ABSTRACT
There are two main classes of telecoms-based digital links in worldwide use for transporting telephone and data signals. One system is called plesiochronous digital hierarchy (PDH) and is based on multiplexing numerous individual channels into higher rate channels. The PDH is not synchronous but “nearly synchronous.” The second link type depends on synchronous communications. The United States and Canada rely on SONET whereas the rest of the world uses the SDH. SONET stands for Synchronous Optical NETwork, and SDH is an acronym for Synchronous Digital Hierarchy. The ITU adopted SONET, with small variations, as the basis for the SDH. The SONET/SDH standards define a hierarchy of interface rates that permit different line rates to be multiplexed as with the PDH method.
