value in instances in which H.261 applications traverse networks with very low bit error rates. FEC is rarely implemented in H.261 coders or decoders. Video Multiplexing Video multiplexing is integral to H.261. It allows a H.261 decoder to accurately scan and decode the flood of bits that represent video data. Multiplexing is performed as a step in the H.261 process by the video multiplex coder portion of the H.261 codec. An H.261-compliant multiplex encoder must present its output bit stream in accordance with the requirements of a hypothetical reference decoder, described in Annex B of H.261. Various aspects of this hypothetical reference decoder (HRD) are set out in the standard: how it clocks to the encoder; what its buffer size is; how often the buffer is examined, what is stored in the buffer and precisely when stored data must be removed. The HRD's buffer size, by definition, depends on the bit rate of the connection. If the bit rate is relatively high, the buffer is relatively large. Standards for Videoconferencing Slower transmission speeds translates to fewer frames delivered in a given second, thus the buffer can be smaller. A HRD is designed to accept the maximum flood of bits permitted in the H.261 Recommendation. The "high water mark" occurs when a high-resolution (CIF-capable) codec is sending images over a very fast transmission link (30 B channels) at a rate of 29.97 per second. The HRD must buffer and decode this fast-moving bit stream without dropping frames. It must also be able to buffer reference frames used during motion compensation. All H.261-compliant decoders must conform to Annex B. H.221 FRAME FORMAT AT 384 Kbps Low band data at 14.4 Kbps Low band data at 64 Kbps Voice Video at at 48 Kbps 326.4 Kbps 56 Kbps High Band (nx64 Kbps where n-0 to 5) SIGNALING AT 1.6 Kbps (bit number 8) TYPICAL SUBSTRATES AT A SPECIFIC TIME -Voice - 56 Kbps -Video - 326.4 Kbps - 64 Kbps data - 262.4 Kbps -No low band data -Hi band data - 64 Kbps (video 272.2 Kbps) Figure 7-6. H.221 Frame Format at 384 Kbps (