This chapter has described how application data is encoded in network packets. Unlike the protocols described earlier in this book, which you can think of as processing messages, these transformations process data. Multimedia data types, such as video, still image, and audio, are increasingly driving developments in this space.
The first issue is presentation formatting, where the problem is formatting the different types of data that application programs compute on, such that they can be transmitted over a network and interpreted correctly by the receiver. Data types such as integers, floating-point numbers, character strings, arrays, and structures must be encoded in some intelligible way. This involves both translating between machine and network byte order and linearizing compound data structures. We outlined the design space for presentation formatting and discussed four specific mechanisms that fall on different points in this design space: XDR, ASN.1, NDR, and the now dominant XML.
The second issue is compression, which is concerned with reducing the bandwidth required to transmit different types of data. Compression algorithms can be either lossless or lossy, with lossy algorithms being most appropriate for image and video data. JPEG, MPEG, and MP3 all make use of lossy compression protocols for still images, video, and audio data, respectively. Video compression and encoding formats such as the MPEG family of standards continue to evolve to meet the demand for higher quality within the limits of available bandwidth.
Our recommended reading list for this chapter includes two papers that give an overview of the JPEG and MPEG standards, respectively. Their main value is in explaining the various factors that shaped the standards. We also recommend the paper on receiver-driven layered multicast as an excellent example of a systems approach to design, embracing the issues of multicast, congestion control, and video coding.
Wallace, G. K. The JPEG still picture compression standard. Communications of the ACM 34(1):30-44, April 1991.
Le Gall, D. MPEG: A video compression standard for multimedia applications. Communications of the ACM 34(1):46-58, April 1991.
McCanne, S., V. Jacobson, and M. Vetterli. Receiver-driven layered multicast. Proceedings of the SIGCOMM '96 Symposium, pages 117-130, September 1996.