In telecommunication, data transmission circuit is the transmission media and the intervening equipment used for the transfer of data between data terminal equipments (DTEs).

Note 1: A data transmission circuit includes any required signal conversion equipment.

Note 2: A data transmission circuit may transfer information in (a) one direction only, (b) either direction but one way at a time, or (c) both directions simultaneously.

Source: from Federal Standard 1037C and from MIL-STD-188