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How Cell Phones Work
From Tibet to Tanzania to Toronto, no matter where you go you'll see someone
talking on his or her cell phone. 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:
tore contact information
!ake task or to"do lists
#eep track of appointments and set reminders
$se the built"in calculator for simple math
end or receive e"mail
%et information &news, entertainment, stock uotes( from the )nternet
*lay games
+atchT
end te-t messages
Take photos and videos
)ntegrate other devices such as *Ds, !*/ playersand %* receivers
0ou might hear terms like 1%, 2T3, %! and 4D! thrown around and wonder what
they refer to. t its most basic, a cell phone is a radio"" an e-tremely sophisticated
radio, but a radio nonetheless. +e'll show you what we mean.
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In half-duplex radio, both transmitters use the same frequency. nly one
party can talk at a time.
Cell-phone !requencies
)n the dark ages before cell phones, people who really needed mobile"
communications ability installed radio telephones in theircars. )n the radio"
telephone system, there was one central antenna tower per city, and perhaps 56
channels available on that tower. This central antenna meant that the phone in your
car needed a powerful transmitter "" big enough to transmit 17 or 67 miles &about
87 kilometers(. )t also meant that not many people could use radiotelephones ""
there 9ust were not enough channels.
The genius of the cellular system is the division of a city into small cells. This allows
e-tensive freuency reuse across a city, so that millions of people can use cell
phones simultaneously.
good way to understand the sophistication of a cell phone is to compare it to a 4
radioor a walkie"talkie.
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In full-duplex radio, the two transmitters use di"erent frequencies, soboth parties can talk at the same time. Cell phones are full-duplex.
!ull-duplex #s. half-duplex" oth walkie"talkies and 4 radios are half-
duplexdevices. That is, two people communicating on a 4 radio use the
same freuency, so only one person can talk at a time. cell phone is a full-
duplexdevice. That means that you use one freuency for talking and a
second, separate freuency for listening. oth people on the call can talk at
once.
Channels" walkie"talkie typically has one channel, and a 4 radio has 17
channels. typical cell phone can communicate on ;,
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3ach cell has a base station that consists of a tower and a small building containing
the radio euipment. +e'll get into base stations later. First, let's e-amine the >cells>
that make up a cellular system.
Cell-phone Channels
single cell in an analog mobile phone system uses one"seventh of the
available duple- voice channels. That is, each cell &of the seven on a he-agonal
grid( is using one"seventh of the available channels so it has a uniue set of
freuencies and there are no collisions:
cell phone carrier typically gets =/5 radio freuencies to use in a city.
3ach cell phone uses two freuencies per call "" a duple- channel"" so there
are typically /?6 voice channels per carrier. &The other 15 freuencies are
used for control channels "" more on this later.(
Therefore, each cell has about 6< voice channels available. )n other words, in anycell, 6< people can be talking on their cell phone at one time. nalog cellular
systems are considered @rst"generation mobile technology, or ;%. +ith digital
transmission methods &5%(, the number of available channels increases. For
e-ample, a TD!"based digital system &more on TD! later( can carry three times
as many calls as an analog system, so each cell has about ;
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ll cell phones have special codesassociated with them. These codes are used to
identify the phone, the phone's owner and the service provider.
2et's say you have a cell phone, you turn it on and someone tries to call you. Bere's
what happens to the call:
+hen you @rst power up the phone, it listens for an (I*&see sidebar( on the
control channel. The control channel is a special freuency that the phone
and base station use to talk to one another about things like call set"up and
channel changing. )f the phone cannot @nd any control channels to listen to,
it knows it is out of range and displays a >no service> message.
+hen it receives the )D, the phone compares it to the )D programmed into
the phone. )f the )Ds match, the phone knows that the cell it is
communicating with is part of its home system.
long with the )D, the phone also transmits a registration reuest, and the
!TC keeps track of the phone's location in a database "" this way, the !TC
knows which cell you are in when it wants to ring your phone.
The !TC gets the call, and tries to @nd you. )t looks in its database to see
which cell you are in.
The !TC picks a freuency pair that your phone will use in that cell to take
the call.
