Cordless Telephones

Cordless telephones are one of those minor miracles of modern
life -- with a cordless phone, you can talk on the phone while
moving freely about your house or in your yard. Long before
cell phones became so cheap that anyone could afford one,
cordless phones gave everyone the freedom to walk and talk
within the privacy of their own homes.

Cordless phones have many of the same features as standard
telephones, and there are many models available. In this
article, we will examine how these cordless telephones work
and see why there are so many different types on the market
today.

The Basics

A cordless telephone is basically a combination telephone and
radio transmitter/receiver (see How Telephones Work and How
Radio Works for details on these two technologies).
A cordless phone has two major parts: base and handset.

* The base is attached to the phone jack through
a standard phone wire connection, and as far as the phone
system is concerned it looks just like a normal phone.
The base receives the incoming call (as an electrical signal)
through the phone line, converts it to an FM radio signal and
then broadcasts that signal.

* The handset receives the radio signal from the base,
converts it to an electrical signal and sends that signal to
the speaker, where it is converted into the sound you hear.
When you talk, the handset broadcasts your voice through
a second FM radio signal back to the base. The base receives
your voice signal, converts it to an electrical signal and
sends that signal through the phone line to the other party.

The base and handset operate on a frequency pair that allows
you to talk and listen at the same time, called duplex
frequency.

A Brief History

Cordless phones first appeared around 1980. The earliest
cordless phones operated at a frequency of 27 MHz. They had
the following problems:

* limited range
* poor sound quality - noisy and ridden with static
because walls and appliances interfered with the signals
* poor security - people could easily intercept signals
from another cordless phone because of the limited number of
channels

In 1986, the Federal Communications Commission (FCC) granted
the frequency range of 47-49 MHz for cordless phones, which
improved their interference problem and reduced the power
needed to run them. However, the phones still had a limited
range and poor sound quality.

Because the 43-50 MHz cordless phone frequency was becoming
increasingly crowded, the FCC granted the frequency range of
900 MHz in 1990. This higher frequency allowed cordless
phones to be clearer, broadcast a longer distance and choose
from more channels. However, cordless phones were still quite
expensive, about $400.

In 1994, digital cordless phones in the 900 MHz frequency
range were introduced. Digital signals allowed the phones to
be more secure and decreased eavesdropping -- it was pretty
easy to eavesdrop on analog cordless phone conversations. In
1995, digital spread spectrum (DSS) was introduced for
cordless phones. This technology enabled the digital
information to spread in pieces over several frequencies
between the receiver and the base, thereby making it almost
impossible to eavesdrop on the cordless conversations.

In 1998, the FCC opened up the 2.4 GHz range for cordless
phone use. This frequency has increased the distance over
which a cordless phone can operate, and brought it out of the
frequency range of most radio scanners, thereby further
increasing security.

In the next article we`ll see the anatomy of the cordless telephone.