Millions of people
in the United States and around the world use cellular
phones. They are such great gadgets -- with a cell phone,
you can talk to anyone on the planet from just about anywhere!
These days, cell phones provide an incredible array of
functions, and new ones are being added at a breakneck pace.
Depending on the cell-phone model, you can:
But have you ever wondered how a cell phone works? What
makes it different from a regular phone? What do all those
confusing terms like PCS, GSM, CDMA and TDMA mean? In this
edition of HowStuffWorks,
we will discuss the technology behind cell phones so that you
can see how amazing they really are.
If you are thinking about buying a cell phone, be sure to
check out How
Buying a Cell Phone Works to learn about everything
you should know before making a purchase.
Let's start with the basics: In essence, a cell phone is a
radio.
The Cell Approach One of the most
interesting things about a cell phone is that it is actually a
radio --
an extremely sophisticated radio, but a radio nonetheless. The
telephone
was invented by Alexander Graham Bell in 1876, and wireless
communication can trace its roots to the invention of the
radio by Nikolai Tesla in the 1880s (formally presented in
1894 by a young Italian named Guglielmo Marconi). It was only
natural that these two great technologies would eventually be
combined!
Cool
Facts
Most newer digital cellular phones have some sort
of entertainment programs on them, ranging from simple
dice-throwing games to memory and logic puzzles.
Approximately 20 percent of American teens (more
girls than boys) own a cellular phone.
Cellular phones are more popular in European
countries than they are in the United States -- more
than 60 percent of Europeans own a cell phone,
compared to about 40 percent of Americans.
In the dark
ages before cell phones, people who really needed
mobile-communications ability installed radio
telephones in their cars. In the radio-telephone system,
there was one central antenna tower per city, and perhaps
25 channels available on that tower. This central
antenna meant that the phone in your car needed a powerful
transmitter -- big enough to transmit 40 or 50 miles (about 70
km). It also meant that not many people could use radio
telephones -- there just were not enough channels.
The genius of the cellular system is the division of a city
into small cells. This allows extensive frequency
reuse across a city, so that millions of people can use
cell phones simultaneously. In a typical analog cell-phone
system in the United States, the cell-phone carrier receives
about 800 frequencies
to use across the city. The carrier chops up the city into
cells. Each cell is typically sized at about 10 square
miles (26 square kilometers). Cells are normally thought
of as hexagons on a big hexagonal grid, like this:
Because cell phones and base
stations use low-power transmitters, the same frequencies can
be reused in non-adjacent cells. The two purple cells can
reuse the same frequencies.
Each cell has a base station that consists of a
tower and a small building containing the radio equipment
(more on base stations later).
A single cell in an analog system uses one-seventh of the
available duplex
voice channels. That is, each cell (of the seven on a
hexagonal grid) is using one-seventh of the available channels
so it has a unique set of frequencies and there are no
collisions:
A cell-phone carrier typically gets 832 radio
frequencies to use in a city.
Each cell phone uses two frequencies per call -- a duplex
channel -- so there are typically 395 voice
channels per carrier. (The other 42 frequencies are used
for control channels -- more on this on the next
page.)
Therefore, each cell has about 56 voice channels
available.
In other words, in any cell, 56 people can be talking on
their cell phone at one time. With digital
transmission methods, the number of available channels
increases. For example, a TDMA-based digital system can
carry three times as many calls as an analog system, so each
cell has about 168 channels available (see this
page for lots more information on TDMA, CDMA, GSM and
other digital cell-phone techniques).
Cell phones have low-power transmitters in them.
Many cell phones have two signal strengths: 0.6 watts and 3
watts (for comparison, most CB radios transmit at 4 watts).
The base station is also transmitting at low power. Low-power
transmitters have two advantages:
The transmissions of a base station and the
phones within its cell do not make it very far outside that
cell. Therefore, in the figure above, both of the purple
cells can reuse the same 56 frequencies. The same
frequencies can be reused extensively across the city.
The power consumption of the cell phone, which is
normally battery-operated, is relatively low. Low power
means small batteries,
and this is what has made handheld cellular phones possible.
The cellular approach requires a large number of
base stations in a city of any size. A typical large city can
have hundreds of towers.
But because so many people are using cell phones, costs remain
low per user. Each carrier in each city also runs one central
office called the Mobile Telephone Switching Office
(MTSO). This office handles all of the phone connections to
the normal land-based phone system, and controls all of the
base stations in the region.
In the next section, you'll find out what happens as you
(and your cell phone) move from cell to cell.
