A method and control system for controlling a signal from each outstation in a multiple access passive optical network including a central node and a plurality of outstations involves monitoring, at the exchange, the value of a parameter of a signal from reach outstation. The value of the parameter is monitored relative to a predetermined discrimination value to detect the signal relative to predetermined first and second reference values defining a range of acceptable values for the parameter. If the value of the parameter for a detected signal falls outside the range, a control signal is transmitted from the node to instruct the relevant outstation to alter the value of the parameter as required to bring the value into the acceptable range for subsequent signals originating from that outstation. The method and system are particularly adapted for controlling the temporal position and the amplitude of outstation signals in a TDM passive optical network. A transceiver for use with such a system is also disclosed.