Introduction to H.323
H.323 is a protocol used for transmission of real-time video and audio data over a packet-switched network like Internet Protocol. It is a protocol standard for multimedia communications. H.323 protocol is maintained by the Study Group 16 within the ITU (International Telecommunication Union) Telecommunication Standardization Sector, also known as ITU-T.
Most VoIP application utilizes H.323 protocol, since it supports call forwarding, call setup and teardown. A key feature of H.323 protocol is QoS (Quality of Service), which allows traffic management and real-time prioritization to be placed over Ethernet. Apart from packet switched network, it also supports multimedia communications over Public Switched Telephone Network (PSTN), 3G mobile networks and Integrated Service Digital Network (ISDN).
In 1996, International Telecommunication Union (ITU) published the first version of H.323 with an importance of enabling videoconferencing over a LAN (Local Area Network). It was quickly implemented by the companies over an IP network (including WAN) as a means of transmitting audio communication. In 1998, a new title “Packet-Based Multimedia System” was given to H.323. In 2006, the current version of H.323 was referred as H.323v6.
Protocols and Codec’s
Protocols used in H.323 systems are –
• H.225.0 RAS (Registration, Admission and Status) to provide admission control services and address resolution between a Gatekeeper and H.323 endpoint.
• H.225.0 Call Signaling to establish communication with the remote entity.
• H.235 for security of media and signaling.
• H.239 for live videos, images and videoconferencing.
• H.245 control protocol for multimedia communication.
• H.450 for various supplementary services.
• H.460 for Firewall traversal (FW) and Network Address Translation (NAT).
• RTP (Real-time Transport Protocol) for receiving and sending multimedia information between any two entities.
Codec’s used in H.323 protocols are –
• Video Codec – H.261, H.263, H.264.
• Text Codec – T.140.
• Audio Codec – G.711, G.729, G.723.1, G.726, G.722, G.728, Speex.
Architecture of H.323 protocol
H.323 system describes several network elements –
• Terminal – It is the most fundamental element in H.323 System. It is an endpoint in the LAN which participates in a real-time two way communication with another terminal.
• Multipoint Control Unit (MCU) – It mixes and controls the audio and video from terminals to create healthy videoconferencing. It also provides three or more gateways and terminals to participate in a multipoint conference.
• Gateways – Gateways are devices that enable communication between H.323 network and other network.
• Gatekeepers – Gatekeeper is an optional component which provides many services like admission control, address resolution, user authentication, endpoint registration and so forth to the gateways, MCU devices and terminals.
H.225 Signaling and Control Function
H.323 Network Signaling functions is based on H.225.0 Call Signaling function to establish communication with the remote entity.
The most common H.225.0 messages are –
• Call Setup
• Call Proceedings
• Alerting
• Connect
• Release Complete
• Facility
• Status inquiry
• Notify
The most common H.225.0 Control Functions are –
• Master/Slave Determination (MSD) – It is used to determine which device is the master and which device is the slave, to prevent confliction between two terminals.
• Fast Connect – It is used to set up a bi-directional media flows for the device.
• Open Logical Channel (OLC) – It is used to open the logical channel for the codification and reception of data.
• Close Logical Channel (CLC) – It is used to close the logical information channel.