From Cell to Cell All cell phones have
special codes associated with them. These codes are
used to identify the phone, the phone's owner and the service
provider.
Cell Phone
Codes
Electronic Serial Number (ESN) - a unique
32-bit number programmed into the phone when it is
manufactured
Mobile Identification Number (MIN) - a
10-digit number derived from your phone's number
System Identification Code (SID) - a unique
5-digit number that is assigned to each carrier by the
FCC
While the ESN is considered a permanent part of the
phone, both the MIN and SID codes are programmed into
the phone when you purchase a service
plan and have the phone activated.
Let's say you have a cell phone, you turn it on and someone
tries to call you. Here is what happens to the call:
When you first power up the phone, it listens for an
SID (see sidebar) on the control channel. The
control channel is a special frequency that the phone and
base station use to talk to one another about things like
call set-up and channel changing. If the phone cannot find
any control channels to listen to, it knows it is out of
range and displays a "no service" message.
When it receives the SID, the phone compares it
to the SID programmed into the phone. If the SIDs match, the
phone knows that the cell it is communicating with is part
of its home system.
Along with the SID, the phone also transmits a
registration request, and the MTSO keeps track of
your phone's location in a database -- this way, the MTSO
knows which cell you are in when it wants to ring your
phone.
The MTSO gets the call, and it tries to find
you. It looks in its database to see which cell you are
in.
The MTSO picks a frequency pair that your phone
will use in that cell to take the call.
The MTSO communicates with your phone over the
control channel to tell it which frequencies to use,
and once your phone and the tower switch on those
frequencies, the call is connected. You are talking
by two-way radio to a friend!
As you move toward the edge of your cell, your cell's
base station notes that your signal strength
is diminishing. Meanwhile, the base station in the cell you
are moving toward (which is listening and measuring signal
strength on all frequencies, not just its own one-seventh)
sees your phone's signal strength increasing. The two base
stations coordinate with each other through the MTSO, and at
some point, your phone gets a signal on a control channel
telling it to change frequencies. This hand off
switches your phone to the new cell.
As you travel, the signal is
passed from cell to cell.
Roaming If the SID on the
control channel does not match the SID programmed into your
phone, then the phone knows it is roaming. The MTSO of
the cell that you are roaming in contacts the MTSO of your
home system, which then checks its database to confirm
that the SID of the phone you are using is valid. Your home
system verifies your phone to the local MTSO, which
then tracks your phone as you move through its cells. And the
amazing thing is that all of this happens within seconds!
Cell Phones and CBs A good way to understand
the sophistication of a cell phone is to compare it to a CB
radio or a walkie-talkie.
Simplex vs. duplex - Both walkie-talkies and CB
radios are simplex devices. That is, two people
communicating on a CB radio use the same frequency,
so only one person can talk at a time. A cell phone is a
duplex device. That means that you use one frequency
for talking and a second, separate frequency for listening.
Both people on the call can talk at once.
Channels - A walkie-talkie typically has one
channel, and a CB radio has 40 channels. A typical cell
phone can communicate on 1,664 channels or more!
Range - A walkie-talkie can transmit about 1 mile
(1.6 km) using a 0.25-watt transmitter. A CB radio, because
it has much higher power, can transmit about 5 miles (8 km)
using a 5-watt transmitter. Cell phones operate within
cells, and they can switch cells as they move around.
Cells give cell phones incredible range. Someone using a
cell phone can drive hundreds of miles and maintain a
conversation the entire time because of the cellular
approach.
In simplex radio, both transmitters use the
same frequency. Only one party can talk at a
time.
In duplex radio, the two transmitters use
different frequencies, so both parties can talk at the
same time. Cell phones are
duplex.
In the next section, you'll get a good look inside a
digital cell phone.
Inside a Cell Phone On a "complexity per
cubic inch" scale, cell phones are some of the most intricate
devices people play with on a daily basis. Modern digital cell
phones can process millions of calculations per second
in order to compress and decompress the voice stream.
The parts of a cell phone
If you take a cell phone apart, you find that it contains
just a few individual parts:
An amazing circuit board containing the brains of the
phone
The circuit board is the heart of the system. Here
is one from a typical Nokia
digital phone:
The front of the circuit
board
The back of the circuit board
In the photos above, you see several computer chips. Let's
talk about what some of the individual chips do. The
analog-to-digital and digital-to-analog
conversion chips translate the outgoing audio signal from
analog to digital and the incoming signal from digital back to
analog. You can learn more about A-to-D and D-to-A conversion
and its importance to digital audio in How Compact Discs
Work. The digital signal processor (DSP) is a
highly customized processor designed to perform
signal-manipulation calculations at high speed.
