Cordless Security: Can Eavesdropping Be Prevented?

by Jim Hanks

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.

Channel hopping
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 chosen.

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.

Higher frequencies
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.

My recommendation
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 channels.

²An interesting note: Channel hopping was invented and patented during World War II by actress Hedy Lamarr and her arranger! Their idea, which came to them at a dinner party, became the basis for secure military communications and spread spectrum technology, nearly 20 years later. (Ms. Lamarr never received payment for her idea.)