How Cell Phones Work - A Simple Guide

Are you one of the many people who just use a cellular phone to make calls, and take for granted the technology involved in making that call? Do you wonder how you can talk to someone on the other side of the Earth on a cell phone? Or are you just curious about exactly how cellular phones work? If you’re in the latter group, then we have the answers you seek. Sit back and relax, and enjoy the fascinating journey into the world of cellular phones – how your call is made, how you can use the features offered you, and simply how cellular phones work.

Overview
Every cellular phone conversation needs space to transmit in the airwaves. Unlike CB or short-wave radios, where everyone talks on the same channel at once, every cellular phone transmission requires a separate channel to effectively carry on a conversation. Cellular phones are actually radios – extremely sophisticated – but nonetheless radios.

Almost everyone knows how to use a CB radio or a walkie-talkie. Two people communicate on the same frequency, so only one person can talk at a time. This is a simplex system. A cellular phone is a duplex system, so it uses one frequency for talking and a separate frequency for listening. A CB system has 40 channels; a cell phone can communicate on 1,664 channels. Cell phones also operate within cells and they can switch between cells as they move around, thereby giving them a much wider range of operation than CBs.

The airwaves, or frequencies, available are limited, so it became the challenge for the cell phone manufacturers to find a system that would support as many simultaneous calls as possible. So they split up the available bandwidth into many channels, each capable of supporting one conversation. The following explanation will give you an educated knowledge of how cell phone manufacturers handled the challenges of meeting the consumers’ needs, and, generally, how cellular phones work.

Analog – the first cellular phone technology
The first cellular phones used an analog system of transmission. Essentially, an analog system broadcasts audio as a series of continuously changing voltage levels representing the amplitude of the voice conversation. Think of it as audio cassettes vs. CDs – audio cassettes are analog and CDs are digital. With analog systems, the audio is modulated directly onto a carrier. This is very much like FM radio.

Digital – the next technology
With digital systems, the audio is converted to digitized samples, or pieces, at about 8000 samples/second. The digital samples are numbers that represent the time-varying voltage level at specific points in time. These samples, or numbers, are transmitted as 1s or 0s. At the other end, the samples are converted back to voltage levels and “smoothed out” so that you get about the same audio signal as was transmitted.

Because the 1s and 0s can’t be easily confused or distorted during transmission, and also because extra data is typically included in the transmission, any errors are detected and corrected, thereby giving a higher quality audio transmission than an analog system. Cellular phone technology was progressing fast! Let’s look now at the basics of how cellular phones work.

Cell phone basics
A cellular system operates by dividing a large geographical service area into cells and assigning the same channels to multiple, nonadjacent cells. This allows channels to be reused, increasing airwave efficiency. As a subscriber travels across the service area, the call is transferred from one cell to another without noticeable interruption. All the cells in a cellular system are connected to a Mobile Telephone Switching Office (MTSO) by landline or microwave links. The MTSO controls the switching between the Public Switched Telephone Network (PSTN) and the cell site for all wireline-to-mobile and mobile-to-wireline calls.

Each cell is served by its own radio telephone and control equipment, and is allocated a set of voice channels and a control channel, with adjacent cells assigned different channels to avoid interference. The control channel transmits data to and from the mobile units. This control data tells the mobile unit that a call is coming from, or to, the MTSO. The MTSO also assigns the call to a particular channel.

How the signal is transmitted
Information is transmitted by cell-phone networks using three technologies:

Frequency division multiple access (FDMA)

Time division multiple access (TDMA)

Code division multiple access (CDMA)

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. Multiple access simply means that more than one user can utilize each cell.

Analog systems exclusively use FDMA technology, splitting the allocated spectrum into many channels, and reserving the frequency channel for the entire duration of the call. Therefore, each analog channel can only be used by one user at a time.

TDMA builds on FDMA by dividing conversations by frequency and time, thus fitting three digital conversations into one FDMA channel.

CDMA systems have no channels, but instead encode each call as a coded sequence across the entire frequency spectrum. Each conversation is modulated with a unique code that makes it distinguishable from the other calls in the frequency spectrum. CDMA systems are the latest technology on the market and are already eclipsing TDMA in terms of cost and call quality.

Let’s follow a signal to see how cellular phones work
When a phone is manufactured, it is assigned a unique Electronic Serial Number (ESN). This number can never be changed. When you buy a phone, you’re assigned a Mobile Identification Number (MIN), which is your actual 10-digit phone number.

When you first turn on a new phone, it listens on the control channel for a System Identification Code (SID), a unique 5-digit number that is assigned to each carrier by the FCC. The control channel is a special frequency that the phone and base station use to talk to one another about things like call setup and channel changing. If the phone can’t find any control channels to listen to, it knows it is “out of range” and displays a “No Service” message.

When the phone receives the SID, it compares it with the SID programmed into the phone. If they 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 uses its database to determine which cell you are in so it can ring your phone. Then the MTSO picks a frequency pair that your phone will use in that cell to make (or take) the call. The MTSO then communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch to those frequencies, the call is connected.

Roaming
If the SID on the control channel doesn’t match the SID programmed into your phone, then the phone knows it’s roaming. The MTSO of the cell that you’re 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 cell phone as you move through its cells. The amazing thing about cell phones is that all these messages and data traveling back and forth over the spectrum are sent within milliseconds.

So now you know how cellular phones work
When you understand something, it’s much easier to remember. And with this knowledge firmly implanted in your brain, you can now feel comfortable buying a cell phone. You know exactly how cellular phones work and you’ll know exactly what the salesperson is referring to when the acronyms start flying at you. You’ll be able to make a knowledgeable and informed decision.

About The Author
Gareth Marples is a successful freelance writer providing valuable tips and advice for consumers purchasing cell phone plans, ring tones and logos and hands free kits. His numerous articles offer moneysaving tips and valuable insight on typically confusing topics.