(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.