of the process and involve expressing strings of zeros and ones using shorthand codes. Another method for categorizing compression is based on the methods used for decompression. Is decompression an exact reverse of the compression process? If so, the compression is symmetric. If the techniques used to compress are more compute- intensive than those used to decompress are, compression is asymmetric. Real-time, interactive video communication applications use symmetric techniques. Video playback applications commonly use asymmetric compression. Most of the resource goes into compression SO that receiving devices of limited sophistication can decompress cheaply and easily. Even symmetric compression does not divide the work equally. Real-time encoding requires complex math, SO the sending codec does more work. However, in interactive videoconferences, both codecs are simultaneously sending and receiving. INTRAFRAME AND INTERFRAME COMPRESSION Frames of motion video are full of duplication, as one can demonstrate with a strip of movie film. One will discern little difference between the successive frames. The background is static, and action takes place incrementally. Moreover, within a single frame many long sequences of constant pixel value exist (e.g., walls, tables, clothing, and hair tend to comprise masses of uniform color). Efficient codecs do not waste bandwidth on repetitive information. They code the unique parts of the signal and describe, using an algorithm, how patterns repeat. Video codecs that compress data within a single frame of video are intraframe codecs. They commonly divide a video frame into blocks and then look for redundant data. When they find it, they eliminate it; they attempt to reduce the content of that frame to entropy. Other types of codecs compare multiple frames of data. These are interframe codecs. They, too, try to achieve entropy through compression. What is entropy? It is a scientific term that refers to the measure of disorder, or uncertainty, in a system. Good compression tech