What is PDH?

In 1988, ITU-T modified the concept, remaned it as synchronous digital hierarchy(SDH), making it not only applies to optical fiber communication, also suitable for microwave and satellite transmission system. SDH has become a new generation of ideal transmission network system with internationally recognized.

The minimum grading of SDH is 155.520 Mb/s, which is known as the basic transmission module, using STM 1 to express. STM - N stands for N * 155.520 Mb/s optical module, N is typically take 1, 4, 16, 64, 256.

 

In the departed telecommunication network, many people use PDH. This series of traditional point-to-point communication has good adaptability. And with the rapid development of fiber optic digital communication, direct point-to-point transmission is on the decreasing , but most of them need to  go through the transfer. So the digital transmission and PDH series cannot meet the needs of modern telecom business development, as well as the needs of modern telecommunication network management. That is how SDH transmission system come out.

1. Unified bit rate. Uniform interface standard make the interconnection between different equipment available.
2. Greatly enhanced network management ability
3. Put forward the new concept of self-healing network. With SDH equipment with self-healing ring network, it can be automatically self-healing network back to normal communication when the main transmission media signal is cut off.
4. Using byte multiplex technology to make the network simple for the branch signal.
5. Flexible multiplexing structure to make various business flexibility.
6. SDH equipment seeks to accommodate a variety of new businesses, for example, broadband ISDN/FDDI/ATM.
7. Arranged in the frame structure of SDH is with abundant overhead ratio, so that the operation of the network maintenance and management capability are greatly strengthened. It is cost saving.

Because the SDH has such significant advantages that it will become one of the basic technology for achieving Information Highway.

1. PDH was designed for voice services for especially, while the trend of modern communication is broadband, intelligent and personalized.
2. PDH transmission lines are mainly the point-to-point connections, which lacking of flexibility of network topology.
3. There are two categories, three regional standards(Japan, North America, Europe). It is difficult to realize the international communication.
4. Lack of unified standard optical interface, unable to realize compatible transverse.
5. Channels for network management are in sufficient. It is difficult to establish centralized transmission network.

With increasingly competition in telecom market, how to make use of network and it’s bandwidth resources has become the goal of each operator. As a kind of effective and economical products, PDH become the apple to solve the last 1KM of the edge network.

But along with the network development and the change of user requirements, especially the big customer’s private network to speed up the construction, that on the edge of the transmission network business interface increased a lot. The traditional PDH + association, in addition to the room conditions and construction cost, also produce new problems such as scheduling layered network security management and business.

1. With SDH group of PDH equipment centralized, to realize SDH/PDH network management.
2. Launching significant PDH equipments(8HS-VC/I), to realize transfer and the combination of PDH.
3. Launching 8 HS - DVD series with Ethernet interface, to realize fiber transceiver and the combination of PDH equipment.
4. Launching 8HS-VE series with a variety of small words and data interface, to realize the combination of PCM and PDH.