This Recommendation defines the operation of the Digital subscriber Signalling System No. 2 (DSS 2) for the support of the User-to-User Signalling (UUS) supplementary service at the TB or the coincident SB and TB reference point of the user to network interface of the Broadband-Integrated Services Digital Network (B-ISDN). The User-to-User Signalling (UUS) supplementary service allows a B-ISDN user to send and receive a limited amount of information to/from another B-ISDN user over the signalling virtual channel in association with a call/connection to the other B-ISDN user