Free Web Site - Free Web Space and Site Hosting - Web Hosting - Internet Store and Ecommerce Solution Provider - High Speed Internet

Working Of Cell Phones CELL PHONES

CELL PHONES

	INTRODUCTION
	The Cell Approach
	From Cell to Cell
	AMPS
	Along Comes Digital
	Cellular Access Technologies
	Difference Between Cellular and PCS
	Dual Band vs. Dual Mode
	Problems with Cell Phone
	Inside a Cell Phone
	Cell Phone Towers

INTRODUCTION

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!

A digital cell phone from Nokia

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?

Here in this page we will discuss the technology behind cell phones so that you can see how amazing they really are.

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 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).

From Cell to Cell
All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider.

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.

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.

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.

In duplex radio, the two transmitters use different
frequencies, so both parties can talk at the same time.
Cell phones are duplex.

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. 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.

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.

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.

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.

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.

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.

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!

That is one tall, ugly tree!

Home | Mail to: suji2k@hotmail.com | About Us