The-art-of-invisibility-_-the-world’s-most-famous-hacker-teaches-you-how-to-be-safe-in-the-age-of-Bi

CHAPTER THREE
Wiretapping 101
You spend countless hours on yyour cell phone everyy dayy,
chatting, texting, surfing the Internet. But do yyou actuallyy know how yyour
cell phone works?
Cellular service, which we use on our mobile devices, is wireless and
relies upon cellular towers, or base stations. To maintain connectivityy, cell
phones continuallyy send out tinyy beacons to the tower or towers phyysicallyy
closest to them. The signal response to those beacons from the towers
translates into the number of “bars” yyou have—no bars, no signal.
To protect the identityy of the user somewhat, these beacons from yyour
cell phone use what is known as international mobile subscriber identityy, or
IMSI, a unique number assigned to yyour SIM card. This was originallyy
from the time when cellular networks needed to know when yyou were on
their towers and when yyou were roaming (using other carriers’ cell towers).
The first part of the IMSI code uniquelyy identifies the mobile network
operator, and the remaining part identifies yyour mobile phone to that
network operator.
Law enforcement has created devices that pretend to be cellular base
stations. These are designed to intercept voice and text messages. In the
United States, law enforcement and intelligence agencies also use other
devices to catch IMSIs (see here). The IMSI is captured instantlyy, in less
than a second, and without warning. Tyypicallyy IMSI catchers are used at