Fundamentals of Digital Communications Systems
During the last ten years, most major communications and broadcast systems and many other systems were converted from analog to digital. Examples of digital systems that we use every day include mobile phones, television, radio, and of course the Internet. CDs and MP3s are replacing records and tapes, and the number of digital cameras sold this year exceeded the number of analog cameras by a factor of three. In this chapter, you will see some of the basic building blocks that make all of these digital systems work.
The material in this chapter is intended to provide a background that will be useful when studying digital communications test and measurement techniques described in later chapters. We start with a discussion of a basic digital communications link, cover the most commonly used clocking architectures, discuss line-coding methods, and conclude with special techniques for high-speed serial transmission systems.
The most important aspect of any digital communications system is the required transmission speed. Just how much data needs to be transmitted, and how fast? The variability is huge, even within a single system: The keyboard interface of a typical PC, for example, runs at several kilobits per second, which is still significantly faster than anyone can type. However, the fastest interface available for graphics adapters is not nearly fast enough for the newest games, even at 40 Gbit/s (which is the accumulated bandwidth of a PCIe x16 link, the current standard for graphics adapters).
The second, equally important aspect is the link distance. How far apart are sender and receiver? Again, there is huge variability: The main processor of a computer communicates with its main memory over a distance that's usually less than 10 cm. But when you type a URL into a Web browser, you communicate with a server that's potentially on a different continent.
Generally, digital transmission becomes harder when the transmission speed and link distance increase. A measure for the effort required to make a digital communications link work is the bandwidth-distance product. An old telegraph, for example, transmitted about 100 bit/s, over a maximum distance of 20 km. The radio downlink from the Voyager spacecraft transmits data slightly faster, at 160 bit/s, but over an incredible distance of 14.821 billion km. The much larger bandwidth-distance product of the spacecraft link can be achieved only with incredible effort.
Every digital link consists of three components: a sender, a transport medium, and a receiver. Usually, the medium is defined first, depending on the required link bandwidth, the distance between transmitter and receiver, and economic considerations. Electrical links are still the most common type; they come in a great variety, ranging from bond wires within an integrated circuit package to printed circuit board traces on a motherboard to Ethernet cables connecting office computers. Fiber-optic cables are used for very high bandwidth connections in network and storage environments, but it seems as if "fiber to the home" might be replaced by wireless links in the near future.