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
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
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,
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.
GSM modem vs IP SMS