Wirebook › Bonus lesson
How does a text message find its way to someone's phone?
You press send and, a second later, your friend's phone buzzes on the other side of town. How did the words get there? Picture the message as a frog. It hops onto a lily pad (a phone tower), and you might guess it keeps hopping from tower to tower all the way across town. But here's the surprise: the frog only hops twice. Once onto a lily pad near you, and once off a lily pad near your friend. Everything in between happens in pipes under the pond.
The frog only hops twice
Phone towers are good at one job: talking by radio to the phones near them, a few hundred metres to a few kilometres away. They don't throw your message from tower to tower across the country. Instead, each tower is plugged into the phone company's network, mostly through fibre-optic cables underground (some towers use a dish that beams to a nearby tower with a cable). So a text goes by radio from your phone to the nearest tower, through cables to the middle of the network, through more cables to the tower near your friend, and by radio onto their phone. Two hops, however far apart you are. That's also why a text to someone in another country still takes about a second.
The post office in the middle
For plain texts (SMS, short message service), the middle is a computer called the message centre. It works like a post office: it takes the message, keeps a copy and tells your phone Sent. Then it has a problem: where is your friend's phone right now? Phones move around, so the network keeps an address book, called the home register, that each phone updates whenever it moves near a new tower. The message centre looks your friend up, sends the message to the right tower, and that tower calls out to the phone, which engineers call paging. The phone answers, the message hops on, and a little note travels back saying it arrived. If you've switched on delivery reports, that's when you see Delivered.
Keeping a copy and passing it on when the next step is ready is called store and forward. It's the reason texts don't just vanish when something goes wrong.
What if a lily pad is missing?
There are three places the frog can get stuck, and each one looks different on your screen:
- No lily pad near you (a lift, a tunnel, flight mode). The frog never leaves your phone. A plain text shows Not delivered and waits for you to tap it again. Chat apps put a little clock next to it and send it by themselves when signal comes back.
- The nearest tower is broken. Usually nothing happens at all, because towers overlap: your phone just hops to the next nearest one, a bit further away, maybe with fewer bars. Phone companies plan it that way.
- No lily pad near your friend, or their phone is off or out of battery. Your screen says Sent (or one tick), because the middle has it. The frog waits there and tries again, and as soon as your friend's phone switches on or finds a tower, it tells the network where it is and the waiting messages come flooding in. But the middle doesn't wait forever. For texts it's often somewhere between a day and a week, depending on the network. Chat apps often keep waiting messages for weeks.
Plain texts and chat apps
Chat apps (the kind with ticks) work the same way, frog and all, with a different middle. The message rides as ordinary internet data, over mobile data or Wi-Fi, to the app company's server. Your friend's phone keeps a quiet line open to that server, so it can push the message down straight away. One tick usually means the server has it; two ticks mean it reached their phone. Because they use the internet, chat apps also work over home Wi-Fi with no phone signal at all, which plain texts can't. Many chat apps also lock the message so that only the two phones can read it (end-to-end encryption), so the server is a post office that carries sealed envelopes. Plain texts aren't sealed that way, which is why banks warn you never to share a code that arrives by text.
The plain-text alphabet (called GSM-7) uses 7 bits per letter, so 160 letters fit in 140 bytes. It has ordinary letters, numbers, some accents and symbols, but not emoji or curly quotes. Put just one of those in and the whole message switches to a bigger alphabet where every letter takes 16 bits, so only 70 fit, and most emoji take two slots. Long messages are split into parts, and each part gives up a few slots for a label like "part 2 of 3", which is why the limit per part drops to 153 (or 67). Your phone glues the parts back together so you see one message. Want to know how emoji get their numbers in the first place? See why emojis turn into boxes.
Related lessons: why a video call freezes when you walk to the kitchen · how a phone knows where it is · how apps get told about new messages · how big files travel in numbered pieces.
Grown-up twin: Tooltern's help pages on Wi-Fi calling not working and what flight mode turns off, plus an SMS length counter and what Sent but not Delivered means.