his section by focusing on common SONET and SDH topologies. Overview Both SONET and SDH represent optical technologies based on byte multiplexing. This means that each SONET or SDH node includes a time division multiplexer that is responsible for forming and demulti- plexing the applicable optical signal. Although the cost of such equip- ment considerably declined during the late 1990s, it is still substantial. In addition, as we will note while reviewing the composition of SONET and SDH frames, their structure is quite complex and includes a consid- erable amount of overhead. Therefore, some carriers are beginning to bypass SONET and SDH and transport data directly over multiple opti- cal channels created via WDM and DWDM. However, both SONET and SDH provide plenty of bang for the buck because they can support a ring topology, where a fiber cut can be almost instantaneously compen- sated for by rerouting of the optical signal in a reverse direction around the ring. This restore capability is critical for customers of many com- munications carriers since the Joe Backhoe effect could otherwise dis- rupt the ability of hundreds of thousands to millions of customers. Thus, we can expect both SONET and SDH to remain viable technolo- gies for the foreseeable future. The SONET Transmission Structure SONET can be viewed as having a two-dimensional structure. In the horizontal dimension, SONET defines the method of data transmission between repeaters and other devices where an optical signal is transmit- ted and received. In the vertical dimension SONET defines a bit-rate hierarchy formed through the process of byte multiplexing Figure 6.4 illustrates the line structure of SONET. The SONET line structure was developed in recognition of the fact that an optical signal can flow through many intermediate devices on its path between Chapter Six source and destination. This resulted in the development of the layered line structure shown in Figure 6.4. Note that this structure enables the subdivision of the transmission path i