What is the X in X-mas?
(fdx, from telegraphy) 1. A type of duplex communications channel which carries data in both directions at once.
On purely digital connections, full-duplex communication requires two pairs of wires. On analog networks or in digital networks using carriers, it is achieved by dividing the bandwidth of the line into two frequencies, one for sending, and the other for receiving.
2. An obsolete term for remote echo.
Compare simplex, half-duplex, double-duplex.