Abstract: A synchronous optical digital transmission system comprises a local interface unit having a phase locked loop circuit that senses and adjusts the phase of the clock signals arriving from the remote unit to match the phase of the principal or local clock so that arriving data bit streams can be directly applied to the local system for further processing. The system is also designed to transmit and receive high speed and low speed data over a single fiber operating in a diplex mode without adverse crosstalk effects by employing non-standard line coding techniques.