The !TC communicates with your phone over the control channel to tell it
which freuencies to use, and once your phone and the tower switch on those
freuencies, the call is connected. ow, you are talking by two"way radio to afriend.
s you move toward the edge of your cell, your cell's base station notes that
your signal strength is diminishing. !eanwhile, the base station in the cell
you are moving toward &which is listening and measuring signal strength on
all freuencies, not 9ust its own one"seventh( sees your phone's signal
strength increasing. The two base stations coordinate with each other
through the !TC, and at some point, your phone gets a signal on a control
channel telling it to change freuencies. This handoE switches your phone to
the new cell.
s you travel, the signal is passed from cell to cell. 2et's say you're on the phone
and you move from one cell to another "" but the cell you move into is covered by
another service provider, not yours. )nstead of dropping the call, it'll actually be
handed oE to the other service provider. )f the )D on the control channel does not
match the )D programmed into your phone, then the phone knows it is roamin%.
The !TC of the cell that you are roaming in contacts the !TC of your home
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system, which then checks its database to con@rm that the )D of the phone you are
using, is valid. 0our home system veri@es your phone to the local !TC, which then
tracks your phone as you move through its cells. nd the amazing thing is that all of
this happens within seconds.
The less amazing thing is that you may be charged insane rates for your roamingcall. Cn most phones, the word >roam> will come up on your phone's screen when
you leave your provider's coverage area and enter another's. )f not, you'd better
study your coverage maps carefully "" more than one person has been unpleasantly
surprised by the cost of roaming. 4heck your service contract carefully to @nd out
whether you're paying when you roam. !ost of the larger phone companies do not
charge for roaming within the $.., but some of the discount companies do.
)nternationally is another story. The roaming rates can be very high, assuming you
have a phone that can work in multiple countries. DiEerent countries use diEerent
cellular access technologies. !ore on those technologies later. First, let's get some
background on analog cell phonetechnology so we can understand how theindustry has developed.
+nalo% Cell Phones
)n ;?=/, the analog cell phone standard called +&P(&dvanced !obile *hone
ystem( was approved by the $.. Federal 4ommunications 4ommission &F44( and
@rst used in 4hicago. !* uses a range of freuenciesbetween =51 megahertz
&!Bz( and =?1 !Bz for analog cell phones. )n order to encourage competition and
keep prices low, the $. . governmentreuired the presence of two carriers in every
market, known as and carriers. Cne of the carriers was normally the local-
exchan%e carrier&234(, a fancy way of saying the local phone company.
4arriers and are each assigned =/5 freuencies: 8?7 for voice and 15 for data.
pair of freuencies &one to transmit and one to receive( is used to create one
channel. The freuencies used in analog voice channels are typically /7 kilohertz
&kBz( wide "" /7 kBz was chosen as the standard size because it gives you voice
uality comparable to a wired telephone.
The transmit and receive freuencies of each voice channel are separated by 16
!Bz to keep them from interfering with each other. 3ach carrier has /?6 voice
channels, as well as 5; data channels to use for housekeeping activities like
registration and paging.
version of !* known as arrowband +d#anced &obile Phone
(er#ice&+&P(( incorporates some digital technology to allow the system to
carry about three times as many calls as the original version. 3ven though it uses
digital technology, it is still considered analog. !* and !* only operate in the
=77"!Bz band and don't oEer many of the features common in digital cellular
service, such as e"mail and +eb browsing.
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+lon% Comes *i%ital
The @rst digital cell phones were the second generation &5%( of cellular technology.
Digital phones use the same radio technology as analog phones, but they use it
diEerently. nalog systems don't fully use the signal between the phone and the
cellular network "" analog signals can't be compressed and manipulated as easily astrue digital signals. This is why cablecompanies switched to digital "" to @t more
channels within a given bandwidth.
Digital phones convert your voice into binaryinformation &;s and 7s( and then
compress it &see Bow nalog"Digital ecording +orksfor details on the conversion
process(. This compression allows between three and ;7 digital cell phone calls to
occupy the space of a single analog call.
