![]() Supported A5 ciphers for decryption are A5/1, A5/2 and A5/3. The program supports both cfile and burst captures, and decoding of BCCH, SDCCH, TCH/F. This program provides capability to decode GSM messages from saved captures, for example recorded with airprobe_rtlsdr_capture.py: We will add more information about capturing using this program later, for more information about the available options you can start the program with the -h flag: It supports both raw data in cfile format and gr-gsm's burst format, which are pre-processed GSM-bursts instead of raw signal data: This program provides capability to save captured data to file. You can use the value set later by passing it as argument of the program: There is intentionally added upper of allowable clock offset - it was done in order to avoid adaptation of the algorithm to neighbour channels that would inevitably lead to instability. If the clock offset is too large the clock offset correction algorithm that is implemented in the program won't work. The slider is responsible for setting devices clock offset correction. ![]() If it doesn't happen, set ppm slider into different positions. After setting the fc slider to a carrier frequency of a broadcasting channel the program should immediately print content of subsequent messages on the standard output. By looking for constant hills on the spectrum of such width you can find a GSM broadcasting channel. The GSM signal has bandwidth of around 200kHz. The central frequency of the signal can be changed by moving fc slider. ![]() The window of the program contains amplitude spectrum of the signal drawn in real-time. It can be started by running from a terminal: This program uses cheap RTL-SDR receivers as a source of the signal. After installation of gr-gsm there are three python executables that will be installed: It is improved replacement of the old Airprobe - the program that lets you receive and decode GSM control messages from timeslot 0 on the broadcasting channel of a BTS. At this moment there is one application that is ready out of the box. There are many possible applications of gr-gsm. The whole process should look something like Figure Don’t forget to add ‘gain’ value again, move back to the frequency field and start pressing the UP/DOWN arrows on your keyboard to start scrolling the frequencies in 200Khz steps until you start seeing some data in your console window. airprobe_rtlsdr.py or load the airprobe_c file using gnuradio-companion and set one of the channel frequencies you just found in the frequency field. Once you know the GSM channels frequencies, you can start gr-gsm by running the python script.
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