ITU-T Q.400-Q.490

Signalling System R2 is used as an international signalling system within international regions. Suitable for both automatic and semi-automatic working, it provides sufficient signals in both directions to permit the transmission of numerical and other information relating to the called and calling subscribers' lines and to increase routing facilities. System R2 is specified for one-way operation on analogue and digital transmission systems and for both-way operation on digital transmission systems. Distinction is made between line signalling (supervisory signals) and interregister signalling (call set-up control signals). Versions of line signalling are specified for use on 4-wire carrier or PCM circuits. Nevertheless, the interregister signalling specified can also be used on 2-wire circuits. The interregister signalling is a compelled multifrequency code system which offers the possibility of end-to-end signalling.