In this ETS the general aspects of the specification of supplementary services at the layer 3 radio interface are given. Technical specifications GSM 04.8x and 04.9x-series specify the procedures used at the radio interface (reference point Um as defined in technical specification GSM 04.02) for normal operation, registration, erasure, activation, deactivation, invocation and interrogation of supplementary services. Provision and withdrawal of supplementary services is an administrative matter between the mobile subscriber and the service provider and cause no signalling on the radio interface. Technical specifications GSM 04.08 and GSM 04.80 specifies the formats and coding for the supplementary services. Definitions and descriptions of supplementary services are given in technical specification GSM 02.04 and GSM 02.8x and 02.9x-series. Technical realization of supplementary services is described in technical specification GSM 03.11 and GSM 03.8x and 03.9x-series. The procedures for Call Control, Mobility Management and Radio Resource management at the layer 3 radio interface are defined in technical specifications GSM 04.07 and GSM 04.08.