incorporating highly valued SDH/SONET-like capabilities, including monitoring performance, providing protection, and provisioning optical channels. As mentioned in Chapter 4, SDH/SONET introduced the network sur- vivability tactic. The dual-counter-rotating rings provide a protected fiber path over which information can be shunted in the opposite direction if a fiber ring is broken. Until recently, DWDM had no such capability. It was deployed as a point- to-point link; if the fiber was cut, communication between the two DWDM sys- tems was lost. But now we are beginning to see the introduction of restoration capabilities in DWDM platforms, which means that SDH/SONET will have a more limited life in the future. Industry forecasts predict that SDH/SONET has perhaps a 10-year lifespan left, after which the benefits of DWDM will override the reliability factors that we today associate with SDH/SONET. Remember that SDH/SONET is a TDM system, and therefore it cannot take advantage of the capacity gains that DWDM systems provide. A different consideration emerges as DWDM systems continue to develop. Because of a combination of nonlinearities and dispersion, the majority of the fiber currently in place around the world-possibly 95% of it-would have trouble car- rying very fast (Tbps speed and Pbps pulses) signals for long distances in a DWDM system. These impairments that exist in current fiber can lead to crosstalk among the different wavelengths, interference between consecutive pulses on any signal wavelength, and degradation in the overall signal-to-noise ratio. This means that much of the fiber we have deployed over the past two decades will have to be replaced in order to take advantage of the new generation of optical equipment. Fiber solutions exist today, but time and financial resources will be needed to deploy them. Where DWDM Fits in the Network Architecture The core network was the first place DWDM was deployed because that is where the economics made the most sense. Increases in intercity traffic required ca