Difference Between PDH and SDH

I. PDH
  PDH is the Plesiochronous Digital Hierarchy, divided into the European standard and the US/Japan standard.
  The European standard uses 2M as the base group, the second-order group at 8M, the third-order group at 34M, and so on.
  The US/Japan standard uses 1.5M as the base group, the second-order group at 6.3M, the third-order group at 32M, 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 and US/Japan PDH standards can be conveniently mapped into STM frames.
  III. Comparison between PDH and SDH
  1. Shortcomings 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. These mainly include:
  (1) Complex frame structure. For high-order group digital streams, all multiplexing/demultiplexing operations must be performed. Even to insert or extract a single tributary, multiple stages of frame synchronization, bit rate adjustment, and code pattern conversion are required.
  (2) Different multiplexing levels and different standards 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 to overcome the above shortcomings. Its main features include:
  (1) Flexible add/drop of circuits. Lower-order STM streams and PDH streams of any level can be directly inserted into or 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 has integrated transmission and digital cross-connect into a single platform, forming a powerful network transmission platform.
  (3) Powerful network management functions. SDH has abundant overhead bytes, greatly enhancing 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 features 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 Navigation" is not far off.
  If the main difference between SDH and PDH technologies is compared to railway transport, PDH technology is like a bulk freight train, where various goods (services) are piled inside the cars. To remove a specific package (a specific transmission service) at a certain station, all goods on the train must first be unloaded, the required package found, and then the remaining goods along with newly loaded goods at that station must be reloaded one by one and transported away. Therefore, PDH technology requires a large number of multiplexing equipment for each order group wherever add/drop of circuits is needed. SDH technology, on the other hand, is like a container train, where various goods (services) are labeled (various overheads) and loaded into containers. Small boxes are then placed into larger boxes, level by level. 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 accurately indicate which car and which level of box a package is in). Therefore, only in SDH can add/drop of circuits be achieved simply.
  Therefore, it can be confidently stated that the upcoming information superhighway will be largely composed of SDH equipment, while only the branch and side roads connected to the superhighway (SDH) will retain some PDH equipment.

 A digital transmission system transmits and receives digital signals one after another in a synchronized rhythm, which is called synchronization.
  A digital transmission system transmits information using time-division multiplexing after 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 independent 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 may deviate from their nominal values, so they are also called plesiochronous signals.
  In 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 rapid recovery of each tributary to restore the original tributary signals.
  There are two main types of time-division multiplexing transmission systems currently used in communications: the Plesiochronous Digital Hierarchy (PDH) and the Synchronous Digital Hierarchy (SDH).
  PDH has no worldwide optical interface specification. Each manufacturer 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) set up high-precision clocks at each node of the digital communication network, and the signals from these clocks all have a unified standard rate. Although the accuracy of each clock is very high, there are still slight differences. To ensure communication quality, the differences between these clocks must not exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, hence it is called "plesiochronous."

In previous telecommunications networks, PDH equipment was widely used. This series has good adaptability to traditional point-to-point communications. However, with the rapid development of digital communications, point-to-point direct transmission has become increasingly rare, and most digital transmissions now require transit. Thus, the PDH series can no longer meet the needs of modern telecommunications service development or the management requirements of modern telecommunications 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, where it was 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 intelligent network technology. The initial goal was to standardize optical paths to enable products from different manufacturers to interconnect on optical paths, thereby improving network flexibility.

In 1988, the International Telegraph and Telephone Consultative Committee (CCITT) accepted the SONET concept and renamed it the "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 capabilities.

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

1. Network management capabilities are greatly enhanced.

2. A new concept of self-healing networks is proposed. Ring networks with self-healing protection capability composed of 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. Byte-interleaved multiplexing technology makes the add/drop of tributary signals in the network extremely simple.

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