A direct connexion between the output of a transmitting device and the input of an associated receiving device. Note: When used for equipment measurements or testing purposes, such a back-to-back connexion eliminates the effects of the transmission channel or medium. 2. A direct connexion between the output of a receiving device and the input to a transmitting device. Note: The term "direct," as used in both definitions, may be construed as permitting a passive device such as a pad (attenuator) to accommodate power level constraints.