more than frequency-division multiplexing, albeit at very high frequencies. The ITU has standardized a channel separation grid that centers around 193.1 terahertz (THz), ranging from 191.1 THz to 196.5 THz. Channels on the grid are technically sep- arated by 100 GHz, but many industry players today are using 50 GHz separation. The majority of WDM systems operate in the C-Band (third window, 1,550 nm), which allows for close placement of channels and the reliance on EDFAs to improve signal strength. Older systems, which spaced the channels 200 GHz (1.6 nm) apart, were referred to simply as WDM sys- items; the newer systems are referred to as Dense WDM systems because of their tighter channel spacing. Modern systems routinely pack forty 10 Gbps channels across a single fiber, for an aggregate bit rate of 400 Gbps. How DWDM Works As Figure 7-24 illustrates, a WDM system consists of multiple input lasers, an ingress multiplexer, a transport fiber, an egress multiplexer, and of course, customer receiving devices. If the system has eight chan- nels, such as the one shown in the diagram, it has eight lasers and eight receivers. The channels are separated by 100 GHz to avoid fiber non- linearities, or closer if the system supports the 50 GHz spacing. Each channel, sometimes referred to as a lambda (1, the Greek letter and uni- versal symbol used to represent wavelength), is individually modulated, and ideally, the signal strengths of the channels should be close to one another. Generally speaking, this is not a problem, because in DWDM Figure 7-24 channel separation Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 systems the channels are closely spaced and therefore do not experience significant attenuation variation from channel to channel. There is a significant maintenance issue that faces operators of DWDM-equipp