V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

PDH
       Plesiochronous Digital Hierarchy (ATM)


The Free On-line Dictionary of Computing (30 December 2018):

Plesiochronous Digital Hierarchy
PDH

    (PDH) A transmission system for voice
   communication using plesiochronous synchronisation.

   PDH is the conventional multiplexing technology for network
   transmission systems.  The transmitter adds dummy information
   bits to allow multiple channels to be bit interleaved.  The
   receiver discards these bits once the signals have been
   demultiplexed.

   PDH combines multiple 2 Mb/s (E1) channels in Europe and
   1.544 Mb/s (DS1) channels in the US and Japan.

   PDH is being replaced by SONET and other SDH (Synchronous
   Digital Hierarchy) schemes.

   (2003-09-30)