An H.323 Gatekeeper serves the purpose of

Call Admission Control Call Admission Control (CAC) prevents oversubscription of VoIP networks. CAC is used in the call set-up phase and applies to real-time media traffic as opposed to data traffic. CAC mechanisms complement and are distinct from the capabilities of qua ...

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 Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...

. 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 # Endpoint A dials 1123 from the system. # Endpoint A sends ARQ (Admission Request) to the Gatekeeper. # Gatekeeper returns ACF (Admission Confirmation) with IP address of endpoint B. # Endpoint A sends Q.931 call setup messages to endpoint B. # Endpoint B sends the Gatekeeper an ARQ, asking if it can answer call. # Gatekeeper returns an ACF with IP address of endpoint A. # Endpoint B answers and sends Q.931 call setup messages to endpoint A. # IRR sent to Gatekeeper from both endpoints. # Either endpoint disconnects the call by sending a DRQ (Disconnect Request) to the Gatekeeper. # Gatekeeper sends a DCF (Disconnect Confirmation) to both endpoints. The gatekeeper allows calls to be placed either: Directly between endpoints (Direct Endpoint Model), or Route the call signaling through itself (Gatekeeper Routed Model).

Sources

Cisco Technotes: Understanding H.323 Gatekeepers

Microsoft TechNet: H.323 Gatekeeper

Packetizer: A Primer on the H.323 Series Standard
ITU-T recommendations Videotelephony {{Network-software-stub

See also

Sources