SONET, the acronym for synchronous optical network, is currently the prevailing standard for high-speed digital transmission in North America. Introduced in the 1980s, it replaced an earlier standard, the plesiochronous digital hierarchy (PDH), which had been in place for more than two decades prior to the introduction of the SONET standard [Ballart+89]. The most frequently used lower levels of the PDH system are the DS1 (1.544 Mbps, designed to carry 24 64-Kbps digitized voice signals plus synchronizing overhead) and DS3, running at 44.736 Mbps. An architecture similar to SONET, the synchronous digital hierarchy (SDH), is currently used in Europe and Japan, replacing an earlier European hierarchy similar to the PDH system. SONET can carry PDH bit streams as well as many other types of digital traffic (e.g., ATM cells) as part of its payload. One of the most important features of SONET is its highly organized protection capability [Wu92].

The basic building block (i.e., the first level) of the SONET signal hierarchy is called the synchronous transport signal-level 1 (STS-1). STS-1 has a bit rate of 51.84 Mbps and is divided into two portions, transport overhead and information payload, and the transport overhead is divided further into line and section overheads. (A line is composed of one or more sections in series, separated by electronic regenerators.) The line overhead is terminated at SONET terminals and add/drop multiplexers (ADMs), and the section overhead is terminated at regenerators.