USSD Basics

Unstructured Supplementary Services Data (USSD) is a GSM service which allows high speed interactive communication between the subscribers and applications. It is similar to SMS in the way that it allows to send and receive short text messages. It is different in the way that it is session oriented.

Since USSD is session oriented, it gives a very short delay between sending the query and receiving the response. This makes it ideal to query information from the network and to provide text content as a service.

An USSD Gateway is a platform that enables Operators to introduce messaging services with USSD as the bearer, enabling faster response times. The platform is complementary to SMS service and an SMS gateway. An USSD gateway acts as a Gateway between applications and the GSM network and lets you deliver USSD messages of up to 182 characters on a network between mobile stations and applications.

Both SMS and USSD technology use the signaling channel as the bearer by default. (SMS can also use GPRS). While SMS provides a store and forward functionality, USSD is not like this. It is a real time session-oriented service, which means that when a user accesses a service with USSD, a session is established and the radio connection stays open until the user, application, or time out releases it. This provides faster response times for interactive applications. An USSD session needs to be allocated to each and every interaction.

A typical USSD Gateway uses the same application programming interface that the Short Message Service Center does. Therefore, it is easy to port services based on SMS to utilize USSD as the bearer. In practice, only USSD specific modifications are needed to external applications.

An USSD gateway is usually equipped with session management capabilities, that can handle cell switching, to make sure an USSD Session is preserved even when the subscriber changes cells.

A big advantage of USSD comes while the mobile client is roaming. This is because USSD services are well available in roaming networks and all the USSD messages are directed towards the subscriber's Home Network itself, thus, same set of services that are available in home network can be given in visited network too, giving subscribers a Virtual Home Environment (VHE).

An USSD gateway communicates with GSM network entities, such as the HLR using SS7 (Signaling System 7) protocol stack's MAP (Mobile Application Part). It receives and sends out session IDs from the session ID pool, and maintains and destroys the sessions The MAP (Mobile Application Part) layer is present both on the server and on the mobile phones.

The USSD gateway's primary task is to wait for messages from the MAP layer and to route these messages into a content provider system through SMPP or HTTP or any other way. The content provider system can return an USSD response. Allthough the USSD gateway's primary task is to wait for messages, it can also push messages, because it allows both user initiated (pull) and service initiated (push) sessions. Thanks to supported 1-way and 2-way mobile sessions the USSD Gateway enables simple notifications as well as interactive dialogs.

The service provided by the USSD Gateway depends on the ability of the delivery agent or the source to send and receive USSD messages. The Home Network subscriber or the inbound roamer can send an USSD string to maintain or open an interactive session. The MSC connects through to the HLR in the home network (via the SS7 network). The HLR routes the request to the USSD Gateway, which in turn routes the request to the USSD content server. The content server provides the call handling instructions response back through the same path to the serving MSC in the visited or home network.

For each transaction, the USSD Gateways knows what session is going on and if you work with a locator, you also have information about which cell is the GSM phone located at.

The following terms are also important to mention: PSSR, USSN and USSR. Unstructured Supplementary Service Request (USSR) is a message, that can be used to initiate a session by the USSD gateway to the mobile user.

Today almost all mobile networks support the SS7 standard CAMEL for prepaid charging and interoperability. If CAMEL Phase 2 based charging interface is implemented in the USSD Gateway, it is able to perform real-time billing on almost any mobile network. CAMEL based charging should be supported for both network and mobile initiated USSD sessions.

Ozeki NG SMS Gateway is not an USSD gateway. It can act as a client for USSD services by sending and receiving USSD messages on supported GSM modems. Read more about USSD support in Ozeki NG SMS Gateway at the USSD message type page.

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