!any digital cellular systems rely on frequency-shift keyin%&F#( to send data
back and forth over !*. F# uses two freuencies, one for ;s and the other for
7s, alternating rapidly between the two to send digital information between the cell
tower and the phone. 4lever modulation and encoding schemes are reuired to
convert the analog information to digital, compress it and convert it back again
while maintaining an acceptable level of voice uality. ll of this means that digital
cell phones have to contain a lot of processing power.
Inside a *i%ital Cell Phone
Cn a >comple-ity per cubic inch> scale, cell phones are some of the most intricate
devices people use on a daily basis. !odern digital cell phones can process millions
of calculations per second in order to compress and decompress the voice stream.
)f you take a basic digital cell phone apart, you @nd that it contains 9ust a few
individual parts:
circuit board containing the brains of the phone
n antenna
liuid crystal display&24D(
keyboard ¬ unlike the one you @nd in aT remote control(
microphone
speaker
battery
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Inside a Cell phone Ima%e allery
The circuit board is the heart of the system. 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. 0ou can learn more about "to"
D and D"to" conversion and its importance to digital audio in Bow 4ompact Discs
+ork. The digital signal processor &D*( is a highly customized processor designed
to perform signal"manipulation calculations at high speed.
The microprocessorhandles 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 C!and Gash memorychips provide storage for the phone's operating
systemand customizable features, such as the phone directory. The radio
freuency&F( and power section handles power management and recharging, and
also deals with the hundreds of F! channels. Finally, the $! ampliers handle
signals traveling to and from the antenna.
The displayhas grown considerably in size as the number of features in cell phones
has increased. !ost current phones oEer built"in phone directories, calculators,
games, calendars, notes, +eb browsers, and cameras, as well as countless other
applications, or apps, to serve practically any need or want.
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'he (I& card on the circuit board
ome phones store certain information, such as the )D and !) codes, in internal
Flash memory, while others use e-ternal cards that are similar to mart!ediacards.
4ell phones have such tiny speakers and microphones that it is incredible how well
most of them reproduce sound. s 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. peaking of the watch battery, this is used by the cell phone's internal
clock chip.
+hat is amazing is that all of that functionality "" which only /7 years ago would
have @lled an entire Goor of an oAce building "" now @ts into a package that sits
comfortably in the palm of your handH
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In !*&+, each phone uses a di"erent frequency.
Cell Phone etwork 'echnolo%ies/ 0
4ell phone networks fall into three categories: 5%, /% and 1%. )n 5% networks, there
are three common technologies used for transmitting information:
!requency di#ision multiple access&FD!(
'ime di#ision multiple access&TD!(
Code di#ision multiple access&4D!(
lthough these technologies sound very intimidating, you can get a good sense of
how they work 9ust by breaking down the title of each one.
The @rst word tells you what the access method is. The second word, >division,> lets
you know that it splits calls based on that access method.
FD! puts each call on a separate freuency.
TD! assigns each call a certain portion of time on a designated freuency.
4D! gives a uniue code to each call and spreadsit over the available
freuencies.
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The last part of each name is >multiple access.> This simply means that more than
one user can use each cell.
!*&+separates the spectrum into distinct voice channels by splitting it into
uniform chunks of bandwidth. To better understand FD!, think of radio stations:
3ach station sends its signal at a diEerent freuency within the available band.FD! is used mainly for analog transmission. +hile it is certainly capable of
carrying digital information, FD! is not considered to be an eAcient method for
digital transmission.
'*&+is the access method used by the 3lectronics )ndustry llianceand
theTelecommunications )ndustry ssociationfor )nterim tandard 61 &)"61( and
)nterim tandard ;/< &)";/
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(& and C*&+
TD! is also used as the access technology for %lobal ystem for !obile
4ommunications&%!(. Bowever, (&implements TD! in a somewhat diEerent
and incompatible way from )";/
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receiver, that same uniue code is used to recover the signal. ecause 4D!
systems need to put an accurate time"stamp on each piece of a signal, it references
the %* system for this information. etween eight and ;7 separate calls can be
carried in the same channel space as one analog !* call. 4D! technology is the
basis for )nterim tandard ?6 &)"?6( and operates in both the =77"!Bz and ;?77"
!Bz freuency bands.
