How Cell Phones Work by nafees

7/25/2019 How Cell Phones Work by nafees

1/17

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 &not 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.
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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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How Cell Phones Work by nafees