# How an SMS Travels to Someone Far Away

How a text message hops from your phone to a tower, through glass fibre cables and a message centre, and onto a friend's phone far away\.

By 薇薇

Topic: Computing, AI & Electronics

[Open the interactive Jig](https://ask.jig.com/j/2ax4KG3mP4CK)

## Transcript

### 1. A message to someone far away

You type a text and press send\. Seconds later it pops up on a phone hundreds of kilometres away\. But it does not fly straight there\. Let's follow its journey across this little world, from your phone on the left to your friend's phone on the right\.

SMS means Short Message Service\. The very first one was sent in 1992, and it said "Merry Christmas"\.

### 2. Packed like a tiny letter

When you press send, your phone wraps your words in a small digital envelope\. On it go your friend's number, your own number and the time\. Press Send to seal it and launch it\.

One text holds up to 160 letters\. Emoji take more room, so a message full of them fits only 70 characters\.

### 3. A radio hop to the nearest tower

Your phone talks to the closest cell tower using radio waves, a kind of invisible light\. The text rides a small control lane that phones also use to set up calls, so it does not need mobile data or Wi\-Fi\.

Radio waves travel at about 300,000 kilometres per second\. In towns, towers are often only 2 to 3 kilometres apart\.

### 4. Down the tower, into glass cables

The tower passes your message down to a cabinet at its foot\. From there it races along fibre\-optic cables: threads of glass that carry flashes of light\. They lead to a big switching building run by your phone company\.

Light in glass fibre covers about 200 kilometres in a single thousandth of a second\.

### 5. The message centre keeps it safe

Inside, a computer called the SMS centre stores your message, like a post office holding a letter\. Then it asks the network's address book, the subscriber register, where your friend's phone is right now\.

This is called store and forward\. If the answer is "phone switched off", the centre simply waits and tries again later\.

### 6. Across the country in a blink

Now the centre knows which tower is near your friend\. It forwards the envelope along long\-distance fibre cables, over hills and valleys, maybe even into another company's network\. The distance hardly matters\.

Most of the few seconds a text takes are spent on computers sorting and checking, not on travelling\.

### 7. The last hop: ding\!

Your friend's tower calls out to the phone by radio, a bit like calling its name\. The phone answers, the message drops in, and it chimes\. Switch the phone off to see what happens when nobody answers\.

While the phone is off, your message waits safely at the message centre and is sent again when the phone comes back\.

### 8. Delivered\!

A tiny delivery report can travel all the way back, so your phone can show that the text arrived\. Tower, cables, message centre, cables, tower: every text you send makes this round trip in just a few seconds\.

A long text is split into parts of up to 153 letters, and the receiving phone joins them back together\.