Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

used by

GSM The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...

cellular telephone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...

s to communicate with the

mobile network operator A mobile network operator (MNO), also known as a mobile network provider, mobile network carrier, mobile , wireless service provider, wireless carrier, wireless operator, wireless telco, or cellular company, is a telecommunications provider of se ...

's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network. The service does not require a messaging app, and does not incur charges. USSD messages are up to 182 alphanumeric characters long. Unlike short message service (SMS) messages, USSD messages create a real-time connection during a USSD session. The connection remains open, allowing a two-way exchange of a sequence of data. This makes USSD faster than services that use SMS. While

is being phased out in the 2020s with 2G and 3G technologies, USSD services can be supported over

LTE LTE may refer to: Science and technology * LTE (telecommunication) (Long-Term Evolution), a mobile telephony standard ** LTE Advanced, an enhancement ** LTE Advanced Pro, a further enhancement * Compaq LTE, a line of laptop computers * Leukotrie ...

and 5G.

Uses

When a user sends a message to the phone company network, it is received by a computer dedicated to USSD. The computer's response is sent back to the phone, generally in a basic format that can easily be seen on the phone display. Messages sent over USSD are not defined by any

standardization Standardization (American English) or standardisation (British English) is the process of implementing and developing technical standards based on the consensus of different parties that include firms, users, interest groups, standards organiza ...

body, so each network operator can implement whatever is most suitable for its customers. USSD can be used to provide independent calling services such as a callback service (to reduce phone charges while roaming), enhance mobile marketing capabilities or interactive data services. USSD is commonly used by prepaid GSM cellular phones to query the available balance. The vendor's "check balance" application hides the details of the USSD protocol from the user. On some pay as you go networks, such as

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, once a user performs an action that costs money, the user sees a USSD message with their new balance. USSD can also be used to refill the balance on the user's

SIM card A typical SIM card (mini-SIM with micro-SIM cutout)A SIM card or SIM (subscriber identity module) is an integrated circuit (IC) intended to securely store an international mobile subscriber identity (IMSI) number and its related key, which are u ...

and to deliver

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s or PIN codes. Some operators use USSD to provide access to real-time updates from social-networking websites including

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and

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. Between 2012 and 2018, the

Wikipedia Zero Wikipedia Zero was a project by the Wikimedia Foundation to provide access to Wikipedia free of charge on mobile phones via zero-rating, particularly in developing markets. The objective of the program was to facilitate access to free knowled ...

project provided access to Wikipedia articles via USSD. USSD is sometimes used in conjunction with SMS. The user sends a request to the network via USSD, and the network replies with an acknowledgement of receipt: :"Thank you, your message is being processed. A message will be sent to your phone." Subsequently, one or more mobile terminated SMS messages communicate the status and/or results of the initial request. In such cases, SMS is used to "push" a reply or updates to the handset when the network is ready to send them. In contrast, USSD is used for command-and-control only.

Technical details

Most GSM phones have USSD capability. USSD is generally associated with real-time or instant messaging services. An SMSC is not present in the processing path, so that the

store-and-forward Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node ...

capability supported by other short-message protocols such as SMS is not available. USSD Phase 1, as specified in GSM 02.90, only supports mobile-initiated ("pull") operations. In the

core network A backbone or core network is a part of a computer network which interconnects networks, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same buildi ...

, the message is delivered over

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, USSD Phase 2, as specified in GSM 03.90. After entering a USSD code on a

handset, the reply from the

operator is displayed within a few seconds. While GSM is being phased out in the 2020s with 2G and 3G, a solution is available for supporting USSD services directly from the

/ 5G/

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network, providing a similar user experience as in GSM.

Format

A USSD message typically starts with an asterisk symbol (*) or a hash symbol (#) and is terminated with a hash symbol (#). A typical message comprises digits for commands or data; groups of digits may be separated by additional asterisks. USSD mode Mobile-initiated * USSD/ PULL or USSD/ P2A * when the user dials a code, e.g. *139# from a GSM mobile handset Network-initiated * USSD/ PUSH or USSD/A2P * when the user receives a push message from the network; primarily used for promotional services

Man-Machine Interface

The codes below are not USSD codes, these are the related Man-Machine Interface (MMI); they are standardized so they are the same on every GSM phone. They are interpreted by the handset first before a corresponding command (not the code itself) is sent to the network. These codes might not always work when using an AT interface; there are standard AT commands defined for each of these actions instead. BS is the type of bearer service, some valid values are: 11 for voice 13 for fax 16 for SMS (only valid for barring) 25 for data

for all.

T is number of seconds for the No Reply Condition Timer, default is 20 seconds if not specified.

References

{{Reflist

External links

GSM 04.90 (ETSI EN 300 957, V7.0.1) Specification (USSD)
Stage 3 – 3GPP

Android USSD codes list (MMI code list)
Mobile telecommunications standards 3GPP standards GSM standard