in the 1,550-nm range that can be multiplexed with other similarly constrained wavelengths. The translation device is sometimes called a transponder. The original DWDM systems were optimized for long-haul interex- change applications; therefore, these WDM systems support mainly point- to-point or linear configurations. Although these systems provide fiber capacity relief, the management of add, drop, and pass-through traffic must be done manually. Current-generation DWDM products can perform linear add/drop multiplexer (ADM) functions in long-haul or interexchange appli- cations. These products also support ring topologies; ring topologies increase the reliability and the survivability of the user-level connection. Products that support ring and/or mesh configurations are more suited to metro applications. However, current-generation systems still lack the full flexibility and affordability that is required in metro applications. Because the wave- lengths that are added and dropped are fixed, there is no mechanism to increase the add/drop capacity without physically reconfiguring the system. Figure 5-21 SONET SONET Terminal Terminal Terminal Terminal Simple WDM system. Optical Line Amplifier SONET SONET Terminal Terminal SONET SONET Terminal Optical Optical Terminal Coupler Splitter/ Transponder Filter Dense Wavelength Division Multiplexing (DWDM) Systems Networking in a TDM network is provided by multiplexers, such as tradi- tional linear and ring ADMs and crossconnects. These NEs must convert the optical signal to an electrical signal before performing their specific function and then convert the result back to an optical signal for transmis- sion on the next span. This O-E-O conversion is a major cost component of these network elements. Photonic networking is the next-generation application of WDM tech- nology. Future systems providing end-to-end photonic networking will have the capability to manage individual nonfixed wavelengths utilizing tunable lasers. Optical add/drop capability offers the ability to ad