in a data communication system involving modems transmitting over a link including terrestrial and one or more satellite links , a nontransmitting modem sends a secondary channel tone during the absence of data transmission . the cessation of said tone is detected by the other modem and used to de - squelch the remote modem receiver . in this manner , the receiver is protected from echoes impinging upon it during long transmission delay times . this method optimizes data throughput by adapting to the transmission delay of the circuit .