Short message

Description

A Short message is the unit of information transported by the Short Message Service (SMS). It is defined in the context of the SMS specifications, principally TS 23.040 (Technical realization of the Short Message Service), with related security aspects in TS 23.048 and the 3GPP TS 31.115 specification covering the Short Message Service at the terminal interface. A short message consists of a payload, usually up to 140 octets in the GSM/UMTS SMS, which is commonly encoded as 160 characters in the 7-bit GSM alphabet, 140 8-bit characters, or 70 16-bit UCS2 characters. Longer content is delivered by concatenating multiple short messages.

Architecturally, SMS uses a store-and-forward model. A mobile-originated short message is submitted by the UE to the network via the Short Message Service Centre (SMSC) using signalling channels — in GSM/UMTS via the control channel using the Mobile Application Part (MAP), and in LTE/5G via the NAS signalling path with SMS over NAS or SMS over IP. The SMSC stores the message and attempts delivery to the recipient. When the recipient is reachable, the message is forwarded to the serving MSC/MME/AMF and delivered to the UE. If the recipient is unreachable, the SMSC retries according to its policy, which is one reason SMS is considered reliable for asynchronous notifications.

The short message itself has several header fields that control how it is processed: the destination address (for example, a telephone number or short code), the protocol identifier, the data coding scheme, the validity period, and flags for delivery reports and status. These fields allow the network and terminal to handle the message correctly, apply charging, and provide delivery confirmation. In later releases, SMS has been extended to support many machine-type and IoT applications, including use by IoT devices for configuration and alerts, and has been integrated into the 5G System.

Security and integrity of short messages are addressed across the specifications. TS 23.048 defines security mechanisms for the (U)SIM Application Toolkit and other applications that use SMS as a bearer, while network-level security relies on the underlying signalling security (for example, NAS security in 5G). SMS is also used to carry binary data for OTA (Over-The-Air) provisioning, which updates terminal and SIM parameters.

Over time, the role of the short message has shifted from primarily person-to-person text messaging to include A2P (application-to-person) messaging, two-factor authentication codes, service notifications, and IoT communications. Despite the rise of IP messaging, SMS remains a ubiquitous, interoperable, and regulator-recognized service, and the short message is still the fundamental payload unit of that service.

Purpose & Motivation

The short message exists to provide a lightweight, store-and-forward way of conveying small amounts of information over the mobile network control infrastructure, without requiring a circuit-switched connection or an end-to-end data session. It was introduced with GSM (adopted in 3GPP as R99 and earlier) to allow subscribers to exchange text and small binary payloads reliably, even when the recipient is temporarily unreachable. This store-and-forward design, centered on the SMSC, decouples message submission from delivery and makes the service robust and asynchronous.

The short message solved several problems: it offered a simple, bandwidth-efficient messaging service over signalling channels; it provided a standardized means for operators to deliver notifications, voicemail alerts, and configuration data; and it became a universal addressing medium because every mobile subscription has an MSISDN that can receive a short message. This universality made SMS a foundation for later services, including premium short codes, mobile marketing, and mobile payments.

As networks evolved, the short message was retained and adapted rather than replaced: it is supported over the packet core in LTE and 5G and remains essential for machine-type communication, OTA provisioning, and authentication. Its persistence demonstrates the value of a simple, interoperable messaging primitive that works across generations and is understood by regulators, operators, and billions of devices.