Early stages of development in digital communication, in order to adapt to the needs of the point-to-point communications, a large number of digital transmission system is quasi-synchronous digital hierarchy system (PDH), quasi-synchronization refers to a predetermined capacity at all levels of the bit rate relative to its standard valuebias, and the timing with the clock signal is not issued by a standard clock to

With the introduction of digital switching, the development needs of the construction of long-distance high-capacity digital circuit led to the development of optical communication technology, as well as network control and Broadband Integrated Services Digital Network (B-ISDN), exposed the existing quasi-synchronous digital the sequence exists some inherent weaknesses. : North America, Japan, Europe, three digital system is not compatible with each other; worldwide standard optical interface specifications, interoperability and deployment in the optical path can not; difficult upper and lower telephone channels; manage complex network maintenance, lack of flexibility, you can not to adapt to the evolving requirements of the telecommunications network.

With the rapid development of optical fiber communication technology and LSI, in 1986 the United States proposed an optical fiber communication based on the concept of Synchronous Optical Network (SONET), as the basic structure of the modern communication network. 1988 ITU-T SONET concept modify, re-named the Synchronous Digital Hierarchy, SDH for short, to become not only applies to optical fiber communication system, also suitable for microwave and satellite transmission. The SDH has now become internationally recognized as a new generation of the ideal transmission network system.

A rate signal for a wide range of information carried in the telecommunications network must first be a level of standardization, and then into a sequence of numbers, that become asynchronous or synchronous digital telecommunications network transmission sequence signal. SDH lowest hierarchical 155.520Mb / s, referred to as the basic transmission module, and with said STM-1. STM-N indicates the rate of the N × 155.520Mb / s transmission module, where N is generally taken 1,4,16,64,256.