In 1985, Bellcore proposed SONET (Synchronous Optical Network) standard, the American National Standards Institute (ANSI) standards through a series of SONET 1989, CCITT SONET accepted concept and developed a SDH (Synchronous Digital Hierarchy) standard, making it not only suitable for fiber optic technology is also suitable for general microwave and satellite transmission system, there are nuances and SONET, SDH / SONET defines a set of rate and format of optical fiber transmission optical signals, generally referred to as the optical synchronous digital transmission network, a broadband one of the foundations integrated digital network of B-ISDN .SDH / SONET uses TDM technology, is a synchronous system, the master clock control, accuracy of 10 ^ -9. both for backbone transport is to follow the application of quasi-synchronous digital series PDH (Plesiochronous Digital Hierarchy) a revolution.

SONET and more for North America and Japan, SDH and more for China and Europe.