What does Plesiochronous Digital Hierarchy (PDH) 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 Multiplexer

PDH Multiplexer, or Plesiochronous Digital Hierarchy multiplexer, is a kind of point-to-point optical transmission equipment used to transport large quantities of data over digital transport media, such as fiber-optic and microwave radio systems. PDH Multiplexer is designed of highly integrated structure and provides 16 standard E1 interfaces together with one channel of order wire, with self-contained alarm and NM functions, as well as self-testing and E1 loop-back testing functions. The device is popularly with telecommunication operator. It is suitable in business for communication operator, government and kinds of entities.

PDH was developed in the early 1960s. It derives its names from the Greek term “plesio,” meaning near, and “chronos,” meaning time. The name refers to the fact that networks using PDH run in a state of almost, but not quite, perfect synchronization. PDH was the first standardized multiplexing hierarchy based on time-division multiplexing. It works by channeling numerous individual channels into higher-level channels.

PDH is now obsolete and has been replaced by synchronous optical networking and synchronous digital hierarchy schemes, which support much higher transmission rates.