GSM Downlink Protocol Analysis and Decoding using Open-Source Hardware and Software

Since its introduction in 1992, Global System for Mobile Communications (GSM) has evolved to become a worldwide standard in mobile telephony. Security in GSM, albeit being one of the most important issues, has not yet been scrutinized to a level it deserves owing mainly to the expensive hardware requirements and lack of thorough understanding of GSM protocols in general. This scenario is changing significantly with the growing rate of development of open-source tools, which can serve to harness the RF signaling and enable processing on a general purpose workstation. In this paper, these open-source tools are utilized to study the feasibility of developing a GSM Protocol Analyzer and investigating the theoretical security protocols and procedures through practical implementation of such system. Universal Software Programmable Radio Peripheral (USRP) was selected as the RF frontend for the protocol analyzer. The USRP is a general purpose, open-hardware transceiver that can be linked to a workstation via an RJ45 Gigabit Ethernet link. Various open-source software, based primarily on GNU Radio, are then utilized to synchronize, demodulate and decode the received signal.