from Wikipedia, the free encyclopedia

H.323 is an H. standard , more precisely a protocol of the H.32X series, which also includes communication via public telephone networks and ISDN .

It is an overarching recommendation by the ITU-T that defines protocols that enable audio-visual communication on any network that carries packets. It is currently implemented in various applications such as NetMeeting and OpenH323 .


H.323 protocol stack

One of the strengths of H.323 was its relatively early availability as a set of standards that not only define the basic call model, but also the features that are required to meet expectations for business communications. H.323 was the first IP telephony standard to adapt the IETF standard RTP to transport audio and video over IP networks.

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

H.323 is based on the ISDN protocol Q.931 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, facilitates the introduction of IP telephony into existing networks consisting of ISDN-based telecommunications systems. A smooth migration towards IP-based telephone systems can thus be planned.

The context of H.323 includes a gatekeeper with additional performance features ( see below ).

H.323 is based on many other ITU-T protocols, for example:

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


A gatekeeper is a device that assumes essential gateway functions between the IP network and the telephone network in an IP telephony installation. It implements the interface functions defined in the H.323 framework standard and mainly serves to emulate the PSTN connection setup via the IP network and to adapt the data streams. To do this, it takes on the signaling, the necessary translation of telephone numbers into IP addresses and vice versa, and later the packaging of the synchronous data stream from the telephone network into IP packets according to the H.225 standard. He is also responsible for the management of a zone, which contains terminals, gateways and multipoint control units .


Open source


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


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


Free software gatekeeper

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

Web links