Therefore, it must span a cell with a LAC different to all neighboring cells, but with a much better signal strength than the other cells. An IMSI catcher needs to force a location update to be able to interact with the phone and derive the desired information. Rationale ¶Ī mobile will only perform a normal location update when changing to a different area, i.e. The LAC of the current base station differs from the LAC of many neighboring cells. Furthermore, an operator may reconfigure the cell to use a different LAC/CID. Multiple base stations operating on the same frequency may be receivable in the same locations, e.g. If the same frequency is used with a different LAC/CID within a certain time frame, this is considered an attack. The frequency and LAC/CID is recorded together with a time stamp. Hence, changes of the LAC/CID for a given frequency are specific to the geographic location of the cell.Īs associating an frequency/LAC/CID combination with a precise location reliably can be challenging, a simpler approach is taken. Note, that for resource efficiency, GSM reuses frequency in different geographic areas very frequently. The change of the LAC on a given frequency can be detected. To force the mobile into a location update, a LAC different from all neighboring stations is chosen by the IMSI catcher. Rationale ¶Īn IMSI catcher may use the frequency of an existing base station that has a weak signal in the area the catcher operates. The LAC/CID recently seen on a frequency suddenly changed.
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