e greatest strengths of SDH/SONET and will The Transport Network Infrastructure Node A Primary Fiber, Clockwise Direction SDH/SONET Node D Node B DLC/ADM DLC/ADM Node C Protect Fiber, SDH/SONET Counterclockwise Direction Figure 4.8 SDH/SONET ring architecture likely keep it operational in networks for another 10 to 20 years. But these types of capabilities are also being introduced in the new generations of standards, such as WDM, and when that occurs, we will start to move away from SDH/SONET because SDH/SONET is a Time Division Multiplexing (TDM) system that does not take advantage of the fact that light can be spatially multiplexed, allowing multiple wavelengths to be carried over one fiber pair. Still, at this point, the flexibility of adding and dropping payload is one of the main reasons telcos still prefer SDH/ SONET over other options: They can more easily integrate SDH/SONET into their billing systems. Although more and more customers are requesting Gigabit Ether- net, few telcos provide it. SDH/SONET is also important because it grooms and routes traffic. Grooming means that SDH/SONET selectively removes channels from a digital facility for routing to a designated remote location via another digital facility; basically, it enables you to drop and add payload flexibly. SDH/SONET also provides for perfor- mance monitoring SO that you can understand the performance of the network, its components, and the congestion levels. The SDH/SONET Signal Hierarchy The SDH/SONET signal hierarchy deals with optical carrier (OC) levels, which refer to the optical aspect of the transmission-the optical pulse as it travels through the fibers. These optical pulses go through electronic muxes, and when the signal is going through these network elements, the bits are packaged in a frame for trans- port across the fiber. In the case of SONET, this frame is called the Synchronous Chapter 4 The PSTN Table 4.3 The SDH/SONET Signal Hierarchy Data Rate Payload Rate OC Level SONET (Mbps) (Mbps) STS-1 STM-0 51.48 50.840 STS-3 STM-1 