The microprocessor
The microprocessor
handles all of the housekeeping chores for the keyboard and
display, deals with command and control signaling with the
base station and also coordinates the rest of the functions on
the board. The ROM and
Flash
memory chips provide storage for the phone's operating
system and customizable features, such as the phone
directory. The radio
frequency (RF) and power section handles power
management and recharging, and also deals with the hundreds of
FM channels. Finally, the RF amplifiers handle signals
traveling to and from the antenna.
The display and keypad contacts
The display has
grown considerably in size as the number of features in cell
phones have increased. Most current phones offer built-in
phone directories, calculators and even games. And many of the
phones incorporate some type of PDA or Web
browser.
The Flash memory card on the circuit
board
The Flash memory card removed
Some phones store certain information, such as the SID and
MIN codes, in internal Flash memory, while others use external
cards that are similar to SmartMedia
cards.
The cell-phone speaker, microphone and
battery backup
Cell phones have such tiny speakers and microphones that it
is incredible how well most of them reproduce sound. As you
can see in the picture above, the speaker is about the size of
a dime and the microphone is no larger than the watch battery
beside it. Speaking of the watch battery, this is used by the
cell phone's internal clock chip.
What is amazing is that all of that functionality -- which
only 30 years ago would have filled an entire floor of an
office building -- now fits into a package that sits
comfortably in the palm of your hand!
AMPS In 1983, the analog cell-phone standard
called AMPS (Advanced Mobile Phone System) was approved
by the FCC and first used in Chicago. AMPS uses a range
of frequencies between 824 megahertz (MHz) and 894 MHz for
analog cell phones. In order to encourage competition and keep
prices low, the U. S. government required the presence of two
carriers in every market, known as A and B carriers.
One of the carriers was normally the local-exchange
carrier (LEC), a fancy way of saying the local phone
company.
Carriers A and B are each assigned 832 frequencies:
790 for voice and 42 for data. A pair of frequencies (one for
transmit and one for receive) is used to create one
channel. The frequencies used in analog voice channels
are typically 30 kHz wide -- 30 kHz was chosen as the
standard size because it gives you voice quality comparable to
a wired
telephone.
The transmit and receive frequencies of each voice channel
are separated by 45 MHz to keep them from interfering
with each other. Each carrier has 395 voice channels, as well
as 21 data channels to use for housekeeping activities like
registration and paging.
A version of AMPS known as Narrowband Advanced Mobile
Phone Service (NAMPS) incorporates some digital technology
to allow the system to carry about three times as many
calls as the original version. Even though it uses digital
technology, it is still considered analog. AMPS and NAMPS only
operate in the 800-MHz band and do not offer many of the
features common in digital cellular service, such as e-mail
and Web browsing.
Along Comes Digital Digital cell
phones use the same radio technology as analog phones, but
they use it in a different way. Analog systems do not fully
utilize the signal between the phone and the cellular network
-- analog signals cannot be compressed and manipulated as
easily as a true digital signal. This is the reason why many
cable
companies are switching to digital -- so they can fit more
channels within a given bandwidth. It is amazing how much
more efficient digital systems can be.
Digital phones convert your voice into binary
information (1s and 0s) and then compress it (see How
Analog-Digital Recording Works for details on the
conversion process). This compression allows between
three and 10 digital cell-phone calls to occupy the space of a
single analog call.
Many digital cellular systems rely on frequency-shift
keying (FSK) to send data back and forth over AMPS. FSK
uses two frequencies, one for 1s and the other for 0s,
alternating rapidly between the two to send digital
information between the cell tower and the phone. Clever
modulation and encoding schemes are required to convert the
analog information to digital, compress it and convert it back
again while maintaining an acceptable level of voice quality.
All of this means that digital cell phones have to contain a
lot of processing power!
Cellular Access Technologies There are three
common technologies used by cell-phone networks for
transmitting information:
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Although these technologies sound very intimidating,
you can get a good sense of how they work just by breaking
down the title of each one.
The first word tells you what the access method is.
The second word, division, lets you know that it splits
calls based on that access method.
FDMA puts each call on a separate frequency.
TDMA assigns each call a certain portion of time
on a designated frequency.
CDMA gives a unique code to each call and spreads
it over the available frequencies.
The last part of
each name is multiple access. This simply means that
more than one user can utilize each cell.
