How cellphones work
by Chris Woodford. Last updated: March 17, 2016.
Walking and talking, working on the
train, always in contact, never out of touch—cellphones have dramatically
changed the way we live and work. No one knows exactly how many little
plastic handsets there are in the world, but the best
guess is that there over 7.4 billion subscriptions. That's more than the planet's population! In developing countries, where
large-scale land line networks (ordinary telephones
wired to the wall) are few and far between, over 90 percent of the phones in use are
cellphones. Cellphones (also
known as cellular phones and, chiefly in Europe, as mobile phones or
mobiles) are radio telephones that route their calls through a
network of masts linked to the main public telephone network. Here's
how they work.
Photo: Most people now use smartphones as their cellphones, which are actually
small computers with cellphone circuitry built in. Back in the 1990s, cellphones were simpler and could only be used for making voice calls. Now networks are faster and capable of handling greater volumes of traffic, smartphones are used as portable communication centers, capable of doing all the things you can do with a telephone, digital camera, MP3 player, GPS "sat nav," and laptop computer.
Cellphones use wireless technology
Photo: Cellphones as they used to be. This Nokia dates from the early 2000s and
has a slide-out keypad. Although it has a camera and a few other basic functions, it doesn't have anything
like the computing power of a modern smartphone.
Although they do the same job, land lines
and cellphones work in a completely different way. Land lines carry
calls along electrical
cables. Cut out all the satellites, fiber-optic cables, switching
offices, and other razzmatazz, and land lines are not that much
different to the toy phones you might have made out of a piece of
string and a couple of baked bean cans. The words you speak ultimately
travel down a direct, wired connection between two handsets. What's
different about a cellphone is that it can send and receive calls without wire
connections of any kind. How does it do this? By using electromagnetic
radio waves to send and receive the sounds that would normally travel down wires.
Whether you're sitting at home, walking down the street, driving a
car, or riding in a train, you're bathing
in a sea of electromagnetic
waves. TV and radio
programs, signals from radio-controlled
cordless phone calls, and even wireless doorbells—all these things
work using electromagnetic energy:
undulating patterns of electricity
and magnetism that zip and zap invisibly through space at the speed of
light (300,000 km or 186,000 miles per second). Cellphones are by far
the fastest growing source of electromagnetic energy
in the world around us.
How cellphone calls travel
When you speak into a cellphone, a tiny microphone in the handset
converts the up-and-down sounds of your voice into a corresponding
up-and-down pattern of electrical signals. A microchip inside the phone
turns these signals into strings of numbers. The numbers are packed up
into a radio wave and beamed out from the phone's
antenna (in some
countries, the antenna is called an aerial). The radio wave races
through the air at the speed of light until it reaches the nearest
Photo: Engineers repair a cellphone mast. Photo by Brien Aho courtesy of US Navy.
The mast receives the signals and passes them on to its base station,
which effectively coordinates what happens inside each local part of the cellphone network, which is called a
cell. From the base station, the calls are routed onward to their destination.
Calls made from a cellphone to another cellphone on the same network travel to their
destination by being routed to the base station nearest to the destination
phone, and finally to that phone itself. Calls made to a cellphone on a
different network or a land line follow a more lengthy path. They may have
to be routed into the main telephone network before they can reach
their ultimate destination.
How cellphone masts help
At first glance, cellphones seem a lot like two-way radios and walkie talkies,
where each person has a radio (containing both a sender and a receiver) that bounces messages back and forth directly, like tennis
players returning a ball. The problem with radios like this is that you can only use so many
of them in a certain area before the signals from one pair of callers start interfering with those
from other pairs of callers. That's why cellphones are much more sophisticated—and work in a completely different way.
A cellphone handset contains a radio transmitter, for sending radio signals onward from the
phone, and a radio receiver, for receiving incoming signals from other
phones. The radio transmitter and receiver are not very high-powered, which means cellphones cannot send signals very far.
That's not a flaw— it's a deliberate feature of their design! All a cellphone has to do is communicate with its local mast and base station; what the base station has to do is pick up faint signals from many cellphones and route
them onward to their destination, which is why the masts are huge, high-powered antennas (often mounted on a hill or tall building).
If we didn't have masts, we'd need cellphones with enormous antennas and giant power supplies—and they'd
be too cumbersome to be mobile. A cellphone automatically communicates with the nearest cell
(the one with the strongest signal) and uses as little power to do so as it possibly can (which makes its battery
last as long as possible and reduces the likelihood of it interfering with other phones nearby).
What cells do
So why bother with cells? Why don't cellphones simply talk to one another directly? Suppose several
people in your area all want to use their cellphones at the same time.
If their phones all send and receive calls in the same way, using the same kind of radio waves, the
signals would interfere and scramble together and it would be impossible to tell one call from another. One way to get around this is
to use different radio waves for different calls. If each phone call uses a slightly different frequency
(the number of up-and-down undulations in a radio wave in one second), the calls are easy to keep separate. They can travel through the air like different radio stations that use different wavebands.
