to solve the problems of sending signaling signals between exchanges which are interconnected by links which have no facility for sending signaling signals conforming to the isdn standard , the necessary signaling channel is simulated on a supplementary channel . in this case , the signaling data formatted in accordance with the isdn standard is converted into data formatted in accordance with said other channel on transmission and restored to its original format on reception . it is shown that this approach can overcome the hybrid nature of the links between two exchanges conforming to the isdn standard . the exchanges include software conforming to the isdn standard for which the link between exchanges becomes transparent .