H.323 Gatekeeper

A H.323 Gatekeeper serves the purpose of Call Admission Control and translation services from E.164 IDs (commonly a phone number) to IP addresses in an H.323 telephony network. Gatekeepers can be combined with a gateway function to proxy H.323 calls and are sometimes referred to as Session Border Controllers. A gatekeeper can also deny access or limit the number of simultaneous connections to prevent network congestion.
H.323 endpoints are not required to register with a gatekeeper to be able to place point to point calls, but they are essential for any serious H.323 network to control call prefix routing and link capacities among other functions.
A typical H.323 Gatekeeper call flow for a successful call may look like:-
 | | | |
 Endpoint A Endpoint B
 1234 1123
The gatekeeper allows calls to be placed either:
Directly between endpoints (Direct Endpoint Model), or
Route the call signaling through itself (Gatekeeper Routed Model).