WDM Broadband WDM can be considered as laying the foundation for advances in the development of techniques to enhance the transmission capacity of optical fiber. As optical filters and laser technology improved, it became both possible and practical to combine additional signals on a fiber. In 1995 the number of channels, each consisting of a separate optical signal at a distinct wavelength, increased to 4 per fiber. Since then advances in technology resulted in the number of channels per fiber increasing to 8, then 16, 32, 40, 64, and even 80, with systems with 128, 256, and even more channels being devel- oped, as we note later in this chapter. Chapter Six As the number of channels in an optical multiplexing system increased, the term dense wavelength division multiplexing (DWDM) came into use in reference to such systems. While there is no magic number of channels where a WDM system becomes a DWDM system, today most optical multiplexing systems are classified as DWDM systems. SYSTEM CAPACITY We can obtain an indication of the growth in the transmission capacity of optical fiber systems by focusing on the technology at certain periods of time. To do this, let's commence our discussion with the AT&T FT3C lightwave transmission system that became operational in 1983. As noted earlier, the FT3C system provided a composite transmission rate of 270 Mbits/s. In 1994, as previously noted, a broadband WDM system was developed through the use of two 2.5-Gbit/s lasers. This system provided a compos- ite transmission capacity of 5 Gbits/s. In 1995 an increase in the number of channels to 4, each operating at 2.5 Gbits/s, resulted in a composite transmission rate of 10 Gbits/s. If we fast-forward our timeline to the beginning of the new millen- nium, by the year 2000 a transmission rate of 10 Gbits/s was obtainable on a per channel basis. With 32 and 40 channel systems representing common offerings, the composite transmission rate obtainable on a single optical fiber increased from 320 to 400 Gbits/s. During the year 2000,