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

The need to reduce network operating costs and increase revenues were the drivers behind the introduction of SDH. The former can be achieved by improving the operations management of networks and introducing more reliable equipment. SDH scores high on both. SDH was designed to allow for flexibility in the creation of products for electronically routing telecommunications traffic.