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 cus tomers. Besides these basic telecommunications services, features whic h 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 na ture compared to classical distribution transmission techniques used i n the fixed networks, such a category of features is related to securi ty aspects. In a GSM PLMN, both the users and the network operator have to be prot ected against undesirable intrusion of third parties. However, measure s should be provided for in order to insure maximum protection of the rights of the individuals concerns. As a consequence, a security featu re 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 de fine the security features which are to be available in a GSM PLMN, to gether with the associated levels of protection. This ETS is only conc erned 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 0 3.20.