H.323 is an ITU (International Telecommunications Union) Telecommunication standard that describes packet-based video, audio, and data conferencing. H.323 is a standard that describes the architecture of calling systems and refers to a set of other related standards (H.245, H.225.0, and Q.931) to describe its actual protocol. H.323 is a standard from the ITU, which is a telephony group. Note that SIP (Session Initiation Protocol), which is a competing standard, was set by the IETF (Internet Engineering Task Force), which is an Internet-based group. SIP now has market momentum despite the fact that H.323 came into existence first.
H.323, which is implemented by voice, videoconferencing and fax equipment manufacturers, is used within Internet-based real-time applications and is strategically used worldwide by enterprises and service providers for voice, fax and video services over Internet Protocol (IP) networks. The usage of T.38 (a FoIP standard set by the ITU) within the H.323 Recommendation is specified in H.323 Annex D. T.38 is not a call setup protocol, so T.38 devices need to use standard call setup protocols to negotiate the T.38 call, like H.323, SIP or MGCP (Media Gateway Control Protocol)
H.323 was first published by the ITU in November 1996 with the idea of enabling videoconferencing over LANs, but was rapidly adopted by the marketplace as a way of transmitting voice calls over IP networks. H.323 was the first VoIP standard to use the IETF standard Real-time Transport Protocol (RTP) to move voice, fax and video over IP networks. RTP is considered a better-suited protocol for handling T.38 fax than UDP (User Datagram Protocol).
The protocol’s syntax is defined as a binary protocol, which means that it can support efficient message processing.
H.323 imitates traditional PSTN (public-switched telephone network) protocols, like Q.931, and as a result, is suited for PSTN interoperability. H.323 is, however, packet-switched and does not use the circuit-switched technology used in the PSTN. So, H.323 is defined by a telephony group but does not use the underlying technology (circuit-switching) of the telephone network.
The H.323 recommendation defines terminals as the most fundamental elements in an H.323 system. Terminals are generally IP phone or FoIP devices. H.323 terminals use protocol stacks, which implement the functionality defined by the H.323 system. The protocol stack offers the basic protocol defined in ITU-T Recommendation H.225.0 and H.245, as well as RTP (real-time protocol) or UDP (user datagram protocol).
Gateways are devices that offer communication between disparate networks like H.323 networks and other networks such as the PSTN. If a person in a call is utilizing a terminal that is not an H.323 terminal, then the call must go through a gateway to enable two parties to communicate.
Again, in the marketplace, H.323 has all but been superseded by SIP, which is endorsed by Internet groups. Internet groups have a stronger standing in the marketplace than telephony groups because it is IP networks that H.323 and SIP enable.