The whole journey: from a tap in Lagos to a screen in London

It is her uncle's birthday. Nkem, sitting in a bus in Lagos traffic, opens WhatsApp, types Happy birthday Uncle! 🎉 and taps Send.

One grey tick. Then two. A minute later, two blue ticks: in a flat in London, more than five thousand kilometres away, her uncle has read it.

At the start of this course, that would have been something that "just happens". Now you can see every step — and at every step, something you have learnt. This first lesson of the capstone walks through the whole journey once, stage by stage, pointing back to where you met each piece. Then the rest of this module asks you to do the thing that proves you own this knowledge: explain it yourself, in your own words, and put it to work in your own professional life.

Read this lesson slowly. Wherever a step feels shaky, go back to the lesson it names. By the end of the module you should be able to tell this story without notes, to anyone.

Words in this lesson

Stage 1: On Nkem's phone — from touch to bytes

The touch. Nkem's finger touches the screen. The touchscreen's grid of sensors detects where, and the operating system works out which on-screen key she touched and which app has the focus — WhatsApp (Lesson 5.4).

The characters. Each character becomes a Unicode number, stored as UTF-8 bytes (Lesson 2.3). "H" is 72 — one byte. The party popper emoji 🎉 is code point U+1F389, which takes four bytes in UTF-8: F0 9F 8E 89. Her whole message — 23 characters — is 26 bytes.

The memory and the processor. Those bytes sit in the phone's RAM (Lesson 4.3), inside WhatsApp's own private memory space, which the operating system protects from other apps (Lesson 5.2). WhatsApp is a program — machine code (Lesson 5.1) — being fetched, decoded and executed by the phone's processor cores (Lessons 3.5 and 4.2), sharing them with other apps in time slices (Lesson 5.2). Every instruction is carried out by gates (Lesson 3.2) built from transistors (Lesson 1.4) — switches made from silicon, purified from sand (Lesson 1.3).

Stage 2: Locked before it leaves — encryption and packets

Encryption. Before anything leaves her phone, WhatsApp encrypts the message end to end. WhatsApp explains that only the sender and the recipient hold the key needed to read it (Lesson 9.2). From here on, everyone else on the journey — the network, the cables, even WhatsApp's own servers — sees only scrambled bytes.

Packets. The encrypted message is placed into packets, each with a header carrying the source and destination IP addresses (Lesson 7.2). Before it could send anything, the app had already used DNS to turn the name of WhatsApp's servers into IP addresses (Lesson 7.5). The rules of TCP will make sure every byte arrives, in order (Lesson 7.2).

Stage 3: Through the air and the router

Radio. The packets leave the phone as radio waves — to the nearest mobile mast, or to a Wi-Fi router if she is on Wi-Fi (Lesson 7.1). Radio carries bits as changes in waves rising and falling billions of times a second (Lesson 7.1).

The first router. Her phone has a private address that means nothing on the wider internet (Lesson 7.2). Her mobile operator — or her home router — stands in for her, sending her packets out under a public address and remembering which device to return the replies to (Lesson 7.2).

Stage 4: Across the country and under the sea

Hop by hop. From the mast, the packets join the mobile operator's network — mostly cables — and pass from router to router, each one reading the destination address and forwarding the packet one hop closer. Each router reduces the packet's TTL by one (Lesson 7.3). No router knows the whole route; together they get it there, like danfo conductors each knowing the next park.

Glass and light. Most of the journey is light in glass fibre (Lesson 7.1). To leave Nigeria for Europe, the packets almost certainly travel through one of the submarine cables that land in Lagos — such as WACS, SAT-3, MainOne, Glo-1, ACE, Equiano or 2Africa (Lesson 7.4). Light in glass travels at about 200,000 kilometres a second, so a round trip between Lagos and London can never take less than about 50 milliseconds (Lesson 7.3). If one cable is cut, as in March 2024, traffic is moved onto others (Lesson 7.4).

Stage 5: Through WhatsApp's computers to London

The server. The packets reach WhatsApp's servers, in a data centre (Lesson 7.4) — buildings full of processors, memory and storage like those in Module 4, with cooling for all the heat their switches make (Lesson 4.1). The servers can deliver the message, but because it is encrypted end to end, they cannot read it (Lesson 9.2).

One grey tick. WhatsApp explains that a single grey tick means the message was successfully sent.

Onward. The server sends the message on towards her uncle's phone: through networks in Europe, router to router, to his internet provider, his home router, and the last few metres by Wi-Fi (Lesson 7.1). His phone's TCP puts the packets back in order and asks for any missing ones again, so that what arrives is an exact copy of what was sent (Lesson 7.2).

Two grey ticks. WhatsApp explains that two grey ticks mean the message has reached the recipient's phone or a linked device.

Stage 6: On her uncle's phone — from bytes to light

Unlocking. His phone uses its key to decrypt the message (Lesson 9.2). The bytes are back to exactly the 26 Nkem's phone produced.

From bytes to letters. WhatsApp reads the bytes as UTF-8, turning them back into characters — 72 into "H", and F0 9F 8E 89 into 🎉 (Lesson 2.3). If the two phones disagreed about the encoding, he would see something like the mojibake of Lesson 2.3. They don't.

From letters to light. The app lays out the text; the operating system combines the windows; the phone's GPU works out the colour numbers for every pixel that changes (Lesson 4.2), using the shapes of the characters from a font; and the screen lights its tiny red, green and blue lights to those numbers (Lesson 2.4).

Two blue ticks. He reads it. WhatsApp explains that two blue ticks appear when the message has been read — and that tiny report travels all the way back to Nkem, through every stage again, in reverse.

Common mistakes

The short version

• On Nkem's phone: touch → OS → WhatsApp (a program running on processor cores built from transistors and sand) → Unicode characters stored as UTF-8 bytes in RAM. 🎉 is U+1F389, four bytes.
• Locked and packed: encrypted end to end, then split into packets with IP addresses; DNS found WhatsApp's servers; TCP guarantees the copy.
• Through the air: radio to a mast or Wi-Fi router, which stands in for her private address.
• Across the world: router to router, hop by hop, mostly as light in glass — through an undersea cable from Lagos — at a minimum of about 50 ms for a round trip to London.
• Through WhatsApp: servers in a data centre deliver what they can't read — one grey tick for sent, two for delivered.
• On his phone: decrypt → UTF-8 back to characters → GPU → red, green and blue lights. Two blue ticks for read travel all the way back.

Try it yourself

Tell the story yourself.

1. Without looking at this lesson, write down the six stages of the journey and two things that happen in each. Then check against the lesson and fill any gaps from the lessons it names.
2. Pick a message you actually sent today. Work out how many bytes it was in UTF-8, including any emoji (Lesson 2.3).
3. Run tracert to a website you believe is in Europe (Lesson 7.3), and mark where your packets leave your network and where they probably cross the ocean.
4. Draw the whole journey on one page, as a diagram a fourteen-year-old could follow, with one label for every lesson it uses.
5. Explain the journey out loud, in under five minutes, to someone at home. Note every point where you hesitated — and go back to those lessons.

You'll know you've got it when you can explain, without notes, everything that happens between Nkem's tap in a Lagos bus and two blue ticks appearing — from the sand in the chip to the light on her uncle's screen.