That's fine if there are only a few people calling at once. But suppose you're in the middle of a big city and millions of people are
all calling at once. Then you'd need just as many millions of separate frequencies—more than are usually available. The solution is to
divide the city up into smaller areas, with each one served by its own masts and base station. These areas are
what we call cells and they look like a patchwork of invisible hexagons. Each
cell has its base station and masts and all the calls made or received inside that cell are routed through them. Cells enable the system to handle many more calls at once, because each cell uses the same set of frequencies as its neighboring cells. The more cells, the greater the
number of calls that can be made at once. This is why urban areas have many more cells than rural areas and why the cells in urban areas are
How cellphone cells handle calls
This picture shows two ways in which cells work.
If a phone in cell A calls a phone in cell B, the call doesn't
pass directly between the phones, but from the first phone to mast A and its base station,
then to mast B and its base station, and then to the second phone.
Cellphones that are moving between cells (when people are
walking along or driving) are regularly sending signals to and from
nearby masts so that, at any given time, the cellphone network always
knows which mast is closest to which phone.
If a car passenger is making a call and the car drives between cells C, D, and E, the phone
call is automatically "handed off" (passed from cell to cell) so the call is not interrupted.
The key to understanding cells is to realize that cellphones and the masts they communicate with are
designed to send radio waves only over a limited range; that effectively defines the size of the cells.
It's also worth pointing out that this picture is a simplification; it's more accurate to say that the masts sit at the intersections of the cells, but it's a little easier to understand things as I've shown them.
Types of cellphones
The first mobile phones used analog technology.
This is pretty much how baked-bean can telephones work too. When you talk on a
baked-bean can phone, your voice makes the string vibrate up and down
(so fast that you can't see it). The vibrations go up and down like
your voice. In other words, they are an analogy of your
voice—and that's why we call this analog technology. Some land lines
still work in this way today.
Most cellphones work using digital technology:
they turn the
sounds of your voice into a pattern of numbers (digits) and then beam
them through the air. Using digital technology has many advantages. It
means cellphones can be used to send and receive computerized data.
That's why most cellphones can now send and receive text (SMS)
messages, Web pages, MP3 music files, and digital
photos. Digital technology means cellphone calls can be encrypted
(scrambled using a mathematical
code) before they leave the sender's phone, so eavesdroppers cannot
intercept them. (This was a big problem with earlier analog phones,
which anyone could intercept with a miniature radio receiver called a
scanner.) That makes digital cellphones much more secure.
The world of cellphones
Cellphones have dramatically changed the way the world connects. In the early 1990s,
only one per cent of the world's population owned a cellphone; today,
in a growing number of countries
people spend more time on their mobiles than on
their landlines. According to the
ITU-T, in 2001, only 58 percent of the world's population had access to a (2G) cellphone network; by 2015, that had risen to 95 percent. By the middle of 2016, there were over 7.4 billion cellphone subscriptions—almost
as many as there are people on the planet. At the end of 2016,
mobile (smartphone and tablet) Internet traffic passed
desktop traffic for the first time ever.
Cellphones are also used in different ways by different people.
Back in the early 2000s, cellphones were used entirely
for voice conversations and sending short "texts" (text messages, also known as SMS messages).
A lot of people owned a mobile phone purely for occasional emergency use;
and that still remains a popular reason for owning a phone (according to the
about 70 percent of all emergency calls in the United States are made
from cellphones). Today, smartphones are everywhere and people use them for emailing, browsing the web,
downloading music, social media, and running all kinds of apps.
Where old-fashioned cellphones relied entirely on a decent signal from a cellphone network, smartphones hop back and forth, as necessary, between ordinary networks and Wi-Fi.
Where old cellphones were literally "mobile phones" (wireless landlines),
modern smartphones are essentially pocket computers that just happen to make phone calls.
Cellphones and mobile broadband
If you want to find out how cellphone networks have evolved from purely voice networks to
form an important part of the Internet, please see our separate article on
It also covers all those confusing acronyms like FDMA, TDMA, CDMA, WCDMA, and HSDPA/HSPA.
Photo: Mobile broadband with a USB modem is an increasingly popular form of wireless Internet.
Do cellphones harm your health?
Photo: A simple, modern Nokia 106 cellphone from 2014. New phones like this generally operate at lower power than older ones, producing less electromagnetic radiation and (theoretically) less risk to health.
People have been asking that question pretty much since cellphones first appeared—and
the debate has intensified over the last decade or so. Why is it even an issue?
As we discovered up above, cellphones communicate using radio waves, and we've all been bathing in massive doses of those
things since radio and TV became popular in the early part of the 20th century.
But the long-wave radio waves used in broadcasting are very different from the
short-wavelength, high-frequency, high-energy radio waves at the opposite
end of the electromagnetic spectrum.
Generally speaking, the shorter the wavelength of radio waves the more dangerous
they are to our health. That's why we take great care with the safety of
microwave ovens and X ray machines.
The trouble with cellphones is that they use waves that are on the border between
the safer, long-wavelength radio waves and the unsafer, short-wavelength ones.
