2 based DWDM system with 0.4-nm signal separation. In SO doing, Nortel transmits alternating wavelength channels in opposite directions to minimize the effect of optical crosstalk. Here optical crosstalk is similar to its electrical counterpart, representing the leakage of one optical channel into another. Figure 6.15 illustrates an example of optical crosstalk. In examining Figure 6.15, you can note one of the problems facing DWDM developers as they attempt to add more channels to produce UDWDM systems. Specifically, as the spacing between channels narrows, crosstalk will increase. One method being examined to circumvent nar- rowing channel space is to use new wavelengths, such as those in the L band. Thus, work on 128-, 256-, and even 1024-channel DWDM systems that are referred to by many people in the industry as UDWDM are based on the use of wavelengths in the L band. Figure 6.15 Optical crosstalk rep- resents the leakage of one optical channel into another. Power Wavelength Chapter Six Alternative Protocol Stacks Now that we have an appreciation for WDM and DWDM, we will con- clude our discussion of fiber in the WAN by briefly examining the emergence of alternative protocol stacks to direct SONET and SDH in the WAN that can result in a more efficient transmission capability. Those alternative protocol stacks include ATM over SONET/SDH, IP over SONET/SDH, and IP directly over fiber. Figure 6.16 illustrates these three approaches to obtaining an enhanced transport capability. The first alternative protocol stack shown in Figure 6.16, ATM over SONET/SDH, adapts all services to ATM cells, which are then transport- ed via an existing SONET/SDH frame structure. Because ATM provides true quality of service (QoS) and represents a cell-based delivery service, it can better handle bursty traffic from LAN internetworking require- ments as well as T- and E-carrier circuit emulation. In fact, Telcordia, for- merly known as Bellcore, developed several standards for ATM over SONET/SDH. Two of those standards are GR-2837, w