A downhole data transmission system communicates data along a downhole string including a communications master selected from the group including a surface interface, a down-hole interface, and a node, and including a communications line including a plurality of trans-mission segments that carry signals along the downhole string, and a plurality of repeaters that periodically refresh and restore signals transmitted along the downhole string. To minimize power consumption and to improve communications efficiency, the surface interface, the node, and the downhole interface communicate over the communications line(s) using pulses of radiofrequency energy. These pulses may be organized in data frames that may include one or more wake-up pulses. The data transmission system may be further characterized in that the repeaters and/or the communications master are connected to the communications line in a fail-safe fashion wherein the pulses of radiofrequency energy bypass or pass through the signal repeater and/or the communications master without amplification when the signal repeater and/or the communications master fails.