ITU-T Q.2955.1

This Recommendation specifies the DSS 2 signalling protocol for the support of the CUG supplementary service in B-ISDN. The protocol can be used at the TB and at the coincident SB and TB reference point. The CUG supplementary service enables users to form groups, to and from which access is restricted. Members of a specific closed user group can communicate among themselves but not, in general, with users outside the group. Specific CUG members can have additional capabilities that allow them to originate calls to destinations outside the group, and/or to receive calls from outside the group. Specific CUG members can have additional restrictions that prevent them from originating calls to other members of the CUG or from receiving calls from other members of the CUG. A specific user may be a member of one or more closed user groups.