cs (standards- based or proprietary) are optimized for a narrow range of network operating speeds. A discriminating evaluator will ask about a system's sweet spot (e.g., what operating bandwidth delivers the best picture resolutions and motion handling). It is unusual for a codec to operate at speeds less than 384 Kbps and consistently deliver 30 fps. Regardless of transmission speed, a scene's motion-component will affect a codec's frame rate. Consequently, manufacturers discuss frame rates in terms of a codec's average and maximum. When there is little motion, a codec may achieve its maximum. When large portions of the picture change, or when intricacies in the image require rigorous computation, the codec must buffer the data while it performs intraframe compression. A codec may fall behind and receive a new frame before it finishes coding the previous one. If so, it will discard frames until it catches up. A codec is sometimes called a compression engine. Like an automobile engine, it can deliver raw horsepower but can also be tuned to use its power in special ways. Motion compensation represents one type of tuning. Motion compensation is based on interframe coding. It is optional in the H.320 specification. An H.320 decoder must interpret a motion-compensated stream of pixels if they are sent, but an H.320 encoder does not have to be able to create such a stream. Motion compensation is computation-intensive, but it provides much better compression at comparable bandwidths (typically 30% better than intraframe-only encoding). Even when two codecs support all the same optional sections of the ITU-T's H.320 Recommendation, tuning (signal pre-processing and post-processing) can improve performance. Powerful codecs take additional steps to enhance the H.261/263 process, and prepare video frames for compression in a way that is transparent to a standards-compliant decoder. This is known as pre-processing. Post-processing is also performed, to make the most of the work done during pre-processing. When a manufacture