1. A method for controlling access of a station to a communications medium that is divided into time slots, the method comprising: transmitting, by a controller of the medium, demarcation signals at regular intervals, each demarcation signal demarcating a respective time slot and containing one or more interrogatory codes; receiving, by the station, a demarcation signal corresponding to a particular time slot; masking, by the station, each of the one or more interrogatory codes from the received demarcation signal with a control mask to produce for each interrogatory code a respective masked interrogatory code; and masking, by the station, a response code held by the station and received from the controller with the control mask to produce a masked response code; wherein the station bars itself from attempting to access the communications medium in the particular time slot if none of the one or more masked interrogatory codes matches the masked response code.