How Cell phone works

Millions of people in the India 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:

• Store contact information

• Make task or to-do lists
• Keep track of appointments and set reminders
• Use the built-in calculator for simple math
• Send or receive e-mail
• Get information (news, entertainment, stock quotes) from the Internet
• Play games
• Watch TV
• Send text messages
• Integrate other devices such as PDAs, MP3 players and GPS receivers

Explore Cell Phones

• iPhone
• Cell
Phone Quiz
• 10
Popular
Phones
But have you ever wondered how a cell phone works? What makes it different from a regular phone? What
do all those terms like PCS, GSM, CDMA and TDMA mean? In this article, 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 what you should know before
making a purchase.
To start with, 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.

Cell-phone Frequencies
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.
A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-
talkie.

• Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex 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 full-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 half-duplex radio, both transmitters use the same frequency. Only one
party can talk at a time.

In full-duplex radio, the two transmitters use different frequencies, so both

parties can talk at the same time.
Cell phones are full-duplex.

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:
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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.
We'll get into base stations later. First, let's examine the "cells" that make up a cellular system.

Cell-phone Channels
A single cell in an analog cell-phone 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
later.)

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. Analog cellular systems are considered first-generation mobile
technology, or 1G. With digital transmission methods (2G), the number of available channels increases.
For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as
an analog system, so each cell has about 168 channels available.
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

Cell-phone Codes

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.
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.
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. Now,
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.

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As you travel, the signal is passed from cell to cell.
Let'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. Instead of dropping the call, it'll actually be handed off to
the other service provider.
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.
The less amazing thing is that you may be charged insane rates for your roaming call. On 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. If not, you'd better study your coverage maps carefully -- more than one person has been
unpleasantly surprised by the cost of roaming. Check your service contract carefully to find out how much
you're paying when you roam.
Note that if you want to roam internationally, you'll need a phone that will work both at home and abroad.
Different countries use different cellular access technologies. More on those technologies later. First, let's
get some background on analog cell-phone technology so we can understand how the industry has
developed

Analog Cell Phones

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
Photo courtesy Motorola, Inc.
original version. Even though it uses digital technology, it is still Old school: DynaTAC cell
considered analog. AMPS and NAMPS only operate in the 800-MHz phone, 1983
band and do not offer many of the features common in digital cellular
service, such as e-mail and Web

Along Comes Digital

Digital cell phones are the second generation (2G) of cellular technology. They 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.
Let's take a good look inside a digital cell phone.

Inside a Digital Cell Phone

On a "complexity per cubic inch" scale, cell phones are some of the most intricate devices people use 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 basic digital cell phone apart, you find that it contains just a few individual parts:

• An amazing circuit board containing the brains of the phone

• An antenna
• A liquid crystal display (LCD)
• A keyboard (not unlike the one you find in a TV remote control)
• A microphone
• A speaker
• A battery

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 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 microprocessor

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 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!
In the next section, we'll get into the cell-phone networking methods.

Cell Phone Network Technologies: 2G

There are three common technologies used by 2G cell-phone networks for transmitting information (we'll
discuss 3G technologies in the 3G section):

• 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
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
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.
GSM
Unlocking Your GSM Phone

Any GSM phone can

work with any SIM card,
but some service
providers "lock" the
phone so that it will only
work with their service. If
your phone is locked, you
can't use it with any other
service provider, whether
locally or overseas. You
can unlock the phone
using a special code -- but
it's unlikely your service
provider will give it to
you. There are Web sites
that will give you the
unlock code, some for a
small fee, some for free.
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 850-MHz and
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.
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 850MHz/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, you can either buy a tri-band or quad-band GSM phone and use it both at home and when
traveling or just buy a GSM 900MHz/1800MHz cell phone for traveling. You can get 900MHz/1800MHz
GSM 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
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.

2G is a cell phone network protocol. Click here to learn about network protocols for Smartphones.

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.
2G is a cell phone network protocol. Click here to learn about network protocols for Smartphones.

Now let's look at the distinction between multiple-band and multiple-mode technologies.

