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Global Call IP Technology Guide — November 2007 37
Dialogic Corporation
IP Overview
A gateway provides a bridge between different technologies; for example, an H.323 gateway (or IP
gateway) provides a bridge between an IP network and the PSTN. Figure 3 shows a configuration
that uses a gateway. User A is at a terminal, while user B is by a phone connected to the PSTN.
Figure 3. Basic H.323 Network with a Gateway
Figure 3 also shows a gatekeeper. The gatekeeper provides network services such as Registration,
Admission, and Status (RAS) and address mapping. When a gatekeeper is present, all endpoints
managed by the gatekeeper must register with the gatekeeper at startup. The gatekeeper tracks
which endpoints are accepting calls. The gatekeeper can perform other functions also, such as
redirecting calls. For example, if a user does not answer the phone, the gatekeeper may redirect the
call to an answering machine.
The call scenario in this example involves the following phases:
• Establishing Contact with the Gatekeeper
• Requesting Permission to Call
• Call Signaling and Data Exchange
• Call Termination
Establishing Contact with the Gatekeeper
The user at endpoint A attempts to locate a gatekeeper by sending out a Gatekeeper Request (GRQ)
message and waiting for a response. When it receives a Gatekeeper Confirm (GCF) message, the
endpoint registers with the gatekeeper by sending the Registration Request (RRQ) message and
waiting for a Registration Confirm (RCF) message. If more than one gatekeeper responds, endpoint
A chooses only one of the responding gatekeepers. The next phase of the call scenario, requesting
permission to call, can now begin.
Requesting Permission to Call
After registering with the gatekeeper, endpoint A must request permission from the gatekeeper to
initiate the call. To do this, endpoint A sends an Admission Request (ARQ) message to the
gatekeeper. This message includes information such as:
• a sequence number
• a gatekeeper assigned identifier
PSTN
Internet or
Intranet
Gateway
Gatekeeper
Terminal
Phone
User A
User B