PDH Overview

PDH optical transmission equipment. In digital communication systems, the transmitted signals are digitized pulse sequences.

When these digital signal streams are transmitted between digital switching devices, their rates must remain perfectly consistent to ensure accurate information transmission. This is called "synchronization."

In digital transmission systems, there are two digital transmission series: one is called the "Plesiochronous Digital Hierarchy," abbreviated as PDH; the other is called the "Synchronous Digital Hierarchy," abbreviated as SDH.

Systems adopting the Plesiochronous Digital Hierarchy (PDH) have high-precision clocks set at each node of the digital communication network, and the signals from these clocks all have a unified standard rate. Although the precision of each clock is very high, there are still some minor differences. To ensure communication quality, these clock differences are required not to exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, hence it is called "plesiochronous."

In previous telecommunication networks, PDH equipment was widely used. This series has good adaptability to traditional point-to-point communication. However, with the rapid development of digital communications, direct point-to-point transmission is becoming increasingly rare, and most digital transmissions require relay. Thus, the PDH series can no longer meet the needs of modern telecommunication service development and modern telecommunication network management. SDH is the transmission system that emerged to meet these new requirements.