This document discusses the security measures of cellular communication systems like GSM. It outlines four main security measures in GSM: PIN codes, SIM card authentication, user authentication by the network including encrypting information over the radio interface, and using a temporary mobile subscriber identity instead of the international mobile subscriber identity over the radio interface. It also describes the four main security services provided: anonymity, authentication, signaling protection, and user data protection. Finally, it discusses some limitations of security in GSM systems and the need for periodic review and improvement of technical security features.