In the last article, we introduced that the development of microwave communication systems has also undergone analog-to-digital conversion like other radio communication systems.
Each transceiver of the analog microwave system can work in 60-channel, 960-channel, 1800-channel or 2700-channel communication, and can be used for microwave circuits of different capacity levels. But due to technological progress, the analog system has completely withdrawn from the market. The digital microwave system uses digital multiplexing equipment to form a primary group based on the principle of time division multiplexing of 30 telephones, and then can form a secondary group of 120, a cubic group of 480, and a quartic group of 1920. After that, it is modulated on the transmitter through a digital modulator, and is restored to a multi-channel telephone at the receiving end through a digital demodulator.
In the digital transmission system, there are two digital transmission systems, one is called “Plesiochronous Digital Hierarchy”(PDH), another one is called “Synchronous Digital Hierarchy”(SDH).
The PDH system is to set high-precision clocks on each node of the digital communication network, and the signals of these clocks have a uniform standard rate. Although the accuracy of each clock is high, there are always some small differences. In order to ensure the quality of communication, it is required that the difference of these clocks cannot exceed the specified range. Therefore, this synchronization method is not strictly speaking true synchronization, so it is called “Plesiochronous“.
There are two standards for the PDH system series, namely the E series in Europe and the T series in the United States. The main purpose is to insert a certain number of pulses into each branch signal to achieve synchronization. The interconnection of the two systems requires corresponding signal processing and conversion.
In the past telecommunication network, PDH equipment is often used, and this series has better adaptability to the traditional point-to-point communication. With the rapid development of digital communication, there are fewer and fewer direct point-to-point transmissions, and most of the digital transmissions have to be switched. Therefore, the PDH system cannot meet the needs of the development of modern telecommunication services and the management of modern telecommunication networks. SDH is a transmission system that emerges to meet this new need.
The disadvantages of PDH increasingly limit the improvement of communication system performance, mainly in the following aspects:
1.Difficulty in international communication
2.There is no unified optical interface standard, and it is difficult for equipment to be laterally compatible
3.Plesiochronous multiplexing method, inconvenient for upper and lower circuits
4.The network management ability is weak, and it is difficult to establish a centralized telecom management network
The concept of SDH was first proposed by the Bell Institute of Communication in the United States, and it is called Optical Synchronous Network (SONET). It is an organic combination of high-speed, large-capacity optical fiber transmission technology and highly flexible intelligent network technology that is easy to manage and control. The original purpose is to achieve standardization on the optical path, so that the products of different manufacturers can communicate with each other on the optical path, thereby improving the flexibility of the network. In 1988, the Consultative Committee for International Telegraph and Telephone (CCITT) accepted the concept of SONET and renamed it “Synchronous Digital Hierarchy (SDH)”, making it applicable not only to optical fiber, but also to the technical system of microwave and satellite transmission, and the network management function is greatly enhanced.
Compared with PDH technology, SDH technology has the following significant advantages:
1.The network management capability has been greatly enhanced.
2.A new concept of self-healing net is proposed.
Benefit from the ring network formed with self-healing protection ability composed of SDH equipment, it can automatically restore normal communication through the self-healing network when the main signal of the transmission medium is cut off.
3.The unified bit rate and the unified interface standard provide the possibility for the interconnection between devices of different manufacturers.
4.The use of byte multiplexing technology makes the upper and lower branch signals in the network very simple.
5.The use of byte multiplexing technology makes the upper and lower branch signals in the network very simple.
As SDH has the above-mentioned remarkable advantages, it replaces PDH and has been widely used in the initial construction of the information highway, but PDH equipment will still be useful in the branch roads and fork roads connected with the information highway.
