H.323 is an umbrella recommendation from the ITU (International Telecommunications Union) for real-time multimedia over Local Area Networks.
Originally approved in 1996, the protocol offers no Quality of Service definition, but is supported without regard to the layer two protocol suite – IP, IPX – or the layer one physical environment – Ethernet or Token Ring for example. The widespread adoption of the Internet eliminated market demand for other protocols – IPX, Token Ring, or ATM at the desktop – for example, and a comprehensive, multi-protocol family has been somewhat over hyped.
Modern manufacturers of software and hardware communications systems rely on Ethernet and IP. Period. There are no other relevant market segments to worry about.
The recommendation was originally designed to enable video conferencing, and included ISDN (Integrated Services Digital Network) and PSTN (Public Switched Telephone Network) as part of the infrastructure.
The vision was to make the network independent and assure interoperability. However, the complexities of combining the signaling features of digital switching networks with Ethernet and other packet networks made it awkward to deliver quality functionality despite the support of major industry players such as Microsoft, Cisco, IBM and others.
The protocol family established preferences for codec compression and decompression. It defined features for bandwidth management and was designed to facilitate multipoint support.
H.323 is the protocol family that the 3Com NBX, the Cisco Call Manager and the Nortel Business Communications Manager rely on, but not for interoperability, or inter-brand signaling. It has a role to play in IP-PBX products, but did not fulfill its vision for broad industry-wide adoption.
The balance of manufacturer needs for proprietary control of elements of their product line with end user needs for device substitution usually leaves the end user with only a trivial set of features supported in a standard, and lots of value residual remaining in the manufacturer's offering. This is the value model that the PBX industry created in the migration to digital.
Many manufacturers built a derivative of ISDN signaling between the handset and the switch fabric to enable features like message lamp indicator, call forward, conferencing and the like. IP versions of this product category follow the same value path.