Gatekeeper Flow Diagrams
The RAS portion of the H.225 protocol is defined by requests and responses that
follow similar formats. Requests always end in the letters “RQ” which indicate
request. Responses always end in “CF” which indicate confirmation, or “RJ” which
indicate rejection.The letter or letters preceding these indicate the actual subject.
Thus “RRQ” indicates registration request,“LCF” indicates location confirmation, and
so on.
The process of gatekeeper discovery, registration, and call signaling for IP
phones using CallManager is shown in Figure 5.1.
Both endpoints (in this case, CallManagers) discover and register with their
gatekeeper (Steps 1 to 4).When Phone 1 places a call, its CallManager sends an
admission request to its gatekeeper to determine if it may place the call (Steps 5 to
6).The CallManager will usually send a bandwidth request to specify the bandwidth
required for the call. Gatekeeper 1 uses a location request to locate the endpoint
for the call (Steps 7 to 8).The CallManager receiving the call (on behalf of
Phone 2) sends an admission request to its gatekeeper to determine if it may
receive the call (Steps 9 to 10). Once the placing and receiving of the call has been
approved, actual call setup does not involve the gatekeepers (Steps 11 to 12).
As discussed earlier, the gatekeeper can reject a call based on many factors,
such as available bandwidth. Figure 5.2, Gatekeeper Call Rejection, displays an
example of this.