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.
Just like digital cellular, there are several incompatible
standards using PCS technology. Two of the most popular are
Cellular Digital Packet Data (CDPD) and GSM.
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!
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