Historically, the H.261 recommendation [ITU93] was developed in the
mid-80s, and was developed for video transmission over ISDN channels. The final
CCITT H.261 recommendations, released
in late 1989, were made for a
kbps video codec, where p is
between 1 and 30, whence came the alternative name p*64 [NH95]. The
H.263 standard [ITU95] was developed in the mid-90s, when speeds near
30 kbps over the Public Switched Telephone Network (PSTN) were available to the
majority of users, and was targeted to such low bitrates. Currently, an improved
version, H.263+, is proposed by the industry to support error resilient very low
bitrate video delivery over wireless networks.
The ITU-T has standardised only the decoder for H.261 and H.263, and leaves the design details of a compatible encoder deliberately unspecified. Both standards are parts of umbrella standards, the H.32x standards, which describe the multiplexing of all data streams that compose a videoconferencing system. These include video, audio, data, control and signalling information, and packet frame structures. In the following paragraphs only the video part is described.