This European Telecommunication Standard (ETS) specifies the procedure s used at the radio interface (reference point Um as defined in techni cal specification GSM 04.02) for normal operation, registration, erasu re, activation, deactivation, invocation and interrogation of communit y of interest supplementary services. The provision and withdrawal of supplementary services is an administrative matter between the mobile subscriber and the service provider and causes no signalling on the ra dio interface. In technical specification GSM 04.10, the general aspects of the speci fication of supplementary services at the layer 3 radio interface are given. Technical specifications GSM 04.80 specifies the formats coding for th e supplementary services. Definitions and descriptions of supplementary services are given in te chnical specifications GSM 02.04 and GSM 02.8x and GSM 02.9x-series. T echnical specification GSM 02.85 is related to the community of intere st supplementary services. Technical realization of supplementary services is described in techni cal specifications GSM 03.11 and GSM 03.8x and 03.9x-series. Technical specification GSM 03.85 is related to the community of interest suppl ementary services. The procedures for Call Control, Mobility Management and Radio Resourc e management at the layer 3 radio interface are defined in technical s pecifications GSM 04.07 and GSM 04.08. Signalling interworking for supplementary services between GSM 09.02 a nd GSM 04.08 and between GSM 09.02 and GSM 04.80 is defined in GSM 09. 11. The following supplementary services belong to the community of intere st supplementary services and are described in this ETS: - Closed User Group (CUG) clause 1.