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 the present document is to define the security features which are to be available in a GSM PLMN@ together with the associated levels of protection. The present document 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 the present document. The implementation aspects of security features are described in GSM 03.20.