H.323 defines packet standards for terminal equipment and
services for multimedia communications over local and wide
area networks communicating with systems connected to
telephony networks such as ISDN. The initial version of this
standard came from the International Telecommunications
Union (ITU) in June 1996.
It defines communication over IP-based local area networks
(LANs). A later version (v2), adopted in January 1998,
extended it over wide area use and general-purpose IP networks.
Several subprotocols are included under H.323 relating
to call setup and signaling.
Four components for a multimedia communication system
as shown in Figure 2-17 include terminals, gateways, gatekeepers,
and multipoint control units (MCU). Gateways and
gatekeepers are used in negotiation for PSTN connections,
whereas MCUs enable multiparty audio and videoconferences.
One drawback of H.323 is that it is somewhat complex and
inflexible. However, it is ISDN-based and relatively easy to
build applications across it. For many applications, H.323 is
satisfactory, but falls short for more advanced implementations
and solutions.
All things considered, the most likely scenario is that multiple
protocols will be used with H.323, such as SIP for
exchange between soft switches and gateways and MGCP for
call setup, because H.323 is too complex and time consuming
to set up a call.