What is SMS? How text messaging actually works

SMS (Short Message Service) is the text-messaging standard built into every mobile phone. It sends short messages — up to 160 characters — over the cellular network, not the internet. That is why an SMS reaches almost any phone, even without data or an app.

What SMS is

The difference between SMS and every messaging app you have used is that SMS is not a product anyone ships. It is part of the mobile network standard, implemented by the carrier and supported by the phone before you unbox it. There is nothing to install, nothing to sign up for, and no username — the phone number is the address.

That is why SMS has outlived every messenger built on top of it. Two people on different continents, different carriers and different operating systems can text each other with no shared account and no agreement about which app to use. For the plain definition and the side-by-side comparison, see what does SMS mean.

How SMS actually works

A message does not go from your phone to theirs. It goes through a piece of carrier infrastructure called the SMSC — the Short Message Service Centre — and the SMSC is the reason SMS behaves the way it does.

1. Your phone hands the message to your carrier's network over the signalling channel — the same low-bandwidth channel the network already uses to track which cell your phone is in.
2. The message lands at the SMSC, which stores it.
3. The SMSC works out where the recipient's number lives and forwards it to that carrier.
4. The recipient's network delivers it to their handset. If the handset is off, out of coverage or in flight mode, the message waits at the SMSC and is retried.

That store-and-forward step is the single most useful thing to understand about SMS. It explains why a text sent to a phone in a tunnel arrives twenty minutes later rather than bouncing, why messages sometimes land in a burst when a phone comes back online, and why an SMS that has not arrived yet is not necessarily an SMS that failed.

What SMS can and can't do

It can:

• Reach a phone with no data plan, no Wi-Fi and no app installed.
• Survive a phone being off, by waiting at the SMSC and retrying.
• Work between any two networks in the world without either party agreeing on software.
• Carry a one-time verification code, which is why most of the internet uses it as a second factor.

It can't:

• Carry a photo, a video or a file. That is MMS, a different service that uses mobile data.
• Be end-to-end encrypted. Both carriers in the path can read the message, and so can anyone who compels or compromises them.
• Tell the sender that a person read it — only, at best, that a device received it.
• Guarantee arrival. Filtering, number reputation and carrier policy all sit between the sender and your screen, and none of them are visible to either end.

SMS, MMS, RCS and iMessage

All four are “texting”. Only one of them is SMS, and the difference is what happens when the internet is not available.

Because the other three need a data connection, they all fall back to SMS when it is missing. On an iPhone that fallback is visible as a green bubble and the words “sent as SMS”.

SMS and verification codes

One-time codes ride on SMS for exactly the reason SMS has survived: it reaches any phone, with no assumption about what the recipient has installed. That universality is also its weakness as a security factor — but for most services it is still the only second factor every single user can receive.

It is also why a phone number has quietly become an identity document. Signing up for something now often means handing over the number your bank and your family use. What is a virtual phone number? covers the products that exist to break that link, and how to get a temporary phone number compares the routes to one.