)deally, TD! and 4D! are transparent to each other. )n practice, high"power
4D! signals raise the noise Goor for TD! receivers, and high"power TD! signals
can cause overloading and 9amming of 4D! receivers.
5% is a cell phone network protocol. 4lick here to learn about network protocolsfor
smart phones.
e-t, we'll look at /%.
Cell-phone etwork 'echnolo%ies/ 1
/% technology came along to support increased data needs. /% stands for >third
generation> "" this makes analog cellular technology generation one and digitalJ*4
generation two. /% technology is intended for the true multimedia cell phone ""
typically called smart phones"" and features increased bandwidth and transfer rates
to accommodate +eb"based applications and phone"based audio and video @les.
/% comprises several cellular access technologies. 4ommon ones include:
4D!5777" based on 5% 4ode Division !ultiple ccess
+4D!&$!T( " +ideband 4ode Division !ultiple ccess
TD"4D!" Time"division ynchronous 4ode"division !ultiple ccess
/% networks have potential transfer speeds of up to / !bps &about ;6 seconds to
download a /"minute!*/song(. For comparison, the fastest 5% phones can achieve
up to ;11#bps &about = minutes to download a /"minute song(. /%'s high data rates
are ideal for downloading information from the )nternet and sending and receiving
large, multimedia @les. /% phones are like mini"laptops and can accommodate
broadband applications like video conferencing, receiving streaming video from the
+eb, sending and receiving fa-es and instantly downloading e"mail messages with
attachments.
/% is a cell phone network protocol. 4lick here to learn about network protocolsfor
smart phones.
e-t we look at the latest network technology: 1%.
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+ woman displays a fourth-%eneration Wi&+2 de#ice durin% a exhibition at
a con#ention centrer in 'aipei in 0343.
Cell-phone etwork 'echnolo%ies/ 5
The >1%> cell phone has been around for years K at least since 577
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!bps Lsources:Tech Target, 1% mericas, uddockM. ntenna improvements may
further increase B*O transmission rates Lsource: 1% mericasM.
Despite the obvious gains in speed, many in the cell phone world consider both
+i!N and B*O to be transitional technologies Lsource: %aylordM. The 1% most
people are waiting for is 2T3.
5 6'7
For decades, the mobile world has pushed for industry"wide standards to sync up
the technology and allow for de@nable advances. !any see 2T3, or 2ong Term
3volution, as the @rst real chance at standardization, as many top carriers have
signed on to adopt the technology Lsource: 1% mericasM.
2T3 is being developed as the 1% standard, which is why you sometimes see >1%
2T3> as opposed to simply >1%.> 1% could mean support for any speed above /%Q 1%
2T3 means support for up to =< !bps based on speci@c technology and software
infrastructures Lsource: 1% mericasM.
The 2T3 network is based on )nternet *rotocol &)*( standards, the kind that delivers
+eb pages to your computer, and adds voice data to the transmission streams
Lsource: 1% mericasM. )t uses a schematic called CFD!, or Crthogonal Freuency
Division !ultiple ccess, which is similar to the CFD! approach in +i!N. CFD!
also separates the bits in a single data transmission into multiple subcarriers to
increase speed, reassembling it at the destination. The 2T3 protocol, though, has the
added ability to assign particular data paths to particular users on the Gy,
optimizing the bandwidth available at any given time Lsource: 1% mericasM.
2T3 can operate on a wide range of radio freuency bands, which will allow many
mobile carriers to switch over to 2T3 without starting from scratch Lsource: 1%
mericasM. The migration has already begun: 1% 2T3 is operating in many $.. and
3uropean cities in 57;/ Lsource: CsborneM. )t doesn't reuire a new phone. 2T3 can
operate alongside 5% and /% networks, and multi"mode phones can access any of
them, using 2T3 where it's available and, say, B* where it's not Lsource: 1%
mericasM.
Baving a multi"mode phone, then, is a huge bene@t as 2T3 towers start popping up
around the country and around the world.
