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Dialing a call on a smartphone with a blue keypad background

How cellphones work

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by Chris Woodford. Last updated: July 19, 2020.

Walking and talking, working on the train, always in contact, never out of touch—cellphones have dramatically changed the way we live and work. No one knows exactly how many little plastic handsets there are in the world, but the current estimate is that there over 8.3 billion subscriptions. That's more than the planet's population! In developing countries, where large-scale land line networks (ordinary telephones wired to the wall) are few and far between, over 93 percent of the phones in use are cellphones. [1] Cellphones (also known as cellular phones and, chiefly in Europe, as mobile phones or mobiles) are radio telephones that route their calls through a network of masts linked to the main public telephone network. Here's how they work.

Photo: Most people now use smartphones as their cellphones, which are actually small computers with cellphone circuitry built in. Back in the 1990s, cellphones were simpler and could only be used for making voice calls. Now networks are faster and capable of handling greater volumes of traffic, smartphones are used as portable communication centers, capable of doing all the things you can do with a telephone, digital camera, MP3 player, GPS "sat nav," and laptop computer.

Cellphones use wireless technology

Nokia cellphone with keypad extended

Photo: Cellphones as they used to be. This Nokia dates from the early 2000s and has a slide-out keypad. Although it has a camera and a few other basic functions, it doesn't have anything like the computing power of a modern smartphone. Phones like this are sometimes called "handhelds" or "feature phones" to distinguish them from iPhones and other smartphones.

Although they do the same job, land lines and cellphones work in a completely different way. Land lines carry calls along electrical cables. Cut out all the satellites, fiber-optic cables, switching offices, and other razzmatazz, and land lines are not that much different to the toy phones you might have made out of a piece of string and a couple of baked bean cans. The words you speak ultimately travel down a direct, wired connection between two handsets. What's different about a cellphone is that it can send and receive calls without wire connections of any kind. How does it do this? By using electromagnetic radio waves to send and receive the sounds that would normally travel down wires.

Whether you're sitting at home, walking down the street, driving a car, or riding in a train, you're bathing in a sea of electromagnetic waves. TV and radio programs, signals from radio-controlled cars, cordless phone calls, and even wireless doorbells—all these things work using electromagnetic energy: undulating patterns of electricity and magnetism that zip and zap invisibly through space at the speed of light (300,000 km or 186,000 miles per second). Cellphone networks are by far the fastest growing source of electromagnetic energy in the world around us.

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How cellphone calls travel

When you speak into a cellphone, a tiny microphone in the handset converts the up-and-down sounds of your voice into a corresponding up-and-down pattern of electrical signals. A microchip inside the phone turns these signals into strings of numbers. The numbers are packed up into a radio wave and beamed out from the phone's antenna (in some countries, the antenna is called an aerial). The radio wave races through the air at the speed of light until it reaches the nearest cellphone mast.

A cellphone mast

Photo: Engineers repair a cellphone mast. Photo by Brien Aho courtesy of US Navy.

The mast receives the signals and passes them on to its base station, which effectively coordinates what happens inside each local part of the cellphone network, which is called a cell. From the base station, the calls are routed onward to their destination. Calls made from a cellphone to another cellphone on the same network travel to their destination by being routed to the base station nearest to the destination phone, and finally to that phone itself. Calls made to a cellphone on a different network or a land line follow a more lengthy path. They may have to be routed into the main telephone network before they can reach their ultimate destination.

How cellphone masts help

At first glance, cellphones seem a lot like two-way radios and walkie talkies, where each person has a radio (containing both a sender and a receiver) that bounces messages back and forth directly, like tennis players returning a ball. The problem with radios like this is that you can only use so many of them in a certain area before the signals from one pair of callers start interfering with those from other pairs of callers. That's why cellphones are much more sophisticated—and work in a completely different way.

A cellphone handset contains a radio transmitter, for sending radio signals onward from the phone, and a radio receiver, for receiving incoming signals from other phones. The radio transmitter and receiver are not very high-powered, which means cellphones cannot send signals very far. That's not a flaw— it's a deliberate feature of their design! All a cellphone has to do is communicate with its local mast and base station; what the base station has to do is pick up faint signals from many cellphones and route them onward to their destination, which is why the masts are huge, high-powered antennas (often mounted on a hill or tall building). If we didn't have masts, we'd need cellphones with enormous antennas and giant power supplies—and they'd be too cumbersome to be mobile. A cellphone automatically communicates with the nearest cell (the one with the strongest signal) and uses as little power to do so as it possibly can (which makes its battery last as long as possible and reduces the likelihood of it interfering with other phones nearby).

What cells do

So why bother with cells? Why don't cellphones simply talk to one another directly? Suppose several people in your area all want to use their cellphones at the same time. If their phones all send and receive calls in the same way, using the same kind of radio waves, the signals would interfere and scramble together and it would be impossible to tell one call from another. One way to get around this is to use different radio waves for different calls. If each phone call uses a slightly different frequency (the number of up-and-down undulations in a radio wave in one second), the calls are easy to keep separate. They can travel through the air like different radio stations that use different wavebands.