FDMA separates the spectrum into distinct voice channels by
splitting it into uniform chunks of bandwidth. To
better understand FDMA, think of radio stations: Each station
sends its signal at a different frequency within the available
band. FDMA is used mainly for analog transmission.
While it is certainly capable of carrying digital information,
FDMA is not considered to be an efficient method for digital
transmission.
In FDMA, each phone uses a different
frequency.
TDMA is the access method used by the Electronics
Industry Alliance and the Telecommunications
Industry Association for Interim Standard 54
(IS-54) and Interim Standard 136 (IS-136). Using TDMA,
a narrow band that is 30 kHz wide and 6.7 milliseconds
long is split time-wise into three time slots.
Narrow band means "channels" in the traditional sense. Each
conversation gets the radio for one-third of the time. This is
possible because voice data that has been converted to digital
information is compressed so that it takes up significantly
less transmission space. Therefore, TDMA has three times
the capacity of an analog system using the same number of
channels. TDMA systems operate in either the 800-MHz
(IS-54) or 1900-MHz (IS-136) frequency bands.
TDMA splits a frequency into time
slots.
TDMA is also used as the access technology for Global
System for Mobile communications (GSM). However,
GSM implements TDMA in a somewhat different and
incompatible way from IS-136. Think of GSM and IS-136 as two
different operating
systems that work on the same processor,
like Windows and Linux both working on an Intel Pentium III.
GSM systems use encryption
to make phone calls more secure. GSM operates in the 900-MHz
and 1800-MHz bands in Europe and Asia, and in the 1900-MHz
(sometimes referred to as 1.9-GHz) band in the United States.
It is used in digital cellular and PCS-based systems.
GSM is also the basis for Integrated Digital Enhanced
Network (IDEN), a popular system introduced by Motorola
and used by Nextel.
Cool
Facts
The GSM standard for digital cell phones was
established in Europe in the mid-1980s -- long before
digital cellular phones became commonplace in American
culture.
It is now possible to locate a person using a
cellular phone down to a range of a few meters,
anywhere on the globe.
3G (third-generation wireless) phones may look
more like PDAs, with features such as
video-conferencing, advanced personal calendar
functions and multi-player gaming.
GSM is the international standard in Europe, Australia and
much of Asia and Africa. In covered areas, cell-phone users
can buy one phone that will work anywhere where the standard
is supported. To connect to the specific service providers in
these different countries, GSM users simply switch subscriber
identification module (SIM) cards. SIM cards are small
removable disks that slip in and out of GSM cell phones. They
store all the connection data and identification numbers you
need to access a particular wireless service provider.
Unfortunately, the 1900-MHz GSM phones used in the United
States are not compatible with the international
system. If you live in the United States and need to have
cell-phone access when you're overseas, the easiest thing to
do is to buy a GSM 900MHz/1800MHz cell phone for traveling.
You can get these phones from Planet
Omni, an online electronics firm based in California. They
offer a wide selection of Nokia,
Motorola
and Ericsson
GSM phones. They don't sell international SIM cards, however.
You can pick up prepaid SIM cards for a wide range of
countries at Telestial.com.
CDMA takes an entirely different approach from TDMA.
CDMA, after digitizing data, spreads it out over the
entire available bandwidth. Multiple calls are overlaid
on each other on the channel, with each assigned a unique
sequence code. CDMA is a form of spread
spectrum, which simply means that data is sent in small
pieces over a number of the discrete frequencies available for
use at any time in the specified range.
In CDMA, each phone's data has a unique
code.
All of the users transmit in the same wide-band
chunk of spectrum. Each user's signal is spread over the
entire bandwidth by a unique spreading code. At the
receiver, that same unique code is used to recover the signal.
Because CDMA systems need to put an accurate time-stamp on
each piece of a signal, it references the GPS system for
this information. Between eight and 10 separate calls can be
carried in the same channel space as one analog AMPS call.
CDMA technology is the basis for Interim Standard 95
(IS-95) and operates in both the 800-MHz and 1900-MHz
frequency bands.
Ideally, TDMA and CDMA are transparent to each other. In
practice, high-power CDMA signals raise the noise floor for
TDMA receivers, and high-power TDMA signals can cause
overloading and jamming of CDMA receivers.
In the next section, you'll learn about the difference
between cellular and PCS services.
Cellular vs. PCS Personal Communications
Services (PCS) is a wireless phone service very similar to
cellular phone service, but with an emphasis on
personal service and extended mobility. The term
"PCS" is often used in place of "digital cellular," but true
PCS means that other services like paging, caller ID and
e-mail are bundled into the service.
