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USSD stands for Unstructured Supplementary Service Data, and is a Global System for Mobile(GSM) communication technology that is used to send text between a mobile phone and an application program in the network or access a service host in a web server. USSD text messages can be up to 182 bytes (182 characters) in length.Messages are transferred directly over the network signaling channels.
Remember an SMS has 160 bytes.USSD is session-oriented, which means that when a user accesses a service with USSD, a session is established (normally by a USSD gateway)and the radio connection stays open until the user, application, or time out releases it. This provides faster response times for interactive applications.
You would notice when you dial a USSD application, and become idle, not interacting with the application, the USSD message disappears. USSD applications are accessed by user request, which lead to a series of textual menus, which are navigated through the corresponding menu numbers.
USSD is similar to Short Messaging Service (SMS), but, unlike SMS, USSD transactions occur during the session only. With SMS, message scan be sent to a mobile phone and stored for several days if the phone is not activated or within range.
How does