Introduction to H.323

What is this all about?

H.323 is a protocol that defines how Internet telephony devices can communicate with one another. Another protocol to accomplish basically the same would be SIP. Devices have to speak the same protocol to be able to talk to each other.

Internet telephony (also called Voice-over-IP, short VOIP) needs a number of different types of devices.

One device type is the "phone", called endpoint in H.323. To connect different types of "phone lines" you need a gateway (eg. to place a call from the internet into the traditional phone system).

To coordinate the different types of devices H.323 uses a so called gatekeeper. It provides many features found in a PBX in traditional phone systems. The GNU Gatekeeper is one implementation of a H.323 gatekeeper. Because it speaks the H.323 protocol, it can talk to all (most) other H.323 devices (commercial or free).

To connect more than two endpoints, eg. for conferencing, you need a multipoint control unit (MCU). The MCU will mix the media streams (audio and video) and send them to all participants.

What does a Gatekeeper do ?

A H.323 gatekeeper controls the H.323 endpoints and it's most important function is address translation between symbolic alias addresses and IP addresses. This way you can call "jan" instead of knowing which IP address he currently works uses.

Here is a list of gatekeeper functions:

  • Address Translation
  • Admissions Control
  • Bandwidth Control
  • Zone Management
  • Call Control Signaling
  • Call Authorization
  • Call Management

In addition to these function the GNU Gatekeeper does some things that are not required of a gatekeeper, but fit in very well.

  • Proxy / NAT Traversal (tunnel H.323 calls through a firewall or NATed network)
  • Local Call Routing (ACD and call-center functions)

More Information

Video Lectures: Introduction to H.323

H.323 Primer at

Last updated: 04. May 2017