engineering

Unstructured Supplementary Service Data

Unstructured Supplementary Service Data is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Unstructured Supplementary Service Data rather than just read about it. In short: Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator'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…

Key takeaways

Unstructured Supplementary Service Data belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Unstructured Supplementary Service Data to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Unstructured Supplementary Service Data from memory before moving on to harder problems.

Reference excerpt

Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator'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 GSM is being phased out in the 2020s with 2G and 3G technologies, USSD services can be supported over LTE 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 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 Tesco Mobile, 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 and to deliver one-time passwords or PIN codes. Some operators use USSD to provide access to real-time updates from social-networking websites including Facebook and Twitter. Between 2012 and 2018, the Wikipedia Zero 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 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, the message is delivered over MAP, USSD Phase 2, as specified in GSM 03.90. After entering a USSD code on a GSM handset, the reply from the GSM 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 LTE/5G/IMS 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 <leave blank> for all. T is number of seconds for the No Reply Condition Timer, default is 20 seconds if not specified.

See also USSD Gateway Ultra Mobile Broadband Supplementary service codes Call forwarding § Mobile (cell) phones SIM Application Toolkit

References

External links GSM 04.90 (ETSI EN 300 957, V7.0.1) Specification (USSD) Stage 3 – 3GPP 3GPP TS 22.030 What’s the difference between USSD, MMI and SS codes?

Illustrations

Worked examples

Example 1 — a first encounter with Unstructured Supplementary Service Data

Start with the simplest possible case. Write down what Unstructured Supplementary Service Data claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Unstructured Supplementary Service Data before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Unstructured Supplementary Service Data ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Unstructured Supplementary Service Data

In research: Unstructured Supplementary Service Data appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Unstructured Supplementary Service Data in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Unstructured Supplementary Service Data is common in secondary-school and first-year university syllabi. It links to neighbouring topics 3GPP standards, GSM standard, Mobile telecommunications standards, so understanding it makes those chapters shorter.
In everyday life: Look for Unstructured Supplementary Service Data outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Unstructured Supplementary Service Data in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Unstructured Supplementary Service Data means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Unstructured Supplementary Service Data out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Unstructured Supplementary Service Data in simple terms?

Why does Unstructured Supplementary Service Data matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Unstructured Supplementary Service Data?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Unstructured Supplementary Service Data.