H.323 is an International Telecommunications Union-Telecommunications
(ITU-T) standard that defines a packet-based multimedia communications system.
H.323 defines a distributed architecture for transporting multimedia
applications over LANs. Because of its early availability and its evolution to
address the needs of VoIP, H.323 is currently the most widely used VoIP
signaling and call-control protocol. International and domestic carriers rely
on H.323 to handle billions of minutes of use each year.
H.323 is
considered an umbrella protocol because it defines all aspects of call
transmission. H.323 defines the Registration, Admission, and Status (RAS)
protocol for call routing, H.225 protocols for call set-up, and H.245 protocols
for capabilities exchange. H.323 is based on the Integrated Services Digital
Network (ISDN) Q.931 protocol, which allows it to easily interoperate with
legacy voice networks such as the PSTN and SS7. The figure shows an example of
an H.323 network. H.323 uses the following types of messages to facilitate
communication, as shown in Figure
:
- Location Request Message (LRQ)
- Location Confirm Message (LCF)
- Admission Confirmation Message (ACF)
- Admission Request Message (ARQ)
- Registration Request Message (RRQ)