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.