Unstructured Supplementary Services Data

Telecommunication Services

The communication capabilities (speech, data, etc) available to the subscribers of a GSM PLMN are termed telecommunication services[1]. These services are defined by standardized protocols and functions and are offered to the customers by the network operators on a subscription basis.

Telecommunication services are categorized into:

-          bearer services[2], that provides the underlying lower layer capability of transmission of signals between access points necessary for transmission to occur within the same or different network.

-          teleservices[3], that provide the subscribers with the complete capability, including any terminal equipment specification, to communicate within a GSM network according to the agreed upon established protocols between network operators.

The above two services are also classified as basic services and they together provide the basic telephony capabilites such as making a voice call, sending a short message or establishing a data session, there by, serving the main purpose of a telecommunication network.

Supplementary Services

Apart from basic services, a GSM network also offers a set of supplementary services[4] (SSs) to enrich the experience of its subscribers. SS modifies or supplements a basic service and, hence, can only be offered together or in association with a basic telecommunication service.

An example of a SS could be a call barring service (CB). A subscriber roaming in a visited network may activate a barring of all incoming calls (BAIC) SS to bar any calls made to the subscriber’s mobile to save the high cost of roaming charges on incoming calls.

Motivation behind USSD

The most basic reason to buy a mobile and a subscription in the GSM network is to probably make voice calls. The establishment of mobile originated (outgoing) and mobile terminated (incoming) voice calls is the basic service offered by the network. On top of this basic service, the susbcriber, who pays the mobile bill,  might want to restrict incoming calls under certain circumstances, such as when he is roaming and he doesn’t wants to bear the cost of high roaming charges. Or the subscriber might want a service such that he is notified of an incoming call when he is busy on another call. This service is known to us as call waiting. These later services, such as, call barring and call waiting, extend or modify the basic services and fall under the category of supplementary services.

As the need of the user grows, he might want additional innovative services. For example, a subscriber might want to know his balance. Another popular service is live cricket scores or weather updates on mobile phone.

Such services are not standardized by GSM. However, services such as these which can be implemented on-the-fly keeping in pace with the mobile market and user demands offer a chance to the operators to earn handsome revenues and offer innovative ways to the susbcriber to get more out of their mobile.

Unstructured supplementary services data (or USSD) is a mechanism built into GSM PLMN and GSM MS that allow network operators to offer non-standard operator-specific services to their subscribers. USSD offers a way to network operators to quickly launch new profitable services without having to wait for the GSM standardization procedure. Rather than as a service itself, USSD can be thought of as a tool or a vehicle provided by GSM to drive a new feature, thus avoiding the lengthy process of technical bodies specifying and standardising a feature before it can be made available to the subscribers. USSD could also facilitate the early introduction of GSM specified services.

USSD is characterized by exchange of text strings between the network and the mobile in a certain format. This allows subscribers a simple real-time interface to interactive content rich applications on the network.

Technical Realization of USSD

... to be continued

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[1] GSM 02.01: "Digital cellular telecommunication system (Phase 2+); Principles of telecommunication  services supported by a GSM Public Land Mobile Network (PLMN)".

[2] GSM 02.02 (ETS 300 904): "Digital cellular telecommunication system (Phase 2+); Bearer Services (BS) supported by a GSM Public Land Mobile Network (PLMN)".

[3] GSM 02.03 (ETS 300 905): "Digital cellular telecommunication system (Phase 2+); Teleservices supported by a GSM Public Land Mobile Network (PLMN)".

[4] GSM 02.04: "Digital cellular telecommunications system (Phase 2+); General on supplementary services".