H.261 – a) Recognizing the need for providing ubiquitous video services using the Integrated Services Digital Network (ISDN), CCITT (International Telegraph and Telephone Consultative Committee) Study Group XV established a Specialist Group on Coding for Visual Telephony in 1984 with the objective of recommending a video coding standard for transmission at m x 384 kbit/s (m=1,2,..., 5). Later in the study period after new discoveries in video coding techniques, it became clear that a single standard, p x 64 kbit/s (p = 1,2,..., 30), can cover the entire ISDN channel capacity. After more than five years of intensive deliberation, CCITT Recommendation H.261, Video Codec for Audio Visual Services at p x 64 kbit/s, was completed and approved in December 1990. A slightly modified version of this Recommendation was also adopted for use in North America. The intended applications of this international standard are for videophone and video conferencing. Therefore, the recommended video coding algorithm has to be able to operate in real time with minimum delay. For p = 1 or 2, due to severely limited available bit rate, only desktop face-to-face visual communication (often referred to as videophone) is appropriate. For p>=6, due to the additional available bit rate, more complex pictures can be transmitted with better quality. This is, therefore, more suitable for video conferencing. The IVS (INRIA Video conferencing System) is software implementation of H.261 codec which also features PCM and ADPCM audio codecs and includes an error control scheme to handle packet losses in the Internet. b) The ITU-T H.261 recommendation embraces video codecs for audio visual services at p x 64 kbit/s data rate, where p is between 1 and 30. Thus, the standard is informally called “p x 64”. It is aimed at low bit rate media, and is used in the H.320 video conferencing recommendation. H.261 provides a resolution of 352 x 288 pixels (CIF) or 176 x 144 pixels (QCIF), independent of bit rate. The H.261 recommendation defines both encoding and decoding. However, it defines, more strictly, how to decode than to encode the bit stream, and has room for options in the encoder. The coding is based on DCT with word-length encoding. H.261 defines both independently coded frames (key frames), and frames that frame by using block-based motion compensation (non-key frames). H.261 also defines error-correction codes, and it allows rate control by varying quantization and by dropping frames and jumping blocks.