Explain the difference between PDH and SDH!

This article explains the difference between PDH and SDH. PDH stands for Plesiochronous Digital Hierarchy and SDH stands for Synchronous Digital Hierarchy. There are lots of digital multiplexers associated with both PDH and SDH terminologies which can be exchanged. There is a combination of different hierarchies having different bit rates.


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The inputs of digital multiplexers are of same bit rate in PDH and are derived from various clocks from various oscillators. There is a difference inside the tolerance of few clock periods. Therefore it is considered plesiochronous.

Bit Interleaving is used in PDH to integrate digital signals.


There are two main standards in PDH that are 30 channel one used in Europe and 24 Channel used in Japan or North America. In North America, the basic rate is 64kbps and in Europe. In 30 channel case the rates derived from 2.048 Mbps basic rate are mentioned below:

2.048 x 4 gives 8.448 Mbps (120 channels)

8.448 x 4 gives 34.368 Mbps (480 channels)

34.368 x 4 gives 139.264 Mbps (1920 channels)

139.264 x 4 gives 564.992 Mbps (7680 channels)


The North American PDH level hierarchy is shown in figure 1 and the European PDH levels are shown in figure 2. In 24 channel case the rates derived from 1.544 Mbps basic rate has been mentioned below:

1.544 x 4 gives 6.312 Mbps (96 channels)

6.312 x 7 gives 44.736 Mbps (672 channels)

44.736 x 6 gives 274.16 Mbps (4032 channels)

44.736 x 3 gives 139.264 Mbps

44.736 x 12 gives 564.992 Mbps


High capacity bandwidth cannot be assisted by PDH and therefore it was quite apt for accommodating traffic. SDH was carved for European Standard and SONET was carved for American Standards. SDH assists topologies like a ring, point to point, linear bus etc. Octet multiplexing and TDM are used by it. The timings used by it are quite precise and it uses both optical and electrical specifications.

The bit rate used by digital multiplexers in SDH are same and are derived from common thus it is in phase.


The basic rate of 155.52 Mbps uses SDH and the fourth multiple of 2488.32 Mbps and 622.08 Mbps. STM-1 is the term used for Basic SDH rate of 155.52. Synchronous Transport Module is the full form of STM. SONET stands for Synchronous Optical Network in the USA because of optical interfaces usage.

Most of PDH rates can be combined with SDH up to 155.52 Mbps as per CCITT.

These are the following merits or advantages of SDH: 

Very simple to develop higher bit rates for enhancing the maintenance and administration process

SDH and synchronous networking is assisted by multipoint networking

Current PDH signals are transported with a capability

  • A more compact multiplexing and demultiplexing technique.
  • Broadband signals can be transported easily
  • Various operators are supported by its multi-vending technology.
  • Video conferencing, interactive multimedia are few of the LAN transport services offered by this network.
  • Without the SDH limit, optical fiber bandwidth can be enhanced.
  • With the help of rings, switching protection to traffic is possible.
  • Faster recovery from failure is assisted by SDH.

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Reference site: Rfwireless World

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