H.323 is a standard H., more particularly to a protocol of the H.32x series, which also contains the communication via the public telephone network and ISDN.

It is an umbrella recommendation of ITU -T, are defined in the protocols, which allow an audio-visual communication on any network which transmits packets. It is currently implemented in various applications such as NetMeeting and OpenH323.


One strength of H.323 was the relatively early availability as a set of standards that define not only the basic call model, but in addition also the performance which are needed to meet the expectations for business communication needs. H.323 was the first IP telephony standard, which has the IETF RTP Standard adapted to transport audio and video over IP networks.

Other protocols for IP telephony are the widely used SIP, Jingle and IAX Skype.

H.323 is based on the ISDN Q.931 protocol and is therefore well suited for interworking scenarios between IP and ISDN, or between IP and QSIG. A call model, similar to the ISDN call model, eases the introduction of IP telephony into existing networks, which consist of ISDN-based PBXs. A smooth migration towards IP -based telephone systems is thus predictable.

Concerning the context of H.323 has a gatekeeper with additional features (see below).

H.323 is based on a host of other ITU-T protocols, for example:

  • H.225.0 call signaling describes the media ( audio and video), the conversion of the data stream into packets, synchronization of the data stream and control of the message format.
  • H.245 describes the messages and procedures for the opening and closing of logical channels which are used for transmission of audio, video and data; and the exchange, the control and display of the transmission capacity.
  • The H.450 standard defines additional telephony features to map, for example, the features of ISDN to IP.
  • H.235 is responsible for security and authentication.


A gatekeeper is a device that takes over the main gateway functionality between IP network and the telephone network in an IP telephony installation. It is defined in the H.323 standard framework interface functions, and is mainly used for emulation of the PSTN connection set-up on the IP network, and the adaptation of the data streams. To this end, he takes over the signaling, the necessary translation of phone numbers into IP addresses and vice versa, and later the packaging of the synchronous data stream from the PSTN into IP packets according to the H.225 standard. In addition, he is responsible for managing a zone which terminals, gateways, and multipoint control units includes.


Open Source

  • Ekiga (Linux, Unix, Windows)
  • OpenH323 ( platform independent )


  • NetMeeting (Windows 9X, 2000, XP )
  • Windows Phone Dialer (Windows 2000, XP)
  • IPB (Mac OS X)


  • LifeSize Softphone
  • Emblaze Vcon vPoint HD
  • Spranto (formerly Pacphone )
  • Polycom PVX

There are also several different client - server products for mobile use. The actual end point is a server in this case. This can be useful in business applications, since only the server needs a connection to the internet and the clients can remain within the organization, behind a firewall. Clients must only be connected to the intranet with the endpoint server. In addition, a part of the video processing are transferred from the server.

Examples of this are the following applications:

  • Tandberg Movi
  • Lotus Sametime
  • LifeSize UVC ClearSea

Free Software Gatekeeper

  • GNU Gatekeeper (Windows, Linux, Mac OS X)