oning, monitoring, and controlling the raw transport capacity of the whole network. The Basis of SONET and SDH Both SONET and SDH standards are designed to transport isochronous traffic channels and are focused very much on layer 1 of the well-known ISO seven-layer OSI protocol hierarchy (see Volume 1, The Cable and Telecommnuications Professionals' Reference). They are based on a hierarchy of continuously repeating, fixed-length frames originally designed to support predominantly voice traffic and to interwork with the earlier Plesiochronous Digital Hier- archy (PDH) standards. As the standards were developed, it was apparent that the increas- ingly widespread deployment of optical fibers would rapidly reduce the cost of raw transmission bandwidth. Hence, during the development of SDH standards, there was rela- tively little pressure to restrict the proportion of bandwidth devoted to multiplexing over- heads, as opposed to the traffic payload. It was necessary to adopt not only a synchronous frame structure but one that also preserved the byte boundaries in the various traffic bitstreams. Consequently, the basic frame structure is one that repeats at intervals of 125 us; that is, SDH and SONET are tailor- made for the transport of 64-kbit/s channels or any higher-rate channels that are an integer multiple of 64 kbit/s. Mapping PDH into Synchronous Containers The SONET standards were designed around the need for efficient transport of the existing North American PDH rates, the most important of which are the 1.5- and 45-Mbit/s rates. This leads to a standard where a 45-Mbit/s signal is loaded into an Synchronous Transport Signal-1 (STS-1), which runs at 51.84 Mbit/s-that is, precisely one-third of the STM-1 rate. As with SDH, there is also a concatenation mechanism for dealing with customer signals that do not easily fit inside any of the already defined VC payload areas. This mechanism is particularly useful for carrying rates such as 140 Mbit/s, where the concatenation results in a SONET NNI signal known as an 