Pleisiosynchronous Digital Hierarchy (PDH) is a technology and term used to describe a set of protocols used primarily in telecommunications networks to combine multiple streams of digital data into a single communications path. PDH transports multiple streams of data from several sources over a single circuit allowing the telecommunications provider to reuse physical infrastructure and allow digital data from multiple customers to travel over the same physical facilities. The process of combining the multiple streams into a single stream of data is called "multiplexing". There are actually two PDH systems, one that is used in the Americas and another that is used in Europe and overseas.
Pleisiosynchronous means "almost synchronous" which refers to the fact that any two circuit switches with a common connection will need to coordinate the timing on the circuit
- T1, T2, T3
- DS1, DS2, DS3
- J1, J2, J3
- E1, E2, E3