Bearer and Teleservices@ as respectively defined in GSM 02.02 and GSM 02.03@ are the objects which the GSM PLMN operators offer to their customers. Besides these basic telecommunications services@ features which aim at up-grading these basic services need also to be offered. Due to the use of radiocommunications in a PLMN@ which are of a special nature compared to classical distribution transmission techniques used in the fixed networks@ such a category of features is related to security aspects. In a GSM PLMN@ both the users and the network operator have to be protected against undesirable intrusion of third parties. However@ measures should be provided for in order to insure maximum protection of the rights of the individuals concerns. As a consequence@ a security feature is either a supplementary service to Tele or Bearer services@ which can be selected by the subscriber@ or a network function involved in the provision of one or several telecommunication services. The purpose of this European Telecommunication Standard (ETS) is to define the security features which are to be available in a GSM PLMN@ together with the associated levels of protection. This ETS is only concerned with those security features which aim at the up-grading of the security in a GSM PLMN. In particular@ end-to-end security is outside the scope of this ETS. The implementation aspects of security features are described in GSM 03.20.