nto distinct entities, thus facilitat- ing SONET operation, administration, maintenance, and provisioning, a set of conditions referred to by the acronym OAM&P. Through this subdivision it becomes easier to refer to a portion of a path instead of a complete path, which facilitates such operations as performance mea- surements, testing, and troubleshooting on a segment basis. This, in turn, can expedite the isolation of problems to a specific area. In examining Figure 6.4, note that the lowest layer of SONET is a sec- tion. A section in the SONET context represents a transmission path between two repeaters or other locations where the optical signal is transmitted and received. At the next-higher layer, the line represents a unit of SONET transmission capacity in terms of a Synchronous Trans- port Signal (STS), whose operating rates we will examine shortly. At the top of the line hierarchy is the path, which represents the end-to-end transmission of an STS payload by SONET. As we will note when we examine the composition of the STS frame format, a mechanism was created within frames to enable various OAM&P functions to be per- formed, such as providing a voice channel to be used by technicians. The SONET Transmission Hierarchy Under SONET the fundamental digital signal is referred to as the Synchro- nous Transport Signal, level 1 (STS-1) and has an operating rate of 51.84 Mbits/s. Higher-level STS signals have bit rates that correspond to multiples of the fundamental STS-1 signal. Those higher-level signals are formed through the process of byte-interleaved multiplexing of lower-level signals. Repeater Repeater Figure 6.4 SONET'S layered line structure. Payload Payload Source Terminal Terminal Destination Section Section Fiber in the WAN The actual STS signals refer to a sequence of bits and should not be considered as representing an electrical or optical signal. Thus, they more approximately represent an abstract signal. For each STS signal, there exists a corresponding optical transmission signal, referred to a