PDH

Plesiochronous Digital Hierarchy:

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunication networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems.

The term plesiochronous is derived from Greek plesio, 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, synchronised.

PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) equipment in most telecommunications networks.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate.

By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute.

However, there is no link between watches to guarantee they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.