In a digital communications system, the signals transmitted are digitized pulse trains. When these digital signal streams are transmitted between digital switching devices, their rates must be exactly the same in order to ensure the accuracy of information transmission, which is called "synchronization".

In the digital transmission system, there are two kinds of digital transmission series, one is called "quasi-synchronous digital series" (Plesiochronous Digital Hierarchy), referred to as PDH; the other is called "synchronous digital series" (Synchronous Digital Hierarchy). (Synchronous Digital Hierarchy), referred to as SDH.

The system using the quasi-synchronous digital hierarchy (PDH ) is set up at each node of the digital communication network with high-precision clocks, and the signals of these clocks have a uniform standard rate. Despite the high accuracy of each clock, there are always some minor differences. In order to ensure the quality of communication, it is required that the difference of these clocks cannot exceed the specified range. Therefore, this type of synchronization is not strictly speaking true synchronization, so it is called "quasi-synchronization".

In the past, PDH equipment was mostly used in telecommunication networks. This type of series has good adaptability to the traditional point-to-point communication. With the rapid development of digital communications, point-to-point direct transmission is less and less, and most of the digital transmission has to go through the transfer, so the PDH series can not be suitable for the development of modern telecommunications services, as well as the needs of the modern telecommunications network management, SDH is adapted to the needs of this new transmission system.

The earliest proposed SDH concept is the U.S. Bell Communications Research Institute, known as optical synchronous network (SONET). It is a high-speed, high-capacity fiber optic transmission technology and highly flexible, but also easy to manage and control the organic combination of smart network technology. The initial purpose is to achieve standardization on the optical path, so that the products of different manufacturers can interoperate on the optical path, thus improving the flexibility of the network.

In 1988, the Consultative Committee for International Telegraph and Telephone (CCITT) accepted the concept of SONET, renamed "synchronous digital series (SDH )", so that it is not only applicable to fiber optic, but also applicable to microwave and satellite transmission technology system, and to make its network management functions greatly enhanced.

SDH technology and PDH technology, compared with the following obvious advantages:

1, a unified bit rate, a unified interface standard for the interconnection between different manufacturers of equipment provides a possibility. The attached figure shows the comparison between SDH and PDH in terms of multiplexing levels and standards.

2. The network management capability is greatly enhanced.

3、The new concept of self-healing network is proposed. The ring network form with self-healing protection capability formed by SDH equipment can automatically restore normal communication through self-healing network when the main signal of transmission media is cut off.

4, the use of byte reconnection technology, so that the network up and down the branch signal becomes very simple.

As SDH has the above significant advantages, it will become one of the basic technologies to realize the information superhighway. But in the branch and fork road connected with the information highway, PDH equipment will still have a place.

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