GNU Radio blocks and tools to receive and demodulate the GSM downlink from an SDR. grgsm_livemon tunes a found ARFCN, demodulates the GMSK bursts, decodes the control channels (BCCH/CCCH/SDCCH) and forwards the frames as GSMTAP over UDP to Wireshark. The de-facto open-source GSM receiver, successor to airprobe.