The scope of this standard covers the user signalling bearer service procedures at the S or T reference points. The user signalling bearer service is a packet mode bearer ser- vice. This standard defines the use of the procedures of ANSI T1.602-1989@ Telecommunications ?C Integrated services digital network (ISDN) ?C Data-link layer signalling specification for application at the user-network inter- face@ and ANSI T1.607-1990@ Telecommunications ?C Integrated services digital network (ISDN) ?C Layer 3 signalling specification for circuit-switched bearer service for digital sub- scriber signalling system number 1 (DSS1)@ on basic rate interfaces and primary rate interfaces for the establishment and release of user signalling bearer service through an ISDN. The user signalling bearer service allows users to communicate by means of user-to-user signalling without setting up a circuit-switched connection (no B-channel). The user signalling bearer service is established and cleared in a manner similar to the control of a circuit-switched connection. Purpose This interface standard was written to provide a set of requirements for User-Network Signalling for ISDN support of user signalling bearer service@ while conforming@ wherever possible@ with the Q- and I- Series Recommendations of the International Telegraph and Telephone Consultative Committee (ITU-T)@ and adhering to the principles of evolution expressed therein. Equipment may be implemented with additional functions and procedures.