SMS glossary
The technical, legal and security concepts of SMS, laid out simply. One sentence to grasp the essentials, one paragraph to go further.
Fundamentals
- SMSAn SMS (Short Message Service) is a text message from the GSM standard capped at 160 characters, carried by the mobile network to the recipient's phone.
- MMSAn MMS (Multimedia Messaging Service) extends the SMS to carry photo, sound or video, freeing itself from the 160-character cap.
- RCSRCS (Rich Communication Services) is the protocol meant to replace the SMS, with read receipts, a typing indicator and enriched messages.
- MVNOAn MVNO (Mobile Virtual Network Operator) is an operator that sells plans without owning antennas, renting the network of an infrastructure operator.
- Number portabilityNumber portability allows a change of operator without losing your number, which makes the prefix useless for deducing the operator that serves the line.
- RoamingRoaming lets you use your mobile on a foreign or partner network, outside your own operator's network.
- eSIMAn eSIM is a SIM chip soldered into the device and activated remotely: it replaces the plastic card without changing how the number itself behaves.
Technical aspects
- GSM-7 alphabetGSM-7 is the 7-bit character set used by default for SMS: it opens up 160 characters per message but leaves out some accented letters.
- UCS-2UCS-2 is the 16-bit encoding triggered as soon as an SMS contains a character foreign to GSM-7: capacity then drops to 70 characters per message.
- Concatenated SMSA concatenated SMS is a long message split into several SMS that the phone reassembles, each segment carrying only 153 characters in GSM-7.
- SMS-C (operator's server)The SMS-C is the operator server that receives, holds and then re-sends SMS as long as the recipient is unreachable.
- Delivery report (DLR)The delivery report, or DLR, is the notice returned by the network stating whether an SMS indeed reached the targeted phone.
- E.164 formatE.164 is the worldwide standard for telephone numbering: the number appears preceded by its country code, for example +33 for France.
- Short codeA short code fits in a few digits, reserved for services, and is used notably for unsubscriptions and reports.
- Alphanumeric senderAn alphanumeric sender, or Sender ID, displays a brand name in place of a number as the originator of an SMS, within a limit of 11 characters.
- SMS gatewayAn SMS gateway is the service that connects a website or an application to the carrier network, injecting messages into it without a SIM card.
- 2G and 3G shutdownFrench carriers are closing their 2G then 3G networks between 2026 and 2030, each on its own timetable, to reassign the spectrum to 4G and 5G.
Legal framework
- Opt-inOpt-in is the prior, free and explicit agreement a person gives to accept receiving commercial messages.
- Opt-outOpt-out is the ability to leave a mailing list, embodied in SMS by the « STOP » mention.
- A2P SMSAn A2P SMS (Application-to-Person) is a message sent automatically by a computer system to a mobile, as opposed to a text written by a human.
Security
- OTP codeAn OTP code (One-Time Password) is a single-use, short-lived code sent by SMS to validate a login or a payment.
- SmishingSmishing is phishing carried out by SMS, seeking to extract personal or bank data by posing as a trusted organisation.
- Reporting to 33700 (SMS spam)33700 is the French service that lets you report an unwanted SMS for free by forwarding it to this short code.
- SIM swappingSIM swapping is having someone's phone number moved onto a SIM card the fraudster controls, so that the victim's text messages arrive on their device.
- SIM box (SIM farm)A SIM box is a rack of SIM cards that sends texts in bulk while constantly rotating numbers — the basic tool of industrial smishing campaigns.
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