INITIALIZING_ACCESS_PROTOCOL...
Scan the spectrum to find active GSM base stations (beacons). The tool tunes the SDR receiver to known cellular downlink bands and searches for Broadcast Control Channel (BCCH) signals, outputting the power levels (RSSI) of nearby towers. The utility compiles a list of available cells and their broadcast carrier numbers.
Demodulate and capture downlink signaling frames. Once tuned to a specific ARFCN, gr-gsm processes the radio signal, performs frequency correction, decodes the GMSK burst data, and extracts logical GSM network channels. It maps raw burst data into structured GSM LAPDm layers, handling synchronization and timing.
Forward the decoded frame stream to Wireshark via a local loopback port. This routes the L3 signaling traffic (such as system parameters and paging requests) directly into your protocol dissectors for live inspection. The real-time stream can be filtered by IMSI, cell ID, or network control commands.
GR-GSM operates exclusively in receive mode — it does not emit radio signals, and capturing GSM downlink broadcasts does not require a spectrum transmission license in most jurisdictions. However, connecting a transmit-capable SDR (such as a HackRF or USRP) in transmit mode to replay frames or impersonate a base station on licensed GSM frequencies without government authorization (FCC, OFCOM, or national equivalent) constitutes unlawful radio interference.
GSM downlink captures processed by GR-GSM may decode TMSI allocations, paging messages, and system information parameters broadcast openly by base stations. Despite being publicly transmitted, retaining or distributing subscriber identity correlations — such as mapping paging group TMSIs to physical locations or MSISDNs — may violate wiretap statutes in your jurisdiction. Restrict all capture sessions to controlled lab environments using private GSM test equipment and dedicated test SIM cards.
You've executed the basic steps for GR-GSM. The TelcoSec Academy offers full hands-on simulation labs, baseband source code auditing, and deep cellular signal processing certification courses.