While cellular was originally created for use in cars, PCS
was designed from the ground up for greater user mobility. PCS
has smaller cells and therefore requires a larger
number of antennas to cover a geographic area. PCS phones
use frequencies
between 1.85 and 1.99 GHz (1850 MHz to 1990 MHz).
Technically, cellular systems in the United States operate
in the 824-MHz to 894-MHz frequency bands; PCS operates in the
1850-MHz to 1990-MHz bands. And while it is based on
TDMA, PCS has 200-kHz channel spacing and eight time
slots instead of the typical 30-kHz channel spacing and
three time slots found in digital cellular.
Now let's look at the distinction between "dual band" and
"dual mode" technologies.
Dual Band vs. Dual Mode If you travel a lot,
you will probably want to look for phones that offer dual
band, dual mode or both. Let's take a look at each
of these options:
Dual band - A phone that has dual-band capability
can switch frequencies. This means that it can
operate in both the 800-MHz and 1900-MHz bands. For example,
a dual-band TDMA phone could use TDMA services in either an
800-MHz or a 1900-MHz system.
Dual mode - In cell phones, "mode" refers to the
type of transmission technology used. So, a phone
that supported AMPS and TDMA could switch back and forth as
needed. It's important that one of the modes is AMPS -- this
gives you analog service if you are in an area that doesn't
have digital support.
Dual band/Dual mode - The best of both worlds
allows you to switch between frequency bands and
transmission modes as needed.
Changing bands or
modes is done automatically by phones that support these
options. Usually the phone will have a default option
set, such as 1900-MHz TDMA, and will try to connect at that
frequency with that technology first. If it supports dual
bands, it will switch to 800 MHz if it cannot connect at 1900
MHz. And if the phone supports more than one mode, it will try
the digital mode(s) first, then switch to analog.
Sometimes you can even find tri-mode phones. This
term can be deceptive. It may mean that the phone supports two
digital technologies, such as CDMA and TDMA, as well as
analog. But it can also mean that it supports one digital
technology in two bands and also offers analog support. A
popular version of the tri-mode type of phone for people who
do a lot of international traveling has GSM service in the
900-MHz band for Europe and Asia and the 1900-MHz band for the
United States, in addition to the analog service.
In the next section, we'll touch on some of the problems
encountered with cellular phones.
Problems with Cell Phones A cell phone, like
any other consumer electronic device, has its problems:
Generally, non-repairable internal corrosion of
parts results if you get the phone wet or use wet
hands to push the buttons. Consider a protective case. If
the phone does get wet, be sure it is totally dry before you
switch it on so you can try to avoid damaging internal
parts.
Extreme heat in a car can damage the battery or
the cell-phone electronics. Extreme cold may cause a
momentary loss of the screen display.
Analog cell phones suffer from a problem known as
"cloning." A phone is "cloned" when someone steals
its ID numbers and is able to make fraudulent
calls on the owner's account.
Here is how cloning occurs: When your phone makes a call,
it transmits the ESN and MIN to the network at the beginning
of the call. The MIN/ESN pair is a unique tag for your phone
-- this is how the phone company knows who to bill for the
call. When your phone transmits its MIN/ESN pair, it is
possible for nefarious sorts to listen (with a scanner)
and capture the pair. With the right equipment, it is fairly
easy to modify another phone so that it contains your MIN/ESN
pair, which allows the nefarious sort to make calls on your
account.
Check out the next section to find out about cell-phone
towers!
Cell-phone Towers A cell-phone tower is
typically a steel pole or lattice structure that rises
hundreds of feet into the air. This cell-phone tower along
I-85 near Greenville, SC, is typical in the United States:
This is a modern tower with three different cell-phone
providers riding on the same structure. If you look at the
base of the tower, you can see that each provider has its own
equipment, and you can also see how little equipment is
involved today (older towers often have small buildings at the
base):
Here is the equipment owned by one of the providers:
The box houses the radio transmitters and receivers
that let the tower communicate with the phones. The radios
connect with the antennae on the tower through a set of thick
cables:
If you look closely, you will see that the tower and all of
the cables and equipment at the base of the tower are heavily
grounded. For example, the plate in this shot with the
green wires bolting onto it is a solid copper grounding plate:
One sure sign that multiple providers share this tower is
the amazing five-way latch on the gate. Any one of five people
can unlock this gate to get in!
Cell-phone towers come in all shapes and sizes, but I do
believe this one in Morrisville, NC, is one of the weirdest
looking!
