First of all, PCM equipment and PDH equipment are completely different devices. PCM is integrated service access equipment, and PDH equipment is optical transmission equipment.

The digital signal is produced by sampling, quantizing and encoding the continuously changing analog signal, which is called PCM (pulse code modulation), that is, pulse code modulation.This kind of electrical digital signal is called a digital baseband signal, which is generated by a PCM electrical terminal. Current digital transmission systems all use pulse-code modulation (Pulse-code modulation) system. PCM was not originally used to transmit computer data, but to have a trunk line between switches instead of only transmitting a telephone signal.

PDH optical transmission equipment, in the digital communication system, the transmitted signals are all digitized 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. This is called “synchronization.” In the digital transmission system, there are two digital transmission series, one is called “Plesiochronous Digital Hierarchy” (Plesiochronous Digital Hierarchy), abbreviated as PDH; the other is called “Synchronous Digital Hierarchy” (Synchronous Digital Hierarchy), abbreviated as SDH.

With the rapid development of digital communications, there are fewer and fewer point-to-point direct transmissions, and most digital transmissions have to be switched. Therefore, the PDH series cannot meet the needs of modern telecommunications business development and the needs of modern telecommunications network management. . SDH is a transmission system that has emerged to meet this new need.