That's fine if there are only a few people calling at once. But suppose you're in the middle of a big city and millions of people are all calling at once. Then you'd need just as many millions of separate frequencies—more than are usually available. The solution is to divide the city up into smaller areas, with each one served by its own masts and base station. These areas are what we call cells and they look like a patchwork of invisible hexagons. Each cell has its base station and masts and all the calls made or received inside that cell are routed through them. Cells enable the system to handle many more calls at once, because each cell uses the same set of frequencies as its neighboring cells. The more cells, the greater the number of calls that can be made at once. This is why urban areas have many more cells than rural areas and why the cells in urban areas are much smaller.

How cellphone cells handle calls

Artwork showing how cells work in a hexagonal arrangement

This picture shows two ways in which cells work.

Simple call

If a phone in cell A calls a phone in cell B, the call doesn't pass directly between the phones, but from the first phone to mast A and its base station, then to mast B and its base station, and then to the second phone.

Roaming call

Cellphones that are moving between cells (when people are walking along or driving) are regularly sending signals to and from nearby masts so that, at any given time, the cellphone network always knows which mast is closest to which phone.

If a car passenger is making a call and the car drives between cells C, D, and E, the phone call is automatically "handed off" (passed from cell to cell) so the call is not interrupted.

The key to understanding cells is to realize that cellphones and the masts they communicate with are designed to send radio waves only over a limited range; that effectively defines the size of the cells. It's also worth pointing out that this picture is a simplification; it's more accurate to say that the masts sit at the intersections of the cells, but it's a little easier to understand things as I've shown them.

Types of cellphones

The first mobile phones used analog technology. This is pretty much how baked-bean can telephones work too. When you talk on a baked-bean can phone, your voice makes the string vibrate up and down (so fast that you can't see it). The vibrations go up and down like your voice. In other words, they are an analogy of your voice—and that's why we call this analog technology. Some land lines still work in this way today.

Most cellphones work using digital technology: they turn the sounds of your voice into a pattern of numbers (digits) and then beam them through the air. Using digital technology has many advantages. It means cellphones can be used to send and receive computerized data. That's why most cellphones can now send and receive text (SMS) messages, Web pages, MP3 music files, and digital photos. Digital technology means cellphone calls can be encrypted (scrambled using a mathematical code) before they leave the sender's phone, so eavesdroppers cannot intercept them. (This was a big problem with earlier analog phones, which anyone could intercept with a miniature radio receiver called a scanner.) That makes digital cellphones much more secure.

The world of cellphones

Cellphones have dramatically changed the way the world connects. In the early 1990s, only one per cent of the world's population owned a cellphone; today, in a growing number of countries people spend more time on their mobiles than on their landlines. According to the ITU-T, in 2001, only 58 percent of the world's population had access to a (2G) cellphone network; by 2019, that had risen to 98.8 percent. Also by 2019, there were over 8.3 billion cellphone subscriptions—slightly more than the number of people on the planet. Cellphones have also powered a big leap in Internet access. At the end of 2016, mobile (smartphone and tablet) Internet traffic passed desktop traffic for the first time ever. By the end of 2019, 83 percent of the world's people had active, cellphone-based, mobile broadband subscriptions, which is over five times as many as have traditional wired broadband (just 14.9 percent). [2]

Pie charts showing the growth in mobile cellphone subscriptions in developed and developing countries

Chart: Cellphone subscriptions: The most dramatic cellphone growth has happened in developing countries, which now represent around 80 percent of subscriptions. Source: Drawn using 28 October 2019 data from International Telecommunications Union (ITU).

Cellphones are also used in different ways by different people. Back in the early 2000s, cellphones were used entirely for voice conversations and sending short "texts" (text messages, also known as SMS messages). A lot of people owned a mobile phone purely for occasional emergency use; and that still remains a popular reason for owning a phone (according to the FCC, about 70 percent of all 911 emergency calls in the United States are made from cellphones). Today, smartphones are everywhere and people use them for emailing, browsing the web, downloading music, social media, and running all kinds of apps. Where old-fashioned cellphones relied entirely on a decent signal from a cellphone network, smartphones hop back and forth, as necessary, between ordinary networks and Wi-Fi. Where old cellphones were literally "mobile phones" (wireless landlines), modern smartphones are essentially pocket computers that just happen to make phone calls. You can see just how much phones have changed internally in the photos in the box below.

Cellphones and mobile broadband

If you want to find out how cellphone networks have evolved from purely voice networks to form an important part of the Internet, please see our separate article on broadband and mobile broadband. It also covers all those confusing acronyms like FDMA, TDMA, CDMA, WCDMA, and HSDPA/HSPA.

What's inside your phone?

A Motorola V66, a Nokia 106, and an LG G series smartphone

Photo: Cellphones past and present. Left: A Motorola V66 from about 2000, a Nokia 106 from about 2010, and an LG G series smartphone. I will be taking apart the Motorola and the LG.

