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!
A digital cell phone from Nokia.
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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? We will discuss the technology
behind cell phones so that you can see how amazing they really are.
To access all of the different parts of this article, choose from
the map below: You may want to start
with How They Work, to learn the basics about cell phones. If you
are thinking about buying one, be sure to check out the features to
learn about which ones will suit your needs.
The Cell Approach One of the most interesting
things about a cell phone is that it is really 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!
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. 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 (see this page for details).
A single cell in an analog system uses one-seventh of the
available duplex voice channels. That is, one cell, plus the six
cells around it on the hexagonal grid, are each using one-seventh of
the available channels so that each cell 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 56 or so voice channels available.
In other words, in any cell, 56 people can be talking on
their cell phones 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 would have 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.
For the next section of "How They Work" click here,
or choose from the map below:
From Cell to Cell
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 turned 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 what 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 will note 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) will be able to see
your phone's signal strength increasing. The two base stations
coordinate themselves 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.
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 one mile using a
0.25 watt transmitter. A CB radio, because it has much higher
power, can transmit about five miles 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.
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In duplex radio, the two transmitters use
different frequencies, so both parties can talk at the same
time. Cell phones are
duplex.
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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 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 another 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.
The reason that 30 kHz was chosen as the standard size is 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, paging, etc.
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 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. The same
reasoning applies to many cable companies that are going 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. (Read How Analog and Digital Recording
Works for details on the conversion process.) This compression
allows between three and ten cell phone calls to occupy the space of
a single analog cell phone voice 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 "1"s and the other for
"0"s, 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 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 and 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 (multiple) can use (access) 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.
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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.
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 else 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 a cell phone access when you're
overseas, the easiest thing to do is 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 bandwidth it has
available. Multiple calls are overlaid over 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 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 will raise the noise floor for
TDMA receivers, and high power TDMA signals can cause overloading
and jamming of CDMA receivers.
What's the Difference Between Cellular and
PCS? Personal Communications Services (PCS) is a
wireless phone service very similar to cellular phone service 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 gigahertz (1850 MHz - 1990 MHz).
Technically, cellular systems in the United States operate in the
824-894 megahertz (MHz) frequency bands; PCS operates in the
1850-1990 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.
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. Lets 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 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. An important factor to
look for is 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 TriMode 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 U.S. in addition to the analog service.
Problems with Cell Phones A cell phone, like any
other consumer electronic device, can break. Here are some of the
preventive measures you can take:
- 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 to avoid damaging
internal parts.
- You can lessen the chance of dropping a phone or damaging the
connectors if you use a belt-clip or a holster. The
use of headsets really makes this consideration important.
- Cracked display screens can happen when an overstuffed
briefcase squeezes the cell phone.
- 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, and it is how
the phone company knows whom 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.
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 various parts of a cell phone.
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If you ever take a cell phone apart, you will 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 we saw 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.
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In the photos above, you see several computer chips. Lets 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 the How Stuff Works article on compact discs. The
Digital Signal Processor (DSP) is a highly customized
processor designed to perform signal manipulation calculations at
high speed.
The microprocessor.
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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 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.
The display and keypad contacts.
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The display has grown considerably in size as the number of
features in cell phones have increased. Most phones currently
available 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.
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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 the entire floor of an office building -
now fits into a package that sits comfortably in the palm of your
hand!
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, S.C. is
typical in the U.S.:
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/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, N.C. is the weirdest one I have ever seen!
What They Can Do Cell phones provide a way of
staying in touch and having instant communication at your
fingertips. With a cell phone, you can:
- Call your significant other to let them know that you are on
your way home.
- Contact the police or hospital if you have an emergency.
- Let the boss know that you are stuck in traffic and will be
late for that big meeting.
- Provide a way for others to contact you if you are always on
the go.
- Call home or work to check your messages while on the road.
- Store contact information (names and phone numbers).
- Make task or to-do lists (some models).
- Keep track and remind you of appointments (date book,
calendar).
- Use the built-in calculator for simple math.
- Send or receive e-mail (some models).
- Get information (news, entertainment, stock quotes) from the
Internet (some models).
- Play simple games (some models).
- Integrate other devices such as PDAs, MP3 players and GPS
receivers (some models).
Features Here is a list of features that should be
considered when looking for a cell phone:
- Service plan
- Mode
- Battery type
- Display
- Included functions
- Special Features
- Size
- Price
Service
Plan Before you set your sights on a particular make or
model of cell phone, you should decide on the service plan that
interests you. Otherwise, you could find that the phone you want is
not supported by the plan you need. We will go in depth about this
subject in a dedicated article on "How Cell Phone Service Plans
Work."
Mode Are you looking for analog
or digital? Do you prefer PCS or cellular? TDMA or CDMA?
Battery type Cell phones use
two main battery technologies:
- NiMH (Nickel Metal Hydride) - high capacity battery that
provides extra power for extended use
- Li-ion (Lithium Ion) - has a lot of power in a lightweight
package but usually costs more than NiMH batteries
Note
both the talk time and standby time when comparing phones. Also,
check to see how long the battery takes to recharge and whether a
rapid charger is available. Most cell phone batteries are removable,
but some of the smaller models have a built-in battery instead.
Display All cell phones have
LCD displays, but the specific features of the display can vary:
- Size - A large multi-line display is typically more
expensive but necessary if you plan to use the phone for wireless
Internet.
- Color vs. monochrome - Most cell phones have
monochrome displays (16 grays), but a few are beginning to
appear that have color. Cell phones with color screens need more
memory and tend to be more expensive.
- Reflective or backlit - Almost all cell phones have
backlit screens, which are good for low light conditions.
- Multiparty calls
- E-mail/text messaging
- Minibrowser
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