In the digital communication transmission system, there are two digital optical transceiver series, one is called “Plesiochronous Digital Hierarchy” (Plesiochronous Digital Hierarchy), referred to as PDH optical transceiver; the other is called “Synchronous Digital Hierarchy” (Synchronous Digital Hierarchy), referred to as SDH optical transceiver.

The system using the plesiochronous digital series (PDH) is to set a high-precision clock on each node of the digital communication network, the signals of these clocks all have a uniform standard rate. Although the accuracy of each clock is very high, there are always some slight differences. In order to ensure the quality of communication, the difference between these clocks must not exceed the specified range. Therefore, this synchronization method is not strictly true synchronization, so it is called “quasi-synchronous.”

In the past telecommunications networks, most of the PDH optical transceiver equipment was used, this series has good adaptability to traditional point-to-point communication.With the rapid development of digital communication, there are fewer and fewer point-to-point direct transmissions, and most digital transmissions have to be transferred, therefore, the PDH series cannot meet the needs of modern telecommunication service development and modern telecommunication network management. SDH is a transmission system that has emerged to meet this new need.