ITU-T Q.2735.1

This Recommendation specifies the stage 3 of the Closed User Group (CUG) supplementary service for the Broadband Integrated Services Digital Network (B-ISDN) at the Network Node Interface (NNI) by means of the Broadband ISDN User Part (B-ISUP) protocol. Closed User Group (CUG) enables users to form groups, to and from which access is restricted. A specific user may be a member of one or more CUGs. Members of a specific CUG can communicate among themselves, but not with users outside the group. Specific CUG members can have additional capabilities, that allow them to originate calls 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.