Gatekeeper Discovery; Address Translation; Admission Control - HP FlexNetwork MSR Series Configuration Manual

Figure 66
shows a simple H.323 network. For all calls, a gatekeeper is the call service control and
central control unit in its administrative zone.
Gateway entities are usually deployed on routers. Configure the IP voice gateway function on the
router at CLI. They interact with the gatekeeper by sending H.225.0 RAS messages.
In the current implementation, the gatekeeper is usually deployed on SUN stations or servers while
gateways are deployed on routers. For the reliability's sake, a backup gateway is configured. In the
event communication with a primary gatekeeper becomes abnormal (for example, timeout occurs) or
the primary gatekeeper becomes unavailable, gateways can turn to the backup gatekeeper for
registration.
Figure 66 H.323 network
Telephone
H.323 fundamentals
This section provides a brief description of how an H.323 network functions.

Gatekeeper discovery

When an endpoint wants to establish a call with another endpoint, it first looks for a gatekeeper with
which it can register for services. This process is called gatekeeper discovery. An RAS signaling
message exchange occurs between endpoints and the gatekeeper. The calling endpoint unicasts or
broadcasts a Gatekeeper_Request (GRQ) message. After receiving the request, the gatekeeper
replies with a Gatekeeper_Confirm (GCF) or Gatekeeper_Reject (GRJ) message.
Registration
When the calling endpoint receives a GCF message from the gatekeeper, it sends a
Registration_Request (RRQ) message, requesting to join the zone governed by the gatekeeper. The
gatekeeper replies with a Registration_Confirm (RCF) or Registration_Reject (RRJ) message to
accept or reject the registration request.
After registration, either the endpoint or the gatekeeper sends an Unregister_Request (URQ)
message. However, it is up to the gatekeeper to determine whether to cancel registration, while the
endpoint can only reply with an Unregister_Confirm (UCF) message to cancel registration.

Address translation

If the calling endpoint knows the alias, but not the call signaling address of the called endpoint, it
sends a Location_Request (LRQ) message to the gatekeeper.

Admission control

If the address of the called endpoint is available, the calling endpoint sends an Admission_Request
(ARQ) message, based on which the gatekeeper decides whether to allow this endpoint to join a call
IP network
Gateway
Gatekeeper
Gateway
218
Telephone