RFC 2429 (rfc2429) - Page 2 of 17
RTP Payload Format for the 1998 Version of ITU-T Rec
Alternative Format: Original Text Document
RFC 2429 H.263+ October 1998 The 1998 version of ITU-T Recommendation H.263 added numerous coding options to improve codec performance over the 1996 version. The 1998 version is referred to as H.263+ in this document. Among the new options, the ones with the biggest impact on the RTP payload specification and the error resilience of the video content are the slice structured mode, the independent segment decoding mode, the reference picture selection mode, and the scalability mode. This section summarizes the impact of these new coding options on packetization. Refer to [4] for more information on coding options. The slice structured mode was added to H.263+ for three purposes: to provide enhanced error resilience capability, to make the bitstream more amenable to use with an underlying packet transport such as RTP, and to minimize video delay. The slice structured mode supports fragmentation at macroblock boundaries. With the independent segment decoding (ISD) option, a video picture frame is broken into segments and encoded in such a way that each segment is independently decodable. Utilizing ISD in a lossy network environment helps to prevent the propagation of errors from one segment of the picture to others. The reference picture selection mode allows the use of an older reference picture rather than the one immediately preceding the current picture. Usually, the last transmitted frame is implicitly used as the reference picture for inter-frame prediction. If the reference picture selection mode is used, the data stream carries information on what reference frame should be used, indicated by the temporal reference as an ID for that reference frame. The reference picture selection mode can be used with or without a back channel, which provides information to the encoder about the internal status of the decoder. However, no special provision is made herein for carrying back channel information. H.263+ also includes bitstream scalability as an optional coding mode. Three kinds of scalability are defined: temporal, signal-to- noise ratio (SNR), and spatial scalability. Temporal scalability is achieved via the disposable nature of bi-directionally predicted frames, or B-frames. (A low-delay form of temporal scalability known as P-picture temporal scalability can also be achieved by using the reference picture selection mode described in the previous paragraph.) SNR scalability permits refinement of encoded video frames, thereby improving the quality (or SNR). Spatial scalability is similar to SNR scalability except the refinement layer is twice the size of the base layer in the horizontal dimension, vertical dimension, or both. Bormann, et. al. Standards Track
