"Introduction Part 2010 describes how the Mobile End System (M-ES) consists of three functional subsystems: a Mobile Application Subsystem (MAS)@ a Subscriber Unit (SU) and a Subscriber Identity Module (SIM). A removable SIM is optional; however@ its function must be supported by all M-ESs. This Part describes the functional characteristics of the SIM@ the contents of the SIM and the interface between the SIM and the SU. This recommendation was originally based on those sections of the GSM specification that defines a Subscriber Identity Module (SIM)@ a form of ""smart card@"" for use in the GSM system [ETSI-GSM-0.2.1.7-V.4.3.1]. It is a requirement that all SIMs are logically and functionally standardized for all CDPD M-ESs@ and it is a requirement that removable SIMs are electrically and physically standardized for those CDPD M-ESs that provide this option. The removable SIM shall contain an active CPU element to provide access security to the contents of the SIM. CDPD service may be one of the applications of a multi-service removable SIM. All specifications of the SIM in this recommendation refer to the part of the SIM that is specific for CDPD mobile services. SIM functions will be updated over time. Forward compatibility shall be assured with regard to the standardization of the network interface functions of the SIM via the SU and with regard to the user-related SIM functions on the SIM-SU interface."