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What is an SMS (and why 160 characters)?

In one sentence: A short plain text sent through the phone network rather than the internet, up to 160 plain letters per text, passed from tower to message centre to tower.

A little more

SMS stands for short message service: the plain text messages every mobile phone can send, with no app and no internet needed. Your phone sends the text by radio to the nearest tower, the network passes it through its cables to a message centre, and the message centre finds the other phone and sends it out through the tower nearest to it. If the other phone is off or out of signal, the message centre keeps the text and tries again later, for anything from a day to about a week depending on the network.

Texts were designed to squeeze into spare room in the network's own signalling, so each one carries just 140 bytes. With the basic 7-bit alphabet (GSM-7) that is 160 characters. One emoji, curly quote or unusual letter switches the whole message to UCS-2, 16 bits per character, which fits 70. Longer messages are split into parts of 153 (or 67) with a small label so the phone can glue them back together. Plain texts aren't end-to-end encrypted, which is one reason codes sent by text should never be shared. Chat apps and RCS send messages over the internet instead.

See it in action

  1. BonusHow does a text message find its way to someone's phone?Send a pretend text as a frog: it hops by radio onto a lily pad (a phone tower), rides underground pipes to the message centre, which looks up where your friend's phone is, then hops off the lily pad next to them.8 min · interactive
  2. BonusWhy do emojis turn into boxes, question marks or gibberish?X-ray any emoji to see the secret numbers and bytes inside it, solve four crime scenes where emojis seem to vanish (empty boxes, ?, mojibake and broken families) and glue your own emoji together.6 min · interactive

Do it for real

Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.

Related words

  • Store and forward Passing a message on by keeping a copy at each stop and sending it to the next one only when that next step is ready, so nothing is lost if the far end is away.
  • Unicode The worldwide list that gives every letter, symbol and emoji its own number, so all computers agree on what was typed.
  • Roaming When a phone or laptop hops from one Wi-Fi base station to another as you move around, without picking a new network.

← All 146 words