 are based on previously coded frames, and therefore within a sequence some frames are related to one another. The sequence of frames is first divided into a group of pictures that describes how the sequence is made up of I, P, and B pictures and how they are related to each other. Below this is the individual picture, which can be either I, P, or B. The picture is subdivided into slices to reduce the propagation of errors. The slices themselves are made up of macroblocks. A macroblock is usually made up of four 8 X 8 blocks of pixels, with the finest granularity being the pixel itself. This is clarified in Figure 3.18. Video Transport over the Internet Video is usually accompanied by audio, and when transporting multimedia over the Inter- net a number of operations are required, including deriving the source media, compressing the content, controlling the rate of generation, packetizing the data, smoothing bursts, network interfacing, multiplexing with other traffic, scheduling, and protocol. These opera- tions are usually transparent to the user and integral to the codec, the operating system, and the network. Video Traffic Types Real-time video traffic is where the video is delivered as it is generated and displayed at the receiver as quickly as possible. The delay between generation and final display is of the order of tens of milliseconds, and two-way communication is also possible. Little or no Chapter 3 Digital Video Transmission packet buffering is used, and there is generally no mechanism to recover lost packets; the application has to deal with late or lost packets and the associated degradation caused. A typical example is the videoconferencing software Skype. Streamed video is the simultaneous delivery and playout of multimedia; a fundamental example is television. Archived or near real-time video is held by the sender (server) and delivered to the receiver (client) by default or by request. Often when using digital media over the Internet, the receiver buffers the data and only plays the video when t