A signalling system comprises a first data signal source (10, 14), a first data signal receiver (12, 16) and a cable 18 comprising two or more wire pairs ({1,2}, {3,6}, {4,5}, {7,8}) coupling the first data signal source to the first data signal receiver. A portion of each wire pair is wound around a magnetic core (28). A further winding (30) is wound around the core (28). A further signal source (24) is coupled to the further winding (30) and a further receiver (36, 26) is coupled to the wires to receive the further signal. The windings around the core apply the further signal to the wire pairs as a common-mode signal. This allows the further signal to be transmitted to the further receiver without affecting the signal transmitted between the source (10, 14) and the receiver (12, 16) and with only minor modification of the cable (18).