A communication system where a central module repetitively sends interrogation optical pulses to endpoint modules in order to determine operational state of the endpoint modules and of the two-way communication path to the central module. Those pulses also determine whether the endpoint module wishes to initiate communication with the central office. In the endpoint module, a modulator whose function is to send data from the customer's terminal is arranged to operate in two distinct modes. When it is "powered off", which is when the customer terminal is inactive, it allows central office signals to loop-back to the central office. In this state, the optical pulses sent by the central office serve the function of optical continuity checking. When an endpoint module wishes to establish a connection, i.e., the customer terminal wishes to "make a call", the modulator is arranged to interpose itself in the loop-back path and inform the central module of its desire, such as by temporarily turning off the path to the central. When the optical interrogation pulses fail to return to the central office, the central module knows that either the endpoint module is wishing to make a call, or a break exists in the communications path. The central module assumes that the endpoint module desires to make a call and responds by sending an acknowledgment. Thereafter, the endpoint module can proceed with communication.