th- coded in the usual way. The I picture forms a marker within the bitstream, a point at which no assumptions have been made about the past or future. Therefore, if other types of predic- tive frames have been used within the video stream and errors have occurred in those, the arrival of an I-frame has the effect of resetting the coder back to normal. However, it should be noted that other frames use the I-frame as the basis of their predictions, and therefore errors in or loss of the I-frame can have cumulative effects for a number of frames to follow. An intracoded frame also has a high bitrate associated with it, making it costly in network terms. Intercoded or P Pictures An interframe-coded picture is a predicted, or P, picture that codes the difference between itself and other neighboring pictures within a sequence. In being dependent on information from other pictures, it will be affected by not only loss in its own information but also loss that has occurred in associated I frames; the P frame assumes the existence at the decoder of the information the prediction is based on. The predicted picture may or may not use motion compensation and will have selected the option that yields the lowest coding expense. The difference between the actual and the predicted picture is DCT-coded in the usual way. Video Transport over the Internet Video Sequence Group of Pictures Picture Slice Macroblock Block 8 pixels 8 pixels Fig. 3.18 Picture structure. MPEG-1 took this idea one step further by introducing a bidirectional picture, or a B picture. A B picture can use past, present, and future information as the basis of its predic- tions. The extreme research suggests that there is merit in buffering hundreds or even thousands of frames for use in motion compensation. It should be noted that while not resilient to error, such predictive frames are cheap to encode. Video Structure A video sequence consists of a sequence of pictures, which are presented at a rate of N frames per second. As seen earlier, predictive pictures