it is terminated.

Device Discovery and Registration

The gatekeeper initiates a "discovery" process to determine the gatekeeper with which the endpoint must communicate, as shown in Figure 1. This discovery can be either a statically configured address or through multicast traffic. Once this is determined, the endpoint or gateway registers with the discovered gatekeeper.

Figure 1: H.323 Gatekeeper Call Control/Signaling: Discovery and Registration

Registration is used by the endpoints to identify a zone with which they can be associated (a zone is a collection of H.323 components managed by a single gatekeeper). H.323 can then inform the gatekeeper of the zones' transport address and alias address.

In Figure 1:

  1. A H.323 gateway (or terminal) sends a request to register (RRQ) message using H.225 RAS on the RAS channel to the gatekeeper.

  2. The gatekeeper confirms or denies the registration by sending a registration confirmation (RCF) or a "Reject registration" message back to the gateway.

Intra-zone Call Placement

Once the registration and discovery process is complete, we can place a call. Figure 2 shows Gateway X placing a intra-zone call to a terminal connected to Gateway Y, Gateway X sends an admission request (ARQ) message to the gatekeeper requesting permission to place a call to a phone number serviced by Gateway Y.

Figure 2: H.323 Gatekeeper Call Control/Signaling: Call Placement (Intra-zone)

In Figure 2:

  1. Gateway X sends an ARQ message using H.225 RAS to the gatekeeper.

  2. Gatekeeper requests direct call signaling by sending an admission confirmation (ACF) to Gateway X.

  3. H.323 call setup is initiated.

Inter-zone Call Placement

The process of placing an inter-zone call is somewhat more complicated and resource–intensive, as the network is larger and divided into multiple zones. In Figure 3, Gatekeeper A controls Zone A, and Gatekeeper B controls Zone B. Gateway X (or Terminal X) is registered with Gatekeeper A, and Gateway Y is registered with Gatekeeper B.

Figure 3: H.323 Gatekeeper Call Control/Signaling: Call Placement (Inter-zone)

To place a call to Gateway Y terminal, Gateway X first sends an ARQ message to the gatekeeper requesting permission to make the call. Since Gateway Y is not registered with the gatekeeper in Zone A, we assume that the gateways (terminals) are already registered.

Figure 3 shows five distinct phases in an inter-zone call placement.

  1. ARQ Gateway X requests a connection to Gateway Y from its local gatekeeper.

  2. Location request (LRQ) Local gatekeeper for Gateway X does not know the IP address of Gateway Y and is requesting the address from Gateway Y's local gatekeeper.

  3. Location confirm (LCF) Gateway Y's local gatekeeper responds to Gateway X's local gatekeeper with the IP address of Gateway Y.

  4. ACF The local gatekeeper responds to Gateway X's request and provides the remote IP address of Gateway Y.

  5. Call established The H.323 call is established between Gateway X and Gateway Y.

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