SMS (Short Message Service) is the mobile network service that sends short text messages of up to 160 characters between phones and applications, using a store-and-forward message center that holds each message until the recipient can receive it. In voice & messaging, SMS is the oldest and most universally supported text channel: it works on virtually every mobile phone and network without an app or a data connection.
SMS was standardized as part of GSM and has been carried forward through every later generation, from 3G to LTE and 5G, where it travels over IMS or over network signaling. Besides person-to-person texting, it remains essential for verification codes, alerts and machine-to-machine messages, even as richer channels such as RCS and internet messaging apps dominate everyday chat.
How an SMS travels through the network
When a user sends a text, the phone delivers it to the Short Message Service Center (SMSC) of its operator. The SMSC stores the message, queries the recipient’s home network database, the HLR or in 5G the UDM, to find where the recipient is registered, and forwards the message to the serving network, which pages the phone and delivers it.
If the recipient is switched off or out of coverage, the SMSC keeps the message and retries until a validity period expires, which may range from hours to several days depending on settings. A delivery report can be sent back to the sender once the message reaches the handset. This store-and-forward design is why texts sent to a phone that was off often arrive in a burst when it is switched on.
In 2G and 3G networks, SMS travels over the signaling channels used to set up calls, which is why a text can arrive during a call or in weak coverage. In LTE, operators deliver SMS either over IMS as SMS over IP or through the older SGs interface to a circuit-switched core, and in 5G the core network can carry it inside NAS signaling through the SMS function.
The table lists the main network elements involved in SMS delivery.
| Element | Function | Network generation |
|---|---|---|
| SMSC | Stores and forwards messages | All generations |
| HLR or UDM | Locates the recipient | HLR in 2G to 4G, UDM in 5G |
| MSC | Delivers SMS over circuit-switched signaling | 2G and 3G |
| IP-SM-GW | Delivers SMS over IMS | 4G and 5G with IMS |
| SMSF | Delivers SMS over NAS signaling | 5G core |
| SMS gateway or aggregator | Connects applications to operators | Application-to-person traffic |
Message length, encoding and concatenation
A single SMS carries a payload of 140 bytes. With the default GSM 7-bit alphabet, which covers Latin letters, digits and common symbols, this yields 160 characters. If a message contains any character outside that alphabet, such as many accented letters, non-Latin scripts or emoji, the phone switches to UCS-2 encoding with 16 bits per character, and the limit drops to 70 characters.
Longer texts are split into segments and reassembled by the receiving phone using a concatenation header. The header takes part of each segment, so the per-segment limit falls to 153 characters in GSM 7-bit or 67 in UCS-2. Each segment is billed and delivered as a separate SMS, which matters for businesses sending messages at scale.
| Encoding | Single message | Per segment when concatenated | Typical trigger |
|---|---|---|---|
| GSM 7-bit | 160 characters | 153 characters | Basic Latin text |
| 8-bit data | 140 bytes | 134 bytes | Binary and configuration messages |
| UCS-2 | 70 characters | 67 characters | Non-Latin scripts, emoji, some accents |
SMS compared with MMS, RCS and messaging apps
SMS sits at the simplest end of mobile messaging. Its strengths are universal reach and independence from mobile data; its weaknesses are plain text only, no end-to-end encryption and no typing indicators or read receipts.
| Channel | Content | Needs data connection | Reach |
|---|---|---|---|
| SMS | Plain text | No | Practically every phone |
| MMS | Images, audio, short video | Yes, mobile data | Most phones, limited size |
| RCS | Rich media, typing, read receipts | Yes | Supported phones and networks |
| Internet messaging apps | Rich media, often encrypted | Yes | Users of the same app |
Many phones use SMS as a fallback: when an RCS or app message cannot be delivered because the recipient lacks data or support, the message is resent as SMS. This fallback role keeps SMS relevant even where most conversations moved elsewhere.
Application-to-person messaging
A large share of SMS traffic is application-to-person (A2P) messaging: verification codes, delivery notices, appointment reminders, banking alerts and emergency warnings. Businesses send these through SMS aggregators that connect to operators, often with an alphanumeric sender ID instead of a number.
- One-time passcodes remain widespread for login and payment confirmation, despite their security weaknesses.
- Sender registration rules in many countries require businesses to register sender IDs or templates to combat fraud.
- Delivery is best-effort, so time-critical systems should monitor delivery reports and offer alternatives.
- Cell broadcast, not SMS, is generally preferred for mass public alerts because it does not congest the network.
Security weaknesses and misuse
SMS was designed when mobile networks were treated as trusted, so it offers no end-to-end encryption and limited sender authentication. These gaps have well-known consequences, and security guidance increasingly classifies SMS-based authentication as a weaker option than app-based or hardware methods.
| Threat | How it works | Countermeasure |
|---|---|---|
| Smishing | Fraudulent texts with malicious links | Sender filtering, user caution |
| SIM swap | Attacker takes over the number to receive codes | Authenticator apps, port-out locks |
| Sender ID spoofing | Fake alphanumeric sender names | Sender registration schemes |
| Signaling attacks | Abuse of interconnect signaling to intercept | Signaling firewalls at operators |
Organizations that still depend on SMS for authentication should treat it as one factor among several, limit what an SMS code alone can authorize and encourage users to move sensitive accounts to stronger methods.
The simple explanation
SMS is the classic text message. It works on almost every mobile phone, even an old one without internet. You type a short note, and the mobile network carries it to the other phone.
If the other phone is switched off, the network keeps the message and delivers it later. One message holds up to 160 simple characters, and longer notes are split and joined back together. Because it is so widespread, banks and delivery companies still use SMS for codes and alerts.
How it is used in practice
A regional medical clinic sends appointment reminders to about 2,000 patients each week. Many patients are older and use basic phones without messaging apps, so the clinic chooses SMS through an aggregator with a registered sender name.
The staff first write the reminder with accented letters and a calendar emoji, and the system shows that the text would be split into three UCS-2 segments. They rewrite it in plain GSM 7-bit characters, keeping it under 160 characters, so each reminder is a single message. Delivery reports flag numbers that no longer work, and the reception desk updates those records. The outcome is fewer missed appointments at a third of the original messaging cost, with reminders that reach every patient regardless of phone type.