Increased use of automatic operating techniques has created a greater demand for communications links to be used for telemetry, selective remote control, and supervisory control. Supervisory systems which had gained wide acceptance in electric utility operations have been applied to oil pipe lines. In a manner, these operations are also related to ticket reservation equipment used by railroads and airlines, and to transmitter remote supervision equipment used by standard broadcast stations. There are undoubtedly other examples. Whereever a commercial operation exists requiring very little human effort but constant human surveillance, fail-safe automatic equipment can be developed, and the operation can be supervised from a centralized remote-control location.