On digital radios, there is a Plesiochronous Digital Hierarchy (PDH) system or a Synchronous Digital Hierarchy (PDH) system or a Synchronous Digital Hierarchy (SDH)

system. The term plesiochronous 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.

It basically relates to the data rates, which are equivalent to a certain number of voice circuits.