Multi-band vs. Multi-mode Cell Phones

Dual Band vs. Dual Mode
If you travel a lot, you will probably want to look for phones that offer multiple bands, multiple modes or
both. Let's take a look at each of these options:

• Multiple band - A phone that has multiple-band capability can switch frequencies. For
example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or a 1900-MHz
system. A quad-band GSM phone could use GSM service in the 850-MHz, 900-MHz, 1800-MHz or
1900-MHz band.
• Multiple 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.
• Multiple band/Multiple 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.
You can find both dual-mode and tri-mode phones. The term "tri-mode" can be deceptive. It may mean
that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog. In that
case, it is a true tri-mode phone. 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. Technically, this is a dual-mode phone, and one of
those modes (GSM) supports two bands.
In the next section, we'll take a look at 3G mobile-phone technology.

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.

Cell-phone Network Technologies: 3G

3G technology is the latest in mobile communications. 3G stands for "third generation" -- this makes
analog cellular technology generation one and digital/PCS generation two. 3G technology is intended for
the true multimedia cell phone -- typically called smartphones -- and features increased bandwidth and
transfer rates to accommodate Web-based applications and phone-based audio and video files.

Photo courtesy Amazon.com

Sony Ericsson V800 3G phone

3G comprises several cellular access technologies. The three most common ones as of 2005 are:

• CDMA2000 - based on 2G Code Division Multiple Access (see Cellular Access Technologies)
• WCDMA (UMTS) - Wideband Code Division Multiple Access
• TD-SCDMA - Time-division Synchronous Code-division Multiple Access

3G networks have potential transfer speeds of up to 3 Mbps (about 15 seconds to download a 3-minute
MP3 song). For comparison, the fastest 2G phones can achieve up to 144Kbps (about 8 minutes to
download a 3-minute song). 3G's high data rates are ideal for downloading information from the Internet
and sending and receiving large, multimedia files. 3G phones are like mini-laptops and can accommodate
broadband applications like video conferencing, receiving streaming video from the Web, sending and
receiving faxes and instantly downloading e-mail messages with attachments.
Of course, none of this would be possible without those soaring towers that carry cell-phone signals from
phone to phone.

3G is a cell phone network protocol. Click here to learn about network protocols for Smartphones.
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, North Carolina, is
one of the weirdest looking.
That is one tall, ugly tree!
Like all consumer electronics, cell phones come with their share of problems. In the next section, we'll take
a look at some of the issues facing cell phones

Problems with Cell Phones

A cell phone, like any other 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 individual to
make calls on your account.
For more information about cell phones and related topics, check out the links on the next page and be sure
to read How Buying a Cell Phone Works for loads of helpful consumer tips

On a "complexity per cubic inch" scale, cell phones are some of the most intricate devices people use on a
daily basis. Modern cell phones can process millions of calculations per second in order to compress and
decompress the voice stream. If you have read How Cell Phones Work, you know that they can transmit
and receive on hundreds of FM channels, switching channels in sync with base stations as the phone moves
between cells.

If you ever take a cell phone apart you will find that it contains just a few individual parts:

• A microscopic microphone

Shopping for a
cell phone?

Read Cell Phone Reviews and compare prices at Consumer Guide Products before you
buy.

• A speaker
• An LCD or plasma display
• A keyboard not unlike the one we saw in a TV remote control
• An antenna
• A battery
• An amazing circuit board containing the guts of the phone

The circuit board is the heart of the system. Here is one from a typical Ericsson cell phone:

In this picture several of the components are identified. Starting from the left you the see the Analog-to-
Digital and Digital-to-Analog conversion chips. You can learn more about A-to-D and D-to-A conversion
and its importance to digital audio in How CDs Work. The DSP is a "Digital Signal Processor" -- a highly
customized processor designed to perform signal manipulation calculations at high speed. This DSP is rated
at about 40 MIPS (Millions of Instructions per Second) and handles all the signal compression and
decompression. The microprocessor (Ericsson phones use an ASIC version of the Z-80) and memory
handle all of the housekeeping chores for the keyboard and display, deal with command and control
signaling with the base station and also coordinate the rest of the functions on the board. The RF and power
section handles power management and recharging and also deals with the hundreds of FM channels.
Finally the RF (Radio Frequency) amplifiers handle signals in and out of the antenna.
What is amazing is that all of that functionality -- which only 30 years ago would have filled the entire
floor of an office building -- now fits into a package that sits comfortably in the palm of your hand.
-Sunil
-Santhosh