How bits travel: wires, light and radio

Nkem is sitting in a bus in Lagos traffic. She holds the microphone button on WhatsApp, says "Uncle, happy birthday! We miss you!", and lets go. Two ticks appear. A moment later, in a flat in London, more than five thousand kilometres away, her uncle's phone buzzes, and he hears her voice — the noise of the bus behind it and all.

In Lesson 2.5, you saw how her voice became numbers: the sound wave measured thousands of times a second, each measurement stored as bits. The lesson ended with a promise: once something is numbers, it can travel down a wire, through the air or under the ocean as switches flicking on and off, and you would see exactly how in Module 7.

This is Module 7. In this lesson you will see how a bit — a 1 or a 0 — can leave one device and arrive at another, through copper, through glass and through the air, and how billions of devices are joined into one network that can carry Nkem's voice from a Lagos bus to London in about the time it takes to blink. The next four lessons follow her voice note the whole way.

Words in this lesson

A bit on the move: signals

Inside a computer, a bit is a voltage on a wire: high for 1, low for 0 (Module 1). That works over the few centimetres between a processor and its memory. To send a bit to another building, another city or another continent, you need something that can travel that far and still be detected at the other end. That is a signal.

There are three main ways the world sends bits today:

Real systems are cleverer than simple on-and-off: they pack several bits into each change of the signal, so that far more bits fit into each second. But the principle is exactly what you know. Somewhere, something is being switched between states that a receiver can tell apart (Lesson 1.2) — and those states mean 1s and 0s.

Copper: electricity down a wire

The simplest way to send bits is the way they already move inside a computer: as changing voltage on a copper wire. Office networks are often wired with Ethernet cables — the ones with the wide plastic plug that clicks into the side of a desktop computer or a router.

Copper is cheap and reliable over short distances, but electrical signals weaken and pick up interference as they travel. That is why copper is used for the last stretch — across a room or a building — while long distances use something else.

Glass: light in a fibre

For long distances, the world uses light.

An optical fibre is a strand of extremely pure glass. Light sent into one end bounces along inside it all the way to the other end. The Fiber Optic Association explains how it stays inside: the glass at the centre of the fibre — the core — is made slightly different from the glass around it, so that light striking the boundary at a shallow angle is reflected back into the core, again and again. The light cannot leak out, even when the cable bends.

To send bits, a laser at one end switches its light, and a sensor at the other end detects the changes — light and dark, 1 and 0, in their simplest form.

Light in glass is fast, but not as fast as light in empty space. The Fiber Optic Association explains that glass slows light to about two-thirds of its speed in a vacuum — roughly 200,000 kilometres per second. Remember that number; in Lesson 7.3 you will use it to work out why a message to London can never arrive instantly.

Glass carries enormous amounts of data, over long distances, with very little loss. That is why the cables that cross cities, countries and oceans are made of it — as you will see in Lesson 7.4.

Air: Wi-Fi and mobile data

Nkem's phone has no cable at all. Her bits leave it as radio waves — the same kind of invisible wave that carries FM radio, only at much higher frequencies.

Wi-Fi uses radio in a few frequency bands. The Wi-Fi Alliance, which certifies Wi-Fi equipment, describes Wi-Fi operating mainly in the 2.4 GHz and 5 GHz bands, with newer equipment also using 6 GHz. You know what GHz means from Lesson 3.4: billions of times per second. A 2.4 GHz radio wave rises and falls about 2.4 billion times every second, and the bits ride on changes in those waves.

Radio has limits that you have certainly noticed. Walls, floors and distance weaken it, and many devices sharing the same airwaves get in each other's way. Microsoft's advice for Wi-Fi problems even suggests trying both the 5 GHz and 2.4 GHz bands if your router offers both: in general, 2.4 GHz reaches further through walls, while 5 GHz carries more data over shorter distances.

Mobile data works on the same idea at a bigger scale. Your phone talks by radio to the nearest mast — the tall towers you see on rooftops and beside roads. From the mast, your data joins the mobile operator's own network — mostly cables — and from there, the rest of the internet.

Networks: devices that can reach each other

A network is simply devices connected so that they can send data to one another. Your home or office is probably one already: a router (often with Wi-Fi built in) connects your laptop, your phone and perhaps a printer or a TV. That is a small, local network.

The internet is what you get when you connect networks to other networks: your home network to your internet provider's network, your provider's network to other providers', theirs to networks in other countries — thousands upon thousands of separate networks, owned by different companies and organisations, agreeing to pass each other's data along. That is why the internet is often called the network of networks. Nobody owns all of it. It works because everyone follows the same rules for addressing and passing on data — the subject of the next lesson.

Some of those connection points are right here. The Internet Exchange Point of Nigeria (IXPN), with its main hub in Lagos, is a place where Nigerian networks connect directly to each other. IXPN describes its purpose as keeping internet traffic local and reducing routing costs: when a message from one Nigerian network to another can be exchanged in Lagos, it has no need to travel to Europe and back.

The journey of a voice note, in outline

Here is the whole journey of Nkem's voice note, in outline. The rest of this module fills in each step.

Every one of those steps is something you already understand at its core: switches, numbers and signals. The internet is not magic. It is Modules 1 to 6, joined together and stretched across the planet.

Common mistakes

The short version

• To travel, bits become signals: voltage on copper, pulses of light in glass fibre, or changes in radio waves through the air.
• Fibre keeps light inside by reflecting it at the boundary of the core. Light in glass travels at about two-thirds of its vacuum speed — roughly 200,000 km per second.
• Wi-Fi uses radio, mainly in the 2.4 GHz and 5 GHz bands; mobile data goes by radio to a mast, then into cables.
• A network is connected devices; a router links networks. The internet is the network of networks.
• IXPN in Lagos lets Nigerian networks exchange traffic locally instead of sending it abroad.

Try it yourself

Find the carriers around you.

1. Look at your home, school or office. Find one example of each carrier: a copper network cable, a fibre or internet provider's box, and a Wi-Fi router or mobile mast. Write down where each is.
2. Find out whether your Wi-Fi router offers 2.4 GHz, 5 GHz or both (the router's label or your Wi-Fi network names may show it). Stand close to the router and then two rooms away, and note how your connection changes.
3. Draw the journey of a voice note you sent recently, from your phone to the person who received it, using the table in this lesson as a guide. Label which carrier each step uses.
4. Work out how long light in glass takes to travel 5,000 km — roughly the distance from Lagos to London — at 200,000 km per second. Give your answer in milliseconds.
5. Explain to someone why "Wi-Fi" and "the internet" are not the same thing, using your own router as the example.

You'll know you've got it when you can explain how a 1 or a 0 crosses a room, a city and an ocean — and draw the whole journey of a WhatsApp voice note, naming the carrier at every step.