The Relationship between PDH, SDH and PCM

📅Oct 10, 2012

Brief：PDH (Plesiochronous Digital Hierarchy) is a system in which high-precision clocks are respectively set at each node of a digital communication network, and the signals of these clocks all have a unified standard rate. Although the precision of each clock is very high, there are still some slight differences. In order to ensure communication quality, the differences between these clocks are required not to exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, so it is called "plesiochronous".

PDH (Plesiochronous Digital Hierarchy) is a system in which high-precision clocks are respectively set at each node of a digital communication network, and the signals of these clocks all have a unified standard rate. Although the precision of each clock is very high, there are still some slight differences. In order to ensure communication quality, the differences between these clocks are required not to exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, so it is called "plesiochronous". SDH, like PDH, is a transmission network system, representing a transmission rate conversion mechanism and encapsulation structure. SDH (Synchronous Digital Hierarchy) is a comprehensive information transmission network that integrates multiplexing, line transmission and switching functions into one, and is operated by a unified network management system. It was proposed by Bell Communications Research as Synchronous Optical Network (SONET). SDH can achieve synchronization, so it is called "synchronous". Compared with PDH, SDH has the following advantages:
1. Unified bit rate and unified interface standards.
2. Greatly enhanced network management capability.
3. A new concept of self-healing network is proposed. The ring network form composed of SDH equipment with self-healing protection capability can automatically restore normal communication through the self-healing network when the main signal of the transmission medium is cut off.
4. The byte interleaving multiplexing technology is adopted, which makes the add/drop of tributary signals in the network very simple.
5. The SDH frame structure arranges abundant overhead proportions, thus greatly enhancing the operation, administration and maintenance functions of the network, facilitating centralized and unified management, and greatly saving maintenance costs.
  PCM is a pulse code modulation. In an optical fiber communication system, what is transmitted in the optical fiber are binary optical pulses "0" and "1", which are generated by on-off modulation of the light source by binary digital signals. Digital signals are generated by sampling, quantizing and encoding continuously changing analog signals, called PCM (Pulse Code Modulation), i.e., pulse code modulation. This electrical digital signal is called a digital baseband signal, generated by a PCM electrical terminal. Current digital transmission systems all adopt the Pulse Code Modulation system. PCM was not originally used for transmitting computer data, but to enable a trunk line between switches to transmit more than just one telephone signal. PCM has two standards (forms), namely E1 and T1.
  China adopts the European E1 standard. The T1 rate is 1.544Mbit/s, and the E1 rate is 2.048Mbit/s. Pulse code modulation can provide users with multiple services, including digital data dedicated line services at rates from 2M to 155M, as well as voice, image transmission, distance learning and other services. It is particularly suitable for users who require high data transmission rates and need higher bandwidth.

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