H.323 Recommended by ITU-T for implementing packet-based multimedia conferencing over LAN that cannot guarantee QoS. Specifying protocols, methods and network elements to establish point-to-point multimedia connections between two endpoints and multipoint conferences with three or more parties.

Network elements: –Terminals: a device that includes a signaling end point which supports one or more users. H.323 call model is executed between two terminals, a terminal and a gatekeeper, or a terminal and a gateway. – Gatekeepers (GK): the brain of an H.323 zone. An H.323 zone includes terminals, gateways, and MCUs managed by one gatekeeper. Book keeping, access control. – Multicast units (MCUs): perform one to many communications. –Gateways: offer access to different types of segments.

H.323 functions are in three areas: –Registration with a gatekeeper with the registration, authentication, and status (RAS) protocol (H.225.0) Over UDP –Call signaling using H Over TCP –Logical channel capabilities negotiation (media control) with H.245. Over TCP

–H.323 call models: Many variations Directed routed model Gatekeeper routed model Five phases: –Call setup –Initial communication between endpoints and terminal capability –Establishment of audio/visual communication between endpoints –Request and negotiation of call services –Call termination

H.323 QoS: RSVP –transportQoS field in the ARQ RAS. Telephony Features Call forwarding, call waiting, three-way calling, call trace, etc. H.323 allows call re-direction. Protocol robustness TCP is reliable, but connection maintenance can cause problems. Performance issues: Large call setup time (multiple round trips), a new fast connect procedure has been proposed. GK routed calls also have scalability problems.