The supplementary service 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,in general,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.
ITU-T Q.955.1-1992 Referenced Document
ITU-T Q.85.1 STAGE 2 DESCRIPTION FOR COMMUNITY OF INTEREST SUPPLEMENTARY SERVICES SECTION 1 - CLOSED USER GROUP (CUG)
ITU-T Q.955.1-1992 history
1992ITU-T Q.955.1-1992 STAGE 3 DESCRIPTION FOR COMMUNITY OF INTEREST SUPPLEMENTARY SERVICES USING DSS 1 SECTION 1 - CLOSED USER GROUP