Optical add drop multiplex (oadm)

PCT No. PCT/SE95/01490 Sec. 371 Date Jun. 11, 1997 Sec. 102(e) Date Jun. 11, 1997 PCT Filed Dec. 11, 1995 PCT Pub. No. WO96/19884 PCT Pub. Date Jun. 27, 1996A method of configuring subnodes, or configuring a system of subnodes, in an optical network ring against both node and fiber failure using an OADM, Optical Add Drop Multiplexer. The network includes a working ring and a stand-by ring and each subnode includes a selective optical filter, an optical 2x2 switch and an optical amplifier. The method and the system further include steps of monitoring the inputs and outputs of each subnode at the working and stand-by rings using monitor devices which generate alarm signals upon detection of signal loss at a subnode. As a response to the alarm signal, the state of the subnode, that causes the alarm signal is transferred from a first state into one of a number of possible new states as a function of the generated alarm signal, thereby selecting a switch configuration for the subnode according to the new state to clear the error detected.