H.323 Gatekeeper


An H.323 Gatekeeper serves the purpose of Call Admission Control and translation services from E.164 IDs 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
  1. Endpoint A dials 1123 from the system.
  2. Endpoint A sends ARQ to the Gatekeeper.
  3. Gatekeeper returns ACF with IP address of endpoint B.
  4. Endpoint A sends Q.931 call setup messages to endpoint B.
  5. Endpoint B sends the Gatekeeper an ARQ, asking if it can answer call.
  6. Gatekeeper returns an ACF with IP address of endpoint A.
  7. Endpoint B answers and sends Q.931 call setup messages to endpoint A.
  8. IRR sent to Gatekeeper from both endpoints.
  9. Either endpoint disconnects the call by sending a DRQ to the Gatekeeper.
  10. Gatekeeper sends a DCF to both endpoints.
The gatekeeper allows calls to be placed either:
Directly between endpoints, or
Route the call signaling through itself.