H 323

International standard for multimedia communication ( real-time voice, video, and data communication) over packet-switched networks
includes the standards H.323, H.225.0, H.245, the H.450-series documents, and the H.460-series.
T.120 for data collaboration and file transfer.
Ability to utilize network services for address resolution, including ENUM, LDAP, and DNS an provide various supplementary services Call transfer , forward , park/pick-up , Hold , Waiting , Message Waiting Indication , Call Completion on Busy / No-Answer , Call Intrusion etc
Support for T.120 data conferencing, fax and modem relay

Elements of an H.323 System

• Terminals ( Telephones , Video phones , IVR , Voicemail Systems, Soft phones like, NetMeeting)
• Multipoint Control Units (MCUs) – manages the call signaling and optionally have Multipoint Processors to handle media mixing, switching or other media processing got multipoint conferences
• Gateways – MGC handles call signaling and other non media-related functions while MG handles the media
• Gatekeeper – admission control and address resolution
• Border Elements

Protocols in H323 framework

H.225.0 defines the call signaling between endpoints and the Gatekeeper
RTP/RTCP (RFC 3550) is used to transmit media such as audio and video over IP networks
H.225.0 Annex G and H.501 define the procedures and protocol for communication within and between Peer Elements
H.245 control establishment and closure of media channels within the context of a call and to perform conference control
H.450.x is a series of supplementary service protocols
H.460.x is a series of version-independent extensions to the base H.323 protocol
T.120 specifies how to do data conferencing
T.38 defines how to relay fax signals
V.150.1 defines how to relay modem signals
H.235 defines security within H.323 systems
X.680 defines the ASN.1 syntax used by the Recommendations
X.691 defines the Packed Encoding Rules (PER) used to
encode messages for transmission on the network

Ref :
https://hive2.hive.packetizer.com/users/packetizer/papers/h323/overview_of_h323.pdf

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