A broken phone is a wonderful thing if, like me, you enjoy figuring out how things work. Not surprisingly, there's much more going on inside a modern smartphone than inside the kind of basic cellphone people used to carry about 20 years ago. Old phones were just phones; smartphones are computers packed with all kinds of gadgetry, from fingerprint readers to electronic payment chips. But though phones have changed dramatically, the problems of designing a new handset are, in many ways, just the same as they always were: How do you pack all these components into a small enough space, keep their total weight down, and avoid them overheating? How do you ensure critical components like microphones, loudspeakers, and antennas continue to work effectively even when they're miniaturized?

Inside a classic phone

The biggest difference between old phones and new ones is that older ones have keyboards and small LCD screens, while smartphones have touchscreens that do away with the need for a keyboard altogether (they do still need a few buttons for switching the power on and off and controlling speaker volume). In an old phone, the keyboard's typically one of the "membrane" kind: instead of moving keys, it has squashy rubber buttons that push down on electrical contacts on a printed-circuit board (PCB) below.

Membrane keyboard on a Motorola V66 phone

Photo: Left: The top side of an old Motorola phone keyboard is what's called a rubber membrane, a thin sheet of rubbery plastic with "keys" that press down to make electrical contact with the circuit board below. Right: Each key pushes a little round peg against the corresponding part of the circuit board (the small dots). The keyboard is also packed with LEDs (the eight rectangles with white outlines) that make it light up when you make or take a call.

Unfortunately, digital gadgets aren't anything like as interesting (or as easy to figure out) as mechanical things: most of the good stuff happens inside chips, out of sight, and you can't figure out how a chip works just by looking at it. Taking the keyboard off, there's very little of interest in the board beneath, but do notice the gold antenna running all the way around it. That's why a cellphone like this does not need a long, telescopic (pull-out) antenna.

Membrane keyboard on a Motorola V66 phone showing LCDs and contact points

Photo: The main circuit board from a Motorola V66 phone is directly underneath the keyboard and above the battery compartment.

The other side of the circuit board is a little bit more interesting:

  1. LCD screen, connected to the keyboard unit by a ribbon cable.
  2. Earphone socket.
  3. Battery connector
  4. Battery charger and cable connector for hooking up to a computer.
  5. Heatsinks/screening for chips on the circuit board.
  6. Piezoelectric buzzer.
  7. Buzzer control chip
  8. Antenna connector—links a small external antenna to the gold antenna running round the circuit board.

Circuit and gold antenna inside a Motorola V66 phone

Photo: The back of the main circuit board from a Motorola V66 phone.

Inside a smartphone

There's quite a lot more going on inside a smartphone, as you'd expect. I've not taken the screen apart (it's directly below the circuit board on the right-hand side), but here are some of the other things you'll find:

Internal components of an LG G3 smartphone showing the main circuit board.

Photo: The main circuit board from a more modern LG G-series smartphone.

  1. Contact connections between upper (left photo) and lower (right photo) parts of the circuit board.
  2. Heatsink/screening for processor chips. (The gray stuff you can see here is thermal paste—a kind of heat-conducting goo—that helps to improve cooling.) The power on/off button is under here.
  3. NFC antenna connectors (for contactless payments).
  4. Infrared focusing beam for camera.
  5. 13-megapixel rear digital camera.
  6. Flashlight/camera flash.
  7. Quad-core Qualcomm Snapdragon processor chip.
  8. Micro SD card slot (allows storage to be extended to 32GB).
  9. Micro-SIM card slot
  10. Lithium-ion battery (3000 mAh capacity).
  11. Entirely plastic case with a "brushed metal" finish gives the appearance of a metal case with the fingerprint smudges.
  12. Headphone connector.
  13. Microphone.
  14. USB and charging connector.
  15. Loudspeaker.
  16. Screwed-down plastic shim protects the circuit board and components when you open up the case to change the battery.
  17. Screws!
  18. More contact connections between upper and lower boards.

Who invented cellphones?

Drawing from a patent for a radio telephone system filed by Martin Cooper of Motorola in 1973.

Photo: Martin Cooper's original radio telephone system (cellphone) design, submitted as a patent application in 1973. Note how the mobile part forms an entirely separate system (shown in blue, on the right) that communicates with the existing public network (shown on the left in red). Individual cellphones (turquoise on the extreme right) communicate with their nearest masts and base stations using radio waves (yellow zig-zags). Patent drawing courtesy of US Patent and Trademark Office.

How did we get from land lines to cellphones? Here's a quick history:

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References

  1.    Cellphone subscription statistics come from UN International Telecommunication Union (ITU) statistics.
  2.    Except where noted, all statistics in this paragraph are from UN International Telecommunication Union (ITU) statistics.

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Text copyright © Chris Woodford 2006, 2020. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2006/2020) Cellphones. Retrieved from https://www.explainthatstuff.com/cellphones.html. [Accessed (Insert date here)]

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