 precursor to the MPEG video encoding standards a nearly uniform scalar quantizer using a possible 31 step sizes to scale the AC and interframe DC coefficients. The development is the H.261 encoder which contains many of the video coding methods and techniques later resulting quantized coefficient matrix is zigzag scanned adopted by MPEG. The ITU Recommendation H.261 into a vector that is VLC coded using a hybrid modified was adopted in 1990, and specifies a video encoding run length and Huffman coder. Motion compensation is standard for videoconferencing and videophone services optional. Motion estimation is only defined in the for- ward direction because H.261 is limited to real-time for transmission over integrated services digital network (ISDN) at p X 64 kbps, p=1, 30. H.261 describes the videophone and videoconferencing. The recommenda- video compression methods that were later adopted by tion does not specify the motion estimation algorithm or the conditions for the use of intraframe versus interframe the MPEG standards and is presented in the following section. The ITU Experts Group for Very Low Bit-Rate encoding. Video Telephony (LBC) has produced the H.263 Rec- The video multiplex coder creates a H.261 bit stream ommendation for Public Switched Telephone Networks that is based on the data hierarchy described below. The transmission buffer is chosen not to exceed the maxi- (PSTN), which was finalized in December 1995 [19]. It is an extended version of H.261 supporting bidirectional mum coding delay of 150 msec, and is used to regulate motion compensation and sub-QCIF formats. The en- the transmission bit rate via the coding controller. The transmission coder embeds an error correction code coder is based on hybrid DPCM/DCT coding and im- provements targeted to generate bit rates of less than 64 (ECC) into the video bit stream that provides error kbps. resilience, error concealment, and video synchronization. H.261 supports most of the internationally accepted digital video formats. These include, CCIR 60