Tandberg Data N3 - Manual (Page 17)

11
3.5 Call Control
When an end-point wants to call another endpoint it presents the address it wants to call to
the Gatekeeper using a protocol knows as RAS. The Gatekeeper tries to resolve this address
and supplies the calling endpoint with information about the called endpoint. The destination
address can take several forms: IP address, H.323 ID, E.164 alias or a full H.323 URI.
Dialing by IP address is necessary when the destination endpoint is not registered with a
Gatekeeper or Border Controller. If it is registered, then one of the other addressing schemes
should be used instead as they are more flexible.
When an H.323 ID or E.164 alias is used, the Gatekeeper looks for a match between the
dialed address and the aliases registered by its endpoints. If no match is found, it may query
other Gatekeepers and Border Controllers.
When dialing by H.323 URI, the destination address resembles an email address. The
Gatekeeper first follows the procedure for matching H.323 IDs. If that fails it looks for a
Gatekeeper or Border Controller responsible for the domain (the part of the URI following the
@) and queries that device.
NOTES
ARQ, Admission Request. An endpoint request to make or answer a call
LRQ, Location Request. A query between Gatekeepers or Border Controllers to determine the
location of an endpoint.
RAS, Registration, Admission and Status Protocol. Used by endpoints and Gatekeepers to
communicate.
The Figures 1 and 2 illustrate the process the Gatekeeper performs when receiving call
requests.