Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower rates can also be transferred via an electrical interface,Synchronous optical network (SONET) offers cost-effective transport both in the access area and core of the network. telephone or data switches rely on SONET transport for interconnection.The optical layer provides the foundation of transport services for both metro and long-haul applications. It also directly supports data services. The optical layer is now evolving to provide the same level of sophistication that has been achieved with synchronous transmission, such as performance monitoring and network resilience (i.e., the ability to recover quickly from a link being cut). Thus, the SONET layer will remain for some time. Figure 1 illustrates the access, switching, packet multiservice, SONET, and optical layers in the telecommunications network.