H.323 is an international ITU standard for voice, data and video communication over packet-oriented networks that specifies the specific capabilities of end devices in the IP environment. H.323, which is functionally comparable to the Session Initiation Protocol( SIP), was developed for the transmission of multimedia applications and forms the basis for VoIP and video-over-IP.
The H.323 standard is used to define real-time communication in LANs that cannot provide guaranteed quality of service. It consists of several protocols for signaling, exchange of terminal functionality, connection control, exchange of status information and data flow control. The standard has been revised several times and in its third version defines the transmission of features.
The standard is derived from the H.320 multimedia standard for ISDN. It has defined network transitions between ISDN and ATM and guarantees interoperability among vendor products, with four main groups defined: H.323 gateways, gatekeepers, terminals and multipoint control units( MCU).