trical signal. The introduction of EDFAs immediately opened up the opportunity to make use of fiber-optic systems operating at 10Gbps. EDFAs also paved the way to developing wavelength division multiplexers. Before the advent of WDM, we were using only one wavelength of light within each fiber, although the visible light spectrum engages a large number of different wavelengths. WDM takes advantage of the fact that multiple colors, or frequencies, of light can be transmitted simultaneously down a single optical fiber. The data rate that each of the wavelengths supports depends on the type of light source. Today, the most common data rates supported are OC-48, which is shorthand for 2.5Gbps, and OC-192, which is equivalent to 10Gbps. 40Gbps systems are also now commercially available. In the future, we'll go beyond that. Part of the evolution of Chapter 1 Telecommunications Technology Fundamentals WDM is that every year, we double the number of bits per second that can be car- ried on a wavelength, and every year we double the number of wavelengths that can be carried over a single fiber. But we have just begun. Soon light sources should be able to pulse in the terabits-per-second range, and within a few years, light sources that produce petabits per second (1,000Tbps) will emerge. One thing to clarify about the first use of WDM is that unlike the other types of multiplexing, where the goal is to aggregate smaller channels into one larger chan- nel, WDM is meant to furnish separate channels for each service, at the full data rate. Increasingly, enterprises are making use of high-capacity switches and routers equipped with 2.5Gbps interfaces, so there's a great deal of desire within the user community to plug in to a channel of sufficient size to carry a high-bandwidth sig- nal end to end, without having to break it down into smaller increments only to build them back out at the destination. WDM addresses this need by furnishing a separate channel for each service at the full rate. Systems that support more than 8 w