Abstract: A communication system transmits data in a control system for an underwater fluid well. A communications link is located between first and second facilities of the well. A data transmitter for transmitting data from one of the facilities to the other via the communications link has both a relatively high speed modem and a relatively low speed modem.

Abstract: A modem for receiving and transmitting data from and to a conductor comprises an output drive for transmitting data to the conductor, a receiver for receiving data from the conductor and impedance matching means for matching the impedance of the receiver input with the impedance of the conductor, wherein the gain of the output drive, the receiver gain and the impedance of the receiver input are all adjustable.