Although the waves they use are defined as microwaves, they're longer wavelength,
lower frequency, and lower energy waves than the ones used in microwave ovens.
Where a microwave oven can produce perhaps 1000 watts of cooking power,
a cellphone makes fractions of a watt at most—thousands of times less, in
So do cellphones "cook your brain" or "give you cancer"? It's very difficult to answer
that question conclusively. Proving a link between environmental
"risk factors" and cancers of various kinds is very difficult when people are exposed
to many different risks over their lifetime and cancers can develop
years or even decades in the future; cellphones are still a relatively new
technology so there isn't really enough data to go on.
What's the best guess on the safety of cellphones? In 2010, a large international study of over 5000
brain tumor cases called Interphone (coordinated by the International Agency for Research on Cancer, IARC, in Lyon) revealed that there was
no increased risk of brain tumors for average cellphone use, though very heavy users of cellphones
(30 minutes a day for a decade) did seem to be at greater risk. In May 2011, the World Health Organization also published its view that the electromagnetic fields produced by cellphones are "possibly carcinogenic to humans,"
which means there is a "credible" link but "chance, bias or confounding cannot be ruled out with reasonable confidence."
Two months later, a team of "expert" scientists from the Institute of Cancer Research concluded: "Although there remains some uncertainty, the trend in the accumulating evidence is increasingly against the hypothesis that mobile phone use can cause brain tumors in adults."
The position is less certain for mobile phone use in children and adolescents, though a number of studies are now underway. The WHO was planning to review the evidence on possible health effects from
electromagnetic fields by the end of 2016, but has not yet reported its results.
Find out more
Who invented cellphones?
Photo: Martin Cooper's original radio telephone system (cellphone) design,
submitted as a patent application in 1973. Note how the mobile part forms an entirely separate system (shown in blue, on the right) that communicates with the existing public network (shown on the left in red). Individual cellphones (turquoise on the extreme right) communicate with their nearest masts and base stations using radio waves (yellow zig-zags). Patent drawing courtesy of US Patent and Trademark Office.
How did we get from land lines to cellphones? Here's a quick history:
- 1873: British physicist James Clerk Maxwell
(1831–1879) published the theory of electromagnetism, explaining how how
electricity can make magnetism and vice-versa. Read more about his work
in our main article on magnetism.
- 1876: Scottish-born inventor Alexander Graham Bell
(1847–1922) developed the first telephone while living in the United States
(though there is some dispute about whether he was actually the original inventor).
Later, Bell developed something called a "photophone" that would send and receive phone calls using light beams.
Since it was conceived as a wireless phone, it was really a distant ancestor of the modern mobile phone.
- 1888: German physicist Heinrich Hertz
(1857–1894) made the first electromagnetic radio waves in his lab.
- 1894: British physicist Sir Oliver Lodge
(1851–1940) sent the first message using radio waves in Oxford, England.
- 1899: Italian inventor Guglielmo Marconi
(1874–1937) sent radio waves across the English Channel. By 1901. Marconi had sent radio
waves across the Atlantic, from Cornwall in England to Newfoundland. Marconi is remembered
as the father of radio, but pioneers such as Hertz and Lodge were no less important.
- 1906: American engineer Reginald Fessenden
(1866–1932) became the first person to transmit the human voice using radio waves.
He sent a message 11 miles from a transmitter at Brant Rock,
Massachusetts to ships with radio receivers in the Atlantic Ocean.
- 1920s: Emergency services began to experiment with cumbersome
- 1940s: Mobile radio telephones started to become popular with
emergency services and taxis.
- 1946: AT&T and Southwestern Bell introduced their Mobile
Telephone System (MTS) for sending radio calls between vehicles.
- 1960s: Bell Laboratories (Bell Labs) developed Metroliner mobile
cellphones on trains.
- 1973: Martin Cooper (1928–) of
Motorola made the first cellphone call using his 28-lb prototype DynaTAC phone.
- 1975: Cooper and his colleagues were granted a patent for their
radio telephone system. Their original design is shown in the artwork you can see here.
- 1978: Analog Mobile Phone System (AMPS) was introduced in Chicago
by Illinois Bell and AT&T.
- 1982: European telephone companies agreed a worldwide standard for
how cellphones will operate, which was named Groupe Speciale Mobile and
later Global System for Mobile (GSM) telecommunications.
- 1984: Motorola DynaTAC became the world's first commercial
handheld cellphone. Take a look at a picture
of Martin Cooper and his DynaTAC!
- 1995: GSM and a similar system called PCS (Personal
Communications Services) were adopted in the United States.
- 2001: GSM had captured over 70 percent of the world cellphone
- 2000s: Third-generation (3G and 3.5G) cellphones were launched, featuring
faster networks, Internet access, music downloads, and many more
advanced features based on digital technology.
- 2007: Apple's iPhone revolutionized the world of cellphones, packing what is effectively
a touch-controlled miniature computer into a gadget the same since as a conventional cellular phone.
- 2011: World Health Organization published view that cellphones are "possibly carcinogenic" to humans.
- 2013: Cellphones celebrate their 40th anniversary.