What is PDH: Plesiochronous Digital Hierarchy?

Plesiochronous Multiplexing

• Before SDH transmission networks were based on the PDH hierarchy.Plesiochronous means nearly synchronous.
• 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber or radio.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4 DME.
• Alarm and performance management requires separate equipment in PDH.