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.

SDH (Synchronous Digital Hierarchy), defined according to ITU-T recommendations, is the transmission of digital signals at different speeds to provide information on the structure of the corresponding level, including the multiplexing method and mapping method, as well as the associated synchronization method consisting of a technical system.

The main advantage of OTN is fully backward compatible, it can build on existing SONET / SDH management functions on the basis of not only the existence of a fully transparent communication protocol, but also for WDM-end connectivity and networking capabilities, It provides a light layer interconnect specification ROADM, and added sub-wavelength aggregation and grooming.

In 1988, the United States launched the first of a digital transmission standard, the entire network clock synchronization levels are from a very precise master clock (using expensive cesium atomic clock, accuracy better than plus or minus 10 -11 th).