Plesiochronous Digital Hierarchy

What Does Plesiochronous Digital Hierarchy Mean?

The plesiochronous digital hierarchy (PDH) is a telecommunications network transmission technology designed for the transport of large data volumes across large scale digital networks.


The PDH design allows the streaming of data without having isochronous (clocks running at identical times, perfectly synchronized) to synchronize the signal exchanges. PDH clocks are running very close, but not exactly in time with one another so that when multiplexing, signal arrival times may differ as the transmission rates are directly linked to the clock rate.

PDH allows each stream of a multiplexed signal to be bit stuffed to compensate for the timing differences so that the original data stream could be reconstituted exactly as it was sent.

PDH is now obsolete and has been replaced by synchronous optical networking and synchronous digital hierarchy schemes, which support much higher transmission rates.

Techopedia Explains Plesiochronous Digital Hierarchy

The term plesiochronous means "nearly synchronous". PDH supports a data transmission rate of 2048 Kbps. The data rate is controlled by a clock in the device that generates the data.

With multiplexing signals, the clock rate on each stream within the multiplex can vary very slightly. This can occur for a number of reasons, and is sometimes referred to as "jitter". When a multiplexed stream arrives, there has to be a mechanism for reconstituting the various streams into the original signal form. With signals arriving at various different end-times, there has to be a way to get them all to be available for inverse multiplexing in a simultaneous manner, so PDH bit-stuffs the signals until they are all the same length, at which point they can be successfully demultiplexed. The stuffed bits are then discarded.


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Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…