The difference between SDH and PDH

SDH is an upgrade of PDH. PDH (Plesiochronous Digital Transmission System) is the main transmission system used in the Internet. However, with the rapid development of the Internet industry, the amount of information transmitted, exchanged and processed through the network. In the geometrical rise, the PDH transmission system can no longer meet the existing applications, so the SDH transmission system came into being.

PDH optical transmission equipment, in digital communication systems, the signals transmitted are digital pulse sequences. When these digital signal streams are transmitted between digital switching devices, the rate must be completely consistent to ensure the accuracy of the information transmission. This is called "synchronization".

In the digital transmission system, there are two series of digital transmissions, one called "Plesiochronous Digital Hierarchy" (PDH); the other is called "Synchronous Digital Hierarchy" (SDH).

The difference between the two is in the degree of synchronization. In general, SDH synchronization is complete and accurate, while PDH synchronization is incomplete and inaccurate.

Compared with PDH equipment, SDH equipment has the following advantages:

1. Uniform bit rate, unified interface standard.

2. Network management capabilities have been greatly enhanced.

3. A new concept of self-healing network is proposed. With the ring network form with self-healing protection capability composed of SDH equipment, normal communication can be automatically resumed through the self-healing network when the transmission medium main signal is cut off.

4, using byte multiplexing technology, making the upper and lower branch signals in the network become very simple.

5. The SDH frame structure has a rich overhead ratio, which greatly enhances the operation and maintenance management functions of the network, facilitates centralized and unified management, and greatly saves maintenance costs.

6. Reusable structure with wide adaptability. PDH only supports point-to-point connection, which constitutes a segment of a linear network, and can only perform segment protection. It cannot realize ring protection of multiple routes for unified work, and lacks network topology flexibility.

7.The redundancy function of the network that means when the traffic channel is damaged and the communication is interrupted, the network will automatically switch the service to the channel of the standby service, so that the service can be obtained in a relatively short time to resume normal communication.