&ulti-band #s. &ulti-mode Cell Phones
)f you travela lot, you will probably want to look for phones that oEer multiple
bands, multiple modes or both. 2et's take a look at each of these options:
&ultiple band/ phone that has multiple"band capability can switch
freuencies. For e-ample, a dual"band TD! phone could use TD! services
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in either an =77"!Bz or a ;?77"!Bz system. uad"band %! phonecould
use %! service in the =67"!Bz, ?77"!Bz, ;=77"!Bz or ;?77"!Bz band.
&ultiple mode/)n cell phones, >mode> refers to the type of transmission
technology used. o, a phone that supported !* and TD! could switch
back and forth as needed. )t's important that one of the modes is !* "" thisgives you analog service if you are in an area that doesn't have digital
support.
&ultiple band8&ultiple modes/This best of both worlds allows you to
switch between freuency bands and transmission modes as needed.
4hanging bands or modes is done automatically by phones that support these
options. $sually the phone will have a default option set, such as ;?77"!Bz TD!,
and will try to connect at that freuency with that technology @rst. )f it supports
dual bands, it will switch to =77 !Bz if it cannot connect at ;?77 !Bz. nd if the
phone supports more than one mode, it will try the digital mode&s( @rst, then switchto analog.
0ou can @nd both dual"mode and tri"mode phones. The term >tri"mode> can be
deceptive. )t may mean that the phone supports two digital technologies, such as
4D! and TD!, as well as analog. )n that case, it is a true tri"mode phone. ut it
can also mean that it supports one digital technology in two bands and also oEers
analog support. popular version of the tri"mode type of phone for people who do a
lot of international traveling has %! service in the ?77"!Bz band for 3urope and
sia and the ;?77"!Bz band for the $nited tates, in addition to the analog service.
Technically, this is a dual"mode phone, and one of those modes &%!( supports two
bands.
Cf course, none of this would be possible without those soaring towers that carry
cell phone signals from phone to phone.
Problems with Cell Phones
cell phone tower is typically a steel pole or lattice structure that rises hundreds of
feet into the air.
*ictured here is a tower with three diEerent cell phone providers riding on the same
structure. )f you look at the base of a tower, you can see provider euipment.
The bo- houses the radio transmitters and receivers that let the tower communicate
with the phones. The radiosconnect with the antennae on the tower through a set
of thick cables.
)f you look closely, you will see that the tower and all of the cables and euipment
at the base of the tower are heavily grounded.
http://electronics.howstuffworks.com/question537.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/question537.htmhttp://electronics.howstuffworks.com/radio.htm7/25/2019 How Cell Phones Work by nafees
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Cne sure sign that multiple providers share a tower is a @ve"way latch on the gate.
ny one of @ve people can unlock this gate to get in.
2ike all consumer electronics, cell phones come with their share of problems. e-t,
we'll take a look at some of them.
Cell-phone 'owers
cell phone, like any other electronic device, has its problems:
%enerally, non"repairable internal corrosion of parts results if you get the
phone wet or use wet hands to push the buttons. 4onsider a protective case.
)f 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.
3-treme heat in a carcan damage the battery or the cell phone electronics.
3-treme cold may cause a momentary loss of the screen display.
nalog cell phones suEer from a problem known as >cloning.> phone is
>cloned> when someone steals its )D numbers and is able to make fraudulent
calls on the owner's account.
Bere is how cloning occurs: +hen your phone makes a call, it transmits the 3
&electronic serial number( and !) &mobile identi@cation number or phone number(
to the network at the beginning of the call. The !)J3 pair is a uniue tag for
your phone "" this is how the phone company knows who to bill for the call. +hen
your phone transmits its !)J3 pair, it is possible for nefarious sorts to listen
&with a scanner( and capture the pair. +ith the right euipment, it is fairly easy to
modify another phone so that it contains your !)J3 pair, which allows the crookto make calls on your account.
For more information about cell phones and related topics, check out the links on
the ne-t page and be sure to read Bow uying a 4ell phone +orksfor loads of
helpful consumer tips.
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