TABLE OF CONTENTS Although we have extensively described the scalability under JPEG2000 and MPEG-2, in H.263 it is used with different terminology and we visit this subject again. It might be useful to know that scalability in H.263 is not used for distribution purposes, but more as a layering technique. Hence, by unequal error protection on the base layer, this method in conjunction with the other error resilience methods, explained in section 9.7, further improves the robustness of this codec.
Extensions of H.263 also support temporal, SNR and spatial scalability as optional modes [25-O]. This mode is normally used in conjunction with the error control scheme. The capability of this mode and the extent to which its features are supported is signalled by external means such as H.245 [9].
There are three types of enhancement picture in the H.263+ codec that are known as B, EI and EP-pictures [5]. Each of these has an enhancement layer number, ELNUM, which indicates to which layer it belongs, and a reference layer number, RLNUM, which indicates which layer is used for its prediction. The encoder may use any of its basic scalability nodes of temporal, SNR, spatial or their combinations in a multilayer scalability mode. Details of the basic and multilayer scalabilities were given in section 8.5. However, due to the different nature and application of H.263 to MPEG-2, there are some differences.
Temporal scalability is achieved using bidirectionally predicted pictures or B-pictures. As usual, B-pictures use prediction from either or...
