H.245 is a control channel protocol

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

 used with[in] e.g. H.323

H.323

H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

 and H.324

H.324

H.324 is an ITU-T recommendation for voice, video and data transmission over regular analog phone lines. It uses a regular 33,600 bit/s modem for transmission, the H.263 codec for video encoding and G.723.1 for audio....

 communication sessions, and involves the line transmission of non-telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 signals. It also offers the possibility to be tunneled within H.225.0

H.225.0

H.225.0 is part of the H.323 family of telecommunication protocols.The main objective of H.225.0 is the definition of messages and procedures for:* Call Signaling: establish, control and end an H.323 call...

 call signaling messages. This eases firewall traversing.

H.245 is capable of conveying information needed for multimedia communication, such as encryption

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

, flow control

Flow control

In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...

, jitter

Jitter

Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...

 management, preference requests, as well as the opening and closing of logical channels used to carry media streams. It also defines separate send and receive capabilities and the means to send these details to other devices that support H.323.

Handshake issues

One major drawback within the initial version of H.323 was the lengthy, four-way H.245 protocol handshake

Handshaking

In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...

 required during the opening up the logical channels of a telephony session. which has more Later versions of H.323 introduced the Fast Connect procedure, using the fastStart element of an H.225.0 message. Fast Connect brought the negotiation down to a two-way handshake. Another recommendation, H.460.6, Extended Fast Connect Feature, exists that defines a one-way handshake.

External links

• H.323 Tutorial Includes information on the role of H.245
• ITU-T H.245 Recommendation