What is PDH?

Plesiochronous Digital Hierarchy (PDH) is a digital transmission technology used in telecommunications networks. It allows multiple digital signals to be transmitted over the same physical medium, but the signals are not synchronized to a single, common clock. Instead, each signal has its own independent clock, which operates at a rate that is 'plesiochronous' or nearly synchronous with the other signals. This means that the clocks are very close in frequency, but not exactly the same. PDH was widely used for building digital telephone networks before the advent of Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET).