An optical network (1, 2) having a tree-like structure with a main line (3) and a plurality of branches (6.1 to 6.3), at least two of the branches (6.1 to 6.3) comprising a monitoring unit (10.1 to 10.3) for upstream signalling to the main line (3), wherein each of the monitoring units (10.1 to 10.3) comprises a signal generation unit (18.1 to 18.3) with a light source for generation of a pre-defined optical signal (11.1 to 11.3), and the monitoring units (10.1 to 10.3) are construed to generate pre-defined optical signals (11.1 to 11.3) which are different from each other, as well as a monitoring unit for generating a periodic upstream signal in such an optical network (1, 2) and a method for monitoring such an optical network (1, 2).