What is the difference between PDH and SDH?

I. PDH
PDH is the Plesiochronous Digital Hierarchy, which is divided into the European standard and the American/Japanese standard.
The European standard uses 2M as the base group, 8M as the second-order group, 34M as the third-order group, and so on.
The American/Japanese standard uses 1.5M as the base group, 6.3M as the second-order group, 32M as the third-order group, and so on.
The European standard is the internationally accepted standard, and China adopts this standard.
II. SDH
SDH is the Synchronous Digital Hierarchy, for which CCITT has established a globally unified standard.
The basic SDH frame is STM-1, with a rate of 155M; the second-order multiplexing frame is STM-2, with a rate of 622M; the third-order multiplexing frame is STM-3, with a rate of 2.5G; the fourth-order multiplexing frame is STM-4, with a rate of 10G.
SDH is compatible with PDH, and all order groups of both the European PDH standard and the American/Japanese standard can be conveniently mapped into STM frames.
III. Comparison between PDH and SDH

1. Disadvantages of the PDH system
   It should be noted that PDH has contributed to the development of digital networks and will continue to develop for some time. However, in modern digital networks that require flexible structures and intelligent management systems, its shortcomings become evident. The main ones are:
   (1) Complex frame structure. For high-order group digital streams, all multiplexing/demultiplexing operations must be performed. Even to insert/extract just one tributary, it must be done level by level, requiring multiple frame synchronizations, bit rate adjustments, and code pattern conversions.
   (2) Different multiplexing levels and different systems have different frame structures, making direct interconnection impossible.
   (3) There is no standardized auxiliary data (overhead) available to both users and network management personnel.
2. Advantages of SDH
   SDH was proposed precisely to overcome the above shortcomings. Its main features are:
   (1) Flexible add/drop of circuits. Lower-level STM streams and PDH streams of any level can be directly inserted/extracted from an STM stream of any N value, without the need for step-by-step multiplexing/demultiplexing operations.
   (2) Strong digital cross-connect capability. SDH digital cross-connect (DXC) is easier to implement, making it easier to develop networks with self-healing capability. Modern SDH equipment integrates transmission and digital cross-connect into one, forming a powerful network transmission platform.
   (3) Powerful network management functions. SDH has abundant overhead bytes, greatly improving network monitoring and maintenance management capabilities. Network monitoring, add/drop of circuits, and maintenance management are all controlled and operated by software, laying the foundation for centralized maintenance.
   (4) Rich data communication interfaces. The SDH optical equipment installed in our bureau has 10M/100M Ethernet interfaces and can also provide 1000M network interfaces. Optical and electrical interfaces of various capacities provide a network transmission and switching platform for high-capacity data communications. It can be foreseen that with the completion of the SDH optical fiber communication system, "Digital Yangtze River, Intelligent Shipping" is not far off.
   If the main difference between SDH technology and PDH technology is compared to railway transportation, PDH technology is like a bulk train, where various goods (services) are piled in the carriages. If you want to unload a specific package (a specific transmission service) at a certain station, you need to unload all the goods from the train first, find the one you need, and then reload the remaining goods and the new goods loaded at that station onto the train one by one and transport them away. Therefore, with PDH technology, wherever circuits need to be added or dropped, a large number of multiplexing equipment for various order groups must be deployed. SDH technology, on the other hand, is like a container train, where various goods (services) are labeled (various overheads) and loaded into containers. Then small boxes are loaded into larger boxes, level by level. In this way, through the labels at each level, a specific package can be accurately removed from a high-speed moving train without having to "rummage through the entire train" (the labels allow precise knowledge of which carriage and which level of box a specific package is in). Therefore, only with SDH can add/drop of circuits be simply implemented.
   Therefore, it can be said with certainty that the upcoming information superhighway will be essentially built with SDH equipment, and only the branches and side roads connected to the superhighway (SDH) will retain some PDH equipment.

A digital transmission system transmits and receives digital signals synchronously, beat by beat, which is called synchronization.
A digital transmission system transmits information in a time-division multiplexing manner after information encoding.
If the clocks of the tributary signals to be multiplexed come from the same clock source, and the tributary signals and the local timing signal are synchronized (i.e., the same clock source), such tributary multiplexing is called synchronous multiplexing.
If the clocks of the tributary signals to be multiplexed come from different clock sources (i.e., each has its own separate clock), and the tributary signals and the local timing signal are asynchronous, such multiplexing is called asynchronous multiplexing.
For signals from different sources, the bit rates of each tributary signal can deviate from their nominal values, so they are also called plesiochronous signals.
For plesiochronous digital hierarchy multiplexing, the bit rates of each tributary must be adjusted before multiplexing (adjusting the frequency and phase of each tributary signal) to make them synchronous signals before multiplexing. At the receiving end, synchronous separation is performed first, followed by fast recovery of each tributary to restore the original tributary signals.
There are two main types of time-division multiplexing transmission systems used in current communications: the Plesiochronous Digital Hierarchy (PDH) and the Synchronous Digital Hierarchy (SDH).
PDH has no worldwide optical interface specification. Each manufacturer independently develops its own line code patterns, and even at the same level, optical interfaces and rates differ, making horizontal interconnection impossible.
Except for a few low-level signals (North American 1.5Mbps, Japanese 1.5Mbps and 6.3Mbps, European 2Mbps) that use synchronous multiplexing, PDH uses asynchronous multiplexing for digital signals at other levels.
Hierarchical management cannot be achieved.

Systems using the Plesiochronous Digital Hierarchy (PDH) have high-precision clocks set at each node of the 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. To ensure communication quality, these clock differences are required not to exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, hence the name "plesiochronous."

In traditional telecommunication networks, PDH equipment was commonly used. This hierarchy has good adaptability to traditional point-to-point communications. However, with the rapid development of digital communications, direct point-to-point transmission is becoming less common, and most digital transmissions require transit. Therefore, the PDH hierarchy can no longer meet the needs of modern telecommunication service development or the management needs of modern telecommunication networks. SDH is the transmission system that emerged to meet these new needs.

The concept of SDH was first proposed by Bell Communications Research in the United States, called the Synchronous Optical Network (SONET). It is an organic combination of high-speed, high-capacity optical fiber transmission technology and highly flexible, easily manageable and controllable intelligent network technology. The original purpose was to achieve standardization on the optical path, enabling products from different manufacturers to interoperate on the optical path, thereby improving network flexibility.

In 1988, the International Telegraph and Telephone Consultative Committee (CCITT) accepted the SONET concept and renamed it "Synchronous Digital Hierarchy (SDH)," making it applicable not only to optical fiber but also to microwave and satellite transmission systems, while greatly enhancing its network management functions.

Compared with PDH technology, SDH technology has the following obvious advantages:

1. Network management capability is greatly enhanced.

2. A new concept of self-healing networks is proposed. Ring networks with self-healing protection capability formed by SDH equipment can automatically restore normal communication through the self-healing network when the main signal on the transmission medium is cut off.

3. Unified bit rates and unified interface standards make interconnection between equipment from different manufacturers possible. The attached figure shows a comparison of SDH and PDH in terms of multiplexing levels and standards.

4. The byte-interleaved multiplexing technology makes the add/drop of tributary signals in the network very simple.

Because SDH has the above significant advantages, it will become one of the fundamental technologies for realizing the information superhighway. However, on the branches and side roads connected to the information superhighway, PDH equipment will still have its place.