To put it simply, SDH is an upgraded version of PDH. PDH (Pseudo-Synchronous Digital Transmission System) is the main transmission system used in the Internet in the past, but now with the rapid development of the Internet industry, the amount of information transmitted, exchanged, and processed through the network On the rise of the geometric level, the PDH transmission system can no longer meet the existing applications, so the SDH transmission system emerges as the times require.
Introduction of PDH and SDH
PDH (PlesiochronousDigitalHierarchy, quasi synchronous digital series) optical multiplexer is a large-scale integrated circuit consisting of 120 photoelectric integrating transmission equipment, application is usually in pairs, also called the point-to-point applications, capacity is commonly 4 e1, 8 e1, 16 e1; It is suitable for small-capacity switch network, user ring network, mobile communication (base station), private network, DDN network, etc. Its main characteristics are: using a very large-scale integration chip, with low power consumption, and high-reliability characteristics. The E1 remote loopback test function is provided for easy maintenance. 120 Provides 4E1 channels. Complete alarm function, showing local and remote alarms. Alarm information (including power failure alarms) can be reported to the peer end through the NMS channel. Adopt transceiver integrated optical device, stable and reliable performance. The veneer design is compact and easy to use.
SDH optical multiplexer (SynchronousDigitalHierarchy, synchronous digital series) capacity is larger, it is 16 e1 to 4032 e1. SDH synchronous digital series, all network elements have PDH interfaces, which can be said to be compatible with PDH; due to the addition of a large number of management overhead bytes, it can form a loop and can be monitored. In theory, the products of various manufacturers are compatible with each other and can be interconnected for transmission. But there are still many problems in the actual use process.
The difference between PDH and SDH
1. PDH does not have a unified optical interface, and equipment from various manufacturers cannot be interconnected, but SDH can.
2. The PDH optical transceiver generally transmits a maximum signal of 140M (higher rate signals are difficult to transmit), while the minimum signal transmitted by the SDH optical transceiver is 155M.
3. PDH is quasi-synchronous and SDH is synchronous.
4. There is no unified worldwide standard for PDH, while SDH obtains a worldwide unified standard at the STM-1 level.
5. PDH is mainly designed for voice services, while modern communication requires broadband and intelligence.
6. The PDH transmission line is mainly a point-to-point connection, which lacks networking flexibility. It is difficult to realize self-healing; while SDH equipment adopts add-drop multiplexing, and digital cross-connection greatly enhances networking capability and self-healing capability.
Advantages of SDH
1. The network management capability is greatly enhanced.
2. A new concept of a self-healing net is proposed. The ring network form with self-healing protection ability composed of SDH equipment 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. The accompanying drawing is a comparison of multiplexing levels and standards between SDH and PDH.
4. The use of byte multiplexing technology makes the upper and lower branch signals in the network very simple.
Well, the above is the main difference between SDH and PDH. SDH and PDH multiplexers are mainly used by telecom operators. In addition, SDH can replace PDH, but SDH cannot replace PDH.