H221

H.221 is a standard of the International Telecommunication Union ITU-T for the frame structure in audiovisual and multimedia systems. H.221 deals with the frame structure for transmission channels with transmission rates of 64 kbit/s to 1,920 kbit/s. These data rates apply to the ISDN base channel and the multiplex channel.

H.221 is about multiplexing and synchronization of multiple base channels, where the individual base channels are divided into 8-kbit/s channels. The 8-kbit/s channels can be combined into channels with higher data rates for the audio, video and user data as well as for the telematics information by means of channel bundling and dynamically adapted to the bandwidth requirements.

An H.221 frame consists of 80 octets, each with an octet duration of 125 µs. Consequently, each frame consists of 640 bits, which takes 10 ms to transmit. Each single bit position within an octet can be considered as a subchannel of 8 kbit/s. The 8th bit of each octet represents the 8th subchannel, which is called the service channel( SC).

H.221 is used, among other things, for multipexing in H.320 and H.322video conferencing.