The ITU see their remit as providing quality videotelephonny over the typical telephone companies infrastructure - due to the lowish speed of typical access lines, this has led to a tightly specified multiplex for audio and video, and data being added in an external manner.
Typically, ITU protocols (at least on lower speed access netowrks up to around T1 or megastream) run over synchronous networks, so that the clock for the media can be reasoanbly assumed to be that of the transmission system.
The framing structure for H.261 is H.221, which includes a FEC scheme, as shown in the 2 diagrams 4.17 and 4.18 below.
H221 is the most important control standard when considered in the context of equipment designed for ISDN specially current hardware video CODECs. It defines the frame structure for audiovisual services in one or multiple B or H0 channels or single H11 or H12 channel at rates of between 64 and 1920 Kbit/s. It allows the synchronization of multiple 64 or 384 Kbit/s connections and dynamic control over the subdivision of a transmission channel of 64 to 1920 kbit/s into smaller subchannels suitable for voice, video, data and control signals. It is mainly designed for use within synchronized multiway multimedia connections, such as video conferencing.
H221 was designed specifically for usage over ISDN. A lot of problems arise when trying to transmit H221 frames over PSDN.
Due to the increasing number of applications utilizing narrow (3KHz) and wideband (7KHz) speech together with video and data at different rates, a scheme is recommended by this standard to allow a channel accommodates speech and optionally video and/or data at several rates and in a number of different modes. Signaling procedures for establishing a compatible mode upon call set-up, to switch between modes during a call and to allow for a call transfer, is explained in this standard.
Each terminal would transfer its capabilities to the other remote terminal(s) at call set-up. The terminals will then proceed to establish a common mode of operation. A terminal capabilities consist of : Audio capabilities, Video capabilities, Transfer rate capabilities , data capabilities, terminals on restricted networks capabilities and encryption and extension-BAS capabilities.
H.230 standard is mainly concerned with the control and indication signals needed for the transmission of frame-synchronous or requiring rapid response. Four categories of control and indication signals have been defined, first one related to video, second one related to audio, third one related to maintenance purposes and the last one is related to simple multipoint conferences control (signals transmitted between terminals and MCU's, as specified in H.231).
H.320 covers the technical requirements for narrow-band telephone services defined in H.200/AV.120-Series recommendations, where channel rates do not exceed 1920 kbit/s. There is now convergence work between this group and the AVT and MMusic groups in the IETF. This is discussed further in chapter six.