What does SDH mean in communication? Where is PDH?

In the digital communication system, the transmitted signals are all digital pulse sequences. When these digital signal streams are transmitted between digital switching devices, their rates must be completely consistent 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 hierarchy" (PDH for short); The other is called "synchronous digital hierarchy" (SDH for short). In the quasi-synchronous digital hierarchy (PDH) system, high-precision clocks are set at each node of the digital communication network, and the signals of these clocks have a uniform standard rate. Although the accuracy of each clock is high, there are always some subtle differences. In order to ensure the communication quality, the difference between these clocks is required not to exceed the specified range. Therefore, this synchronization method is not really synchronization strictly, so it is called "quasi-synchronization". In the past telecommunication networks, PDH devices were mostly used. This series has good adaptability to traditional point-to-point communication. With the rapid development of digital communication, point-to-point direct transmission is less and less, and most digital transmission has to go through transit, so PDH series can no longer meet the needs of modern telecom business development and modern telecom network management. SDH transmission system seems to meet this new demand. The concept of SDH was first put forward by Bell Communication Research Institute in America, and it is called SONET. It is an organic combination of high-speed, large-capacity optical fiber transmission technology and highly flexible, easy to manage and control intelligent network technology. The original purpose is to standardize the optical path, so that products from different manufacturers can communicate on the optical path, thus improving the flexibility of the network. In 1988, CCITT accepted the concept of SONET and renamed it "Synchronous Digital Series (SDH)", which made it suitable not only for optical fiber, but also for microwave and satellite transmission technology systems, and greatly enhanced its network management function. Compared with PDH technology, SDH technology has the following obvious advantages: 1, unified bit rate and unified interface standard, which makes it possible to interconnect devices from different manufacturers. The attached figure shows the comparison of SDH and PDH in multiplexing levels and standards. 2. The network management ability has been greatly enhanced. 3. A new concept of self-healing network is proposed. The ring network with self-healing protection function composed of SDH equipment can automatically restore normal communication through the self-healing network when the main signal of the transmission medium is cut off. 4. Byte multiplexing technology is adopted to make the signals of the upper and lower branches in the network very simple. SDH will become one of the basic technologies to realize information expressway due to its obvious advantages. However, PDH equipment will still be useful in branches and forks connected with information expressway.