H.239
From Wikipedia, the free encyclopedia
H.239 is an ITU-T recommendation from the H.32x Multimedia Communications' macrofamily of standards for multimedia communications over various networks.
The H.32x family includes:
- H.320 - describes Multimedia Communications via ISDN
- H.321 - ATM
- H.322 - GQOS-LAN, LANs providing quality of service
- H.323 - IP networks
- H.324 - GSTN, system of 3.1 kHz voice channels
- H.324M - Mobile Networks
- H.245 - Control Protocol For Multimedia Communication
The H.239 recommendation is titled "Role management and additional media channels for H.3xx-series terminals". Practical importance of this recommendation is its setting forth a way to have multiple video channels (e.g., one for conferencing, another for presentation) within a single session (call).
Contents |
[edit] History
H.239 builds on the ITU-T's H.32x and H.245 series of standards, called "recommendations" by the ITU.
Polycom introduced the ability to send a computer or video presentation along with a video teleconference session in 2000 with their "People + Content" feature. Polycom offered their proprietary, patented method to other vendors on a royalty-free basis in 2002. By that time Tandberg had deployed their own proprietary standard for unidirectional secondary image content, called "Duo Video". In Feb. 2003 the ITU-T took up a proposal by vendors to create a standard based on Polycom's technology. The ITU-T published this technology as Recommendation H.239 in July 2003.
[edit] Mechanism
When an H.323/H.324 call is connected, H.245 is used to establish the set of capabilities common to all connected endpoints (codecs) and MCU's. Relevant to H.239, this session-specific data determines the capability of connected devices to connect using multiple transport channels. While H.245 defines these transport channels as channels for multiple video streams (for instance, for multipoint video teleconferences), H.239 creates an abstraction layer that allows these transport channels to be used as logical channels, which then can be used for any type of data. The typical application of this is "presentation media" i.e. "rich media" i.e. a secondary channel of encoded video data which is typically output from a computer's video output at XGA resolution (1024 x 768 pixels). Though codecs will synchronize to input signals of 60 Hz (and maybe higher frame rates), the transmitted frame rate is typically 5 to 15 Hz. This varies dependent on codec manufacturer and model, and on the data rate of the connection.
