ditional 100-250 km, or it can extend the distance between repeaters a similar amount. 9.4.8 Wavelength Division Multiplexing Wavelength division multiplexing (WDM) is just another name for frequency division multiplexing (FDM). In this section we will hold with the conventional term, WDM. The WDM concept is illustrated in Figure 9.33. WDM can multiply the transmission capacity of an optical fiber many-fold. For example, if we have a single fiber carrying 10.0 Gbps, and it is converted for WDM operation with 20 WDM channels, where each of these channels carry 10.0 Gbps, the total capacity of this single fiber is now 10.0 20 or 200 Gbps. The 1550-nm band is the most attractive for WDM applications because the aggregate wavelengths of a WDM signal can be amplified with its total assemblage together by a single EDFA. This is a great advantage. On the other hand, if we wish to regenerate the digital derivate of a WDM signal in a repeater, the aggregate must be broken down into its components as shown in Figure 9.34. The cogent question is, How many individual wavelength signals can we multiplex on a single fiber? It depends on the multiplexing/demultiplexing approach employed, the number of EDFAs that are in tandem, the available bandwidth, channel separation (in nm), and the impairments peculiar to light systems operating at higher powers and the WDM techniques used. As we mentioned above, the number of WDM channels that can be accommodated depends on the bandwidth available and the channel spacing among other parameters and Optical Amplifier NxX Gbit/s X Gbit /s each X Gbit /s each Figure 9.33 The WDM concept. 9.4 FIBER-OPTIC COMMUNICATION LINKS Demux Demux L = laser diode R = Regenerator D = PIN detector Figure 9.34 The use of regenerative repeaters on a WDM link. characteristics. For example, in the 1330-nm band about 80 nm of bandwidth is available and in the 1550-nm band, about 100 nm of bandwidth is available, which includes the operational bandwidth and guardbands. 20 Currently, only EDFA amplifiers provide s