H.323

In all telephony networks, specific signaling protocols are invoked before
and during a call to communicate a desire to set up a call, to monitor call
progress, and to gracefully bring a call to a conclusion. Perhaps the best
example is the ISDN User Part (ISUP), a component of the Signaling System
7 (SS7) signaling suite. In VoIP systems, signaling protocols also need
to be used for exactly the same reasons. The first successful set of protocols
for VoIP was developed by the International Telecommunications Union
(ITU). This set is known as H.323 and has the title, “Packet-based Multimedia Communications Systems.” Although recently new protocols have
emerged, notably the Session Initiation Protocol (SIP), H.323 is still the
most widely deployed VoIP signaling system.