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.225.0
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.
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