Each day something like 30,000 people in the United States sign up for and start using a cellular phone. Therefore it is likely that you or someone you know has a cell phone and uses it on a regular basis. They are such great gadgets - with a cell phone you can talk to anyone on the planet from just about anywhere (80% of the U.S. has coverage). But have you ever wondered how a cell phone works? In this edition of How Stuff Works we will discuss the technology behind cell phones so that you can see how amazing they really are.

Cell Phone Plans If you are shopping for a cell phone and are trying to make sense of all the different cell phone offers and plans, this link is useful: Find the Right Phone and Service -- compare and choose from service plans and phones offered by AT&T Wireless, Nextel, PrimeCo and more. If you try this link, please rate it: Great OK Not Helpful Comments:

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. A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie. A CB radio is a simplex device. 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, so it uses one frequency for talking and a second, separate frequency for listening. A CB radio has 40 channels. A cell phone can communicate on 1,664 channels. Cell phones also operate within cells and they can switch cells as they move around. Cells give cell phones incredible range. A walkie-talkie can transmit perhaps a mile. A CB radio, because it has much higher power, can transmit perhaps 5 miles. Someone using a cell phone, on the other hand, can drive clear across a city and maintain a conversation the entire time. Cells are what give a cell phone its incredible range.

In simplex radio (e.g. - CB radios), 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.

The cell phone standard called AMPS (Advanced Mobile Phone System) was approved by the FCC, finalized and first used in Chicago in 1983. The standard established a range of frequencies between 824 megahertz and 894 megahertz for analog cell phones. In order to encourage competition and keep prices low, this standard established the concept of 2 carriers in every market, known as A and B carriers (e.g. - Sprint and Cellular One). Each carrier is given 832 voice frequencies, each with a bandwidth of 30 kilohertz. A pair of frequencies (one for transmit and one for receive) is used to provide a duplex voice channel per phone. The transmit and receive frequencies of each voice channel are separated by 45 megahertz. Each carrier also has 21 data channels to use for housekeeping activities like registration, paging, etc.

In the dark ages before cell phones, people used 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 realization that a city can be chopped up into small cells, and that the cells allow extensive frequency reuse across a city. Frequency reuse is what lets millions of people own cell phones without problems. Here's how it works. The carrier chops up an area (such as a city) into cells. Each cell is typically sized at about 10 square miles (perhaps 3 miles by 3 miles). 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.

Each cell has a base station that consists of a tower and a small building containing the radio equipment. A single cell uses 1/7th of the available 416 duplex voice channels. Therefore, each cell has 59 or so channels available. In other words, in any cell 59 people can be talking on their cell phones at one time.

Cell phones have low-power transmitters in them. Many cell phones have 2 signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 5 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

1. 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 hand-held cellular phones possible.
2. The transmissions of a base station and the phones within its cell do not make it very far outside the cell. Therefore, in the figure above both of the green cells can use the same 59 frequencies. The same frequencies can be reused extensively across the city.

Cloning Cell phones suffer from a problem known as "cloning". When your phone is "cloned" it means that someone has stolen your phone's ID numbers and is able to make fraudulent calls on your account. Here is how cloning occurs. When your phone makes a call, it transmits two pieces of information to the network at the beginning of the call: a MIN (Mobile Identification number) - a 10 digit number derived from your phone's number (both the MIN and SID are programmed into the phone by the dealer) an ESN (Electronic Serial Number) - a unique 32-bit number programmed into the phone when it is manufactured. The MIN/ESN pair is a unique tag for your phone, and it 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, and now the nefarious sort can make calls on your account.

So let's say you have a cell phone, it is turned on, and someone tries to call you. The MTSO gets the call, and it tries to find you. In early (pre-roaming) systems the MTSO found you by paging your phone (using one of the control channels, to which your phone is always listening) in each cell of the region until your phone responded. It then told both your phone and the base station in the cell which of the 59 channels in your cell your phone should be using. At that point you were connected to the base station and you could start talking and listening.

As you move toward the edge of your cell, your cell's base station will note that your signal strength is diminishing. Meantime, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its 1/7th) 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 handoff switches your phone to the new cell.

Roaming makes things a bit more interesting. In modern systems the phones listen for a System ID (SID) on the control channel at power-up. If the SID on the control channel does not match the SID programmed into the phone, then the phone knows it is "roaming". The phone also transmits a registration request and the network 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). As you move between cells, the phone detects changes in the control channel's strength and re-registers itself with the new cell when it changes channels. If the phone cannot find any control channels to listen to it knows it is out of range and displays a "no service" message.

The latest trend is digital cellular phones. They use the same radio technology (in different frequency bands - for example, PCS phones use frequencies between 1.85 and 1.99 gigahertz) but compress your voice into digital 1s and 0s. This compression allows between 3 and 10 cell phone calls to occupy the space of a single analog voice call. PCS digital phones also offer other features like paging and email.

The next time you pick up and use a cell phone - especially one of the new tiny ones that fit into your shirt pocket - keep in mind all of the technology packed into that amazing little device!

Cell Phone Towers Cell phone towers come in all shapes and sizes, but I do believe this one in Morrisville, NC is the weirdest one I have ever seen! That is one ugly tree! Pretty tall, too!

Links

• Inside a Digital Cell Phone - looks inside a digital cell phone
• Cellular Telephone Basics - an extremely thorough look at cellular technology
• PCS systems - a work in progress
• CDMA vs. TDMA Technology

• Find the Right Phone and Service -- compare and choose from service plans and phones offered by AT&T Wireless, Nextel, PrimeCo and more.

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