Abstract: A radio transmission system designed to operate on a single defined frequency channel. The system comprises a central receiver (12), at least one transmitter (14) remote from the receiver and a plurality of repeaters (13) arranged in zones (Z1 to Zn) arround the receiver. Each repeater can receive a signal from a transmitter and re-transmit the message so that it eventually reaches the receiver by way of one or more repeaters. The system includes protection means for protecting the message against loss or corruption, for example by using unique time delays before re-transmission or by addressing a certain sequence of repeaters with alternate paths being provided to guard against the message being lost.

Abstract: A sheltered housing scheme communications system is provided. The system includes a master unit carried by a warden which is able to receive and initiate outside telephone calls. This is accomplished by inserting the master unit into a system cable which controls an auto dialer. The auto dialer is capable of dialing a non-preprogrammed telephone number by operation of the master unit. The master unit also functions to indicate its availability to answer an incoming call to the central control unit.