oved increasingly difficult to achieve economically. By 1995 speeds had increased to 10 Gbit/s, but even with these bit rates, long-haul carriers in the United States were beginning to run out fiber capacity: They were faced with the choice of either laying new cables on long- distance routes or adopting WDM technology. The principles of WDM were well known, and many suitable components had already been developed but until then Part 2 Transmission Media they had not been at a price that could compete with higher-speed electronics. However, competing with the cost of laying new cables was a different matter and, with further speed increases remaining difficult over the next ten years, fiber capac- ity increased mainly by augmenting the number of wavelengths. In that period, the number of wavelengths that could be launched into a fiber increased from 1 to 160 for commercial systems. In the same period, system speeds increased more mod- estly from 10 to 40 Gbit/s. However, there is currently a strong drive to increase system speeds to 100 Gbit/s for Ethernet applications. In recent years there have been substantial developments in radio technologies in the areas of miniature antenna designs, new types of modulation format, and encryption. Many of these are associated with cellular systems, which were intro- duced in volume 1, and broadband access, which will be covered in volume 3. While radio provides much less bandwidth than that offered by fiber, it provides the means for mobile connectivity and can be used for broadband access without the need to lay a new cable infrastructure. The chapters in Part 3 of this volume of The Cable and Telecommunications Professionals' Reference provide an introduction to the principles of radio propagation. Chapter 5 describes the fundmentals of light propagation in optical fibers and the fiber designs that have been developed for short-, medium-, and long-haul transmission. Chapter 6 provides an introduction to fiber transmission systems and their properties, including wavelength 