Source : Free On-Line Dictionary of Computing

Plesiochronous Digital Hierarchy
     
         (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 2-Mbit/s channels to be bit
        interleaved.  The receiver discards these bits once the
        signals have been demultiplexed.  [Is this correct?]
     
        PDH is being replaced by {SONET} and other SDH ({Synchronous
        Digital Hierarchy}) schemes.  Service levels are described
        with terms like {DS1}, as in {ISDN}.
     
        (2001-04-27)