The plesiochronous digital hierarchy (PDH) is a technology used in
telecommunications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, mes ...
s to transport large quantities of data over digital transport equipment such as
fibre optic
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...
and
microwave radio systems. The term ''
plesiochronous
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at ...
'' is derived from Greek ''plēsios'', meaning near, and ''chronos'', time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly,
synchronized.
Backbone transport networks replaced PDH networks with
synchronous digital hierarchy
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting dio ...
(SDH) or
synchronous optical networking
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting dio ...
(SONET) equipment over the ten years ending around the turn of the millennium (2000),
whose floating payloads relaxed the more stringent timing requirements of PDH network technology. The cost in North America was $4.5 billion in 1998 alone,
p. 171.
PDH allows transmission of data streams that are nominal