At software front, GNU Radio and Airprobe in synchrony
with open-source protocol analyser Wireshark were used to
capture and analyse the GSM downlink traffic as shown in
Fig. 5. The analysis can be explicitly done using Wireshark
wherein packet data in form of octet bits of various GSM
logical channels can be obtained. Then after, RTL-SDR instead
of USRP can be used for capturing the raw GSM frames from
Um interface. The GSM frames are captured using RTL-SDR
in conjunction with GNU Radio and the corresponding data
is stored in a file named as .cfile.
Using the following equations and ARFCN channel number
(n) as obtained in Wireshark, the corresponding downlink
frequencies are obtained [11]. During analysis part, ARFCN
from 26 to 37 were captured on Wireshark terminal. These are
indicated in Table I.
For P-GSM (900) band: