Real-time interception systems for the GSM protocol

A GSM protocol interceptor is a device located in a (possibly) closed area that listens to information exchanged between base stations (BSs) and mobile stations (MSs). This paper presents three new interception systems for security purposes. The first one (detector) forces all idle MSs nearby to generate activity, which can be used to activate an alarm of MS presence. The second one (selective interceptor) monitors information exchange between MS and BS and, in case of MS activity, (1) extracts messages containing MS identifiers and (2) checks identifiers in a local cache to decide if an external jamming unit should block individual calls (either incoming or outgoing). The third system (enhanced selective interceptor) combines the previous two systems to improve blocking performance. We analyze transactions of the GSM protocol that are relevant to interception system operation. Those transactions impose real-time constraints that we quantify. We present real tests and simulations that address the feasibility of the systems.