What SMS stands for and what it is
Short Message Service is the text-messaging service defined in the GSM mobile standard and maintained today by 3GPP as TS 23.040. "Short" is literal: a single message holds 160 characters of text and nothing else. There are no images, no buttons, no formatting and no sender name beyond the phone number or short code it came from. Those limits are the reason SMS works everywhere, because a phone needs nothing more than a cellular connection to receive one.
Three properties follow from that and explain why SMS is still in use decades after richer channels arrived:
- Reach. Every mobile phone can receive SMS, from the newest smartphone to a feature phone with no data plan.
- Independence from apps. Nothing has to be installed, updated or logged into. The message lands in the phone's own messaging app.
- Independence from data. SMS is carried by the mobile network itself rather than over an internet connection, so it arrives with one bar of signal and no Wi-Fi.
How an SMS gets from the sender to the phone
An SMS is a store-and-forward message. The sending phone or platform hands it to a short message service centre (SMSC) run by the carrier, the SMSC looks up which network the destination number is on, holds the message until the handset is reachable, then delivers it and returns a delivery receipt to the sender. If the phone is off, the message waits at the SMSC and arrives when the phone comes back.
Two terms come up constantly in business messaging: a mobile-terminated (MT) message is one sent to a phone, and a mobile-originated (MO) message is one a person sends from a phone, such as a reply or a STOP keyword. The receipt an SMS returns says delivered to the handset, not read; there is no read receipt in SMS, which is one of the things RCS adds.
The 160-character limit, and why some messages count as three
The limit is a property of the encoding. The default GSM-7 alphabet fits 160 characters in one message. A character outside it, an emoji or a curly apostrophe for example, switches the whole message to UCS-2 encoding and the limit falls to 70. Longer messages are split into segments that the handset stitches back together, and each segment carries a few bytes of concatenation overhead, so the per-segment capacity drops slightly. Carriers and providers bill per segment rather than per message, which is how a message that looks slightly too long quietly costs three times as much.
| Encoding | Single message | Per segment when split |
|---|---|---|
| GSM-7 (standard Latin characters) | 160 characters | 153 characters |
| UCS-2 (emoji, accented or non-Latin text) | 70 characters | 67 characters |
What is the RCS character limit? has the comparison with RCS, which sends long text without segmentation.
SMS, MMS and RCS
SMS is one of three messaging standards a phone's default messaging app can carry.
| SMS | MMS | RCS | |
|---|---|---|---|
| Content | Plain text | Images, audio, video, longer text | Text, media, cards, carousels, buttons |
| Sender identity | Phone number or short code | Phone number or short code | Verified brand name, logo and checkmark |
| Needs data | No | Yes, to retrieve | Yes |
| Receipts | Delivered | Delivered | Delivered and read |
| Reach | Every mobile phone | Almost every phone | RCS-capable phones, with SMS fallback for the rest |
SMS vs MMS and RCS vs SMS go through each pair in detail. The short version is that MMS added pictures at several times the price, and RCS is the successor to both, adding branding and interactivity while falling back to SMS wherever a phone cannot take it.
SMS for business: A2P messaging
Person-to-person (P2P) SMS is what two people send each other. Application-to-person (A2P) SMS is what a business sends through software: the verification code, the appointment reminder, the delivery update, the marketing offer. In the US, A2P traffic has to come from a registered sender type or carriers filter it:
| Sender type | What it is | How it is approved |
|---|---|---|
| 10DLC | A standard 10-digit local number registered for business traffic | Brand and campaign registration with The Campaign Registry |
| Toll-free number | An 800, 888, 877, 866, 855, 844 or 833 number | Toll-free verification |
| Short code | A 5 or 6 digit number for high volume | Carrier provisioning and review |
Business SMS also needs consent under the TCPA before the first message and a working opt-out after it; how RCS consent works sets out the rules, which are the same for SMS. On SimplyRCS an SMS costs $0.0039 in either direction, with carrier surcharges passed through at cost, and a local or toll-free number rents for $0.50 a month; the full card is on the pricing page.
Where SMS still wins, and where RCS takes over
SMS remains the right channel when the words are the whole payload and reach is everything: one-time passcodes, short confirmations, alerts to a list that includes older phones. It stops being enough when the message needs to prove who sent it, show a product, offer a tap-to-act button or report whether it was read. Those are the jobs RCS was designed for, and because RCS falls back to SMS automatically, a business does not choose between them; it sends RCS where it lands and SMS everywhere else. Is RCS replacing SMS? covers how far that transition has gone.
- SMS stands for Short Message Service, the plain-text messaging standard defined in GSM and maintained by 3GPP as TS 23.040.
- One message holds 160 GSM-7 characters or 70 UCS-2 characters; longer text splits into segments billed individually.
- SMS needs no app or data connection and reaches every mobile phone, which is why it is the fallback beneath RCS.
- SMS returns a delivery receipt but no read receipt and carries no sender branding.
- US business SMS is A2P traffic and must come from a registered 10DLC, verified toll-free or short code sender, with TCPA consent.
- On SimplyRCS an SMS is $0.0039 per message, the same rate as an RCS text.