An H.323 Gatekeeper serves the purpose of Call Admission Control

Call Admission Control

Call Admission Control prevents oversubscription of VoIP networks. It is used in the call set-up phase and applies to real-time media traffic as opposed to data traffic...

 and translation services from E.164

E.164

E.164 is an ITU-T recommendation that defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers. E.164 numbers can have a maximum of fifteen digits and are usually written with a + prefix...

 IDs (commonly a phone number) to IP addresses in an H.323

H.323

H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

 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, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...

.

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 H323 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 (Admission Request) to the Gatekeeper.
3. Gatekeeper returns ACF (Admission Confirmation) 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 (Disconnect Request) to the Gatekeeper.
10. 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).

See also

• GNU Gatekeeper (GnuGK)
  GNU Gatekeeper
  The GNU Gatekeeper is an open-sourced project that implements an H.323 gatekeeper based on the OpenH323 or H323Plus stack...
  
  
• Cisco IOS
  Cisco IOS
  Cisco IOS is the software used on the vast majority of Cisco Systems routers and current Cisco network switches...
  
   release version 11.3(2)NA and 12.0(3)T
• Cisco Multimedia Conference Manager (MCM)
• Emblaze-VCON Media Exchange Manager (MXM)
• TANDBERG Video Communication Server (VCS)