The Free Online Dictionary of Computing (http://foldoc.doc.ic.ac.uk/) is edited by Denis Howe <dbh@doc.ic.ac.uk>.

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Plesiochronous Digital Hierarchy

<communications> (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)