Cordless Security: Can Eavesdropping Be Prevented?
Most of us have picked up our cordless phones at one time or
another, and found ourselves listening in on other people's
conversations. Perhaps we listen for a while (until we realize that
a random phone call isn't very interesting), and then we hang up.
Although many of us prefer not to think about it, this technological
flaw can be very dangerous. Certainly when trade secrets and insider
information can be heard by anyone with a $200 radio scanner, a
company will quickly learn how financially damaging poor cordless
phone security can be.
Several technologies have been invented to thwart unwanted phone
call participants and, in varying degrees, they can be effective at
keeping conversations private. But first, let's take a look at the
nature of eavesdropping.
A real need for security
Cordless telephones are really just small radio transmitters and
receivers. Each handset and its base station communicate with each
other over a pair of frequencies or channels; one channel for the
base-to-handset signal, and one channel for the handset-to-base
signal. When you use a cordless phone with poor security, just about
anybody with a scanner can tune in and hear your entire
conversation—sometimes from more than a mile away!¹
In response to such an invasion of privacy, various technologies
have been invented to protect you. I'll discuss those that are
commonly built in to the cordless phones of today, including channel
hopping, digital security, spread spectrum technology (SST), and the
newer, higher frequencies, 900 MHz and 2.4 GHz.
This technology was invented during World War II to prevent the
enemy from jamming signals that directed torpedoes.² Channel hopping
involves moving (or hopping) a signal transmission to different
random frequencies while the receiver hops in the same manner. This
process makes it much more difficult for enemies (or eavesdroppers)
to keep up with a signal's location. Because signals stay on a
particular frequency only for a short period of time, channel
hopping also makes for clearer transmissions. Any interference you
encounter will affect the call only until the next channel is
If your phone is said to have 25-channel capability, it does not
necessarily mean it employs channel hopping. It merely indicates
that your phone can choose between 25 different channels if it had
to. In actual fact, there are times when it doesn't have to switch
channels at all. Sometimes just one channel is sufficient.
Digital security codes
One of the first measures made possible by the invention of digital
technology was security coding. Every time a handset with this
feature is placed into a base unit, one of up to 100,000 digital
codes is randomly assigned. This code is then continually
transmitted during calls. In order to communicate, a handset and a
base unit must have this code.
Digital security codes do not prevent scanners from picking up
your calls. The codes merely prevent eavesdropping by other people
with similar handsets and make it impossible for them to use your
phone line to make their own calls.
Spread Spectrum Technology (SST)
Since binary code is being transmitted (as opposed to analog
pulses), digital signals make call interception more difficult.
However, eavesdropping is still possible. To further protect
cordless privacy, spread spectrum technology was invented. SST
disassembles a voice signal, "spreads" it over several channels
during transmission, and then reassembles it back into its original
form. It is almost impossible for people to reassemble an SST signal
on their own. And in addition to better security, since the FCC
permits SST phones to use higher output wattage than traditional
cordless phones, SST phones deliver better range.
Most inexpensive radio scanners—the ones you typically find in
electronics stores and/or your local discount drugstore—cannot
access frequencies above 512 MHz. However, if eavesdroppers are
willing to spend a little more money, they can listen in on a 900
MHz frequency, and beyond.
On the other hand, even the pricier scanners can't cover the 2.4
GHz frequencies used by the newest, most advanced cordless phones.
If you want to safeguard your conversations, 2.4 GHz cordless phones
tend to offer maximal security. They operate above the range covered
by scanners, they almost always transmit digitally, and they
generally employ spread spectrum technology, as well. They're
designed to give you the best of everything—including security.
¹The number of available channels for transmission
is different for each type of cordless phone. Phones using the 46-49
MHz range generally have 10 to 25 available channels; 900 MHz phones
have 20 to 60 available channels; and 2.4 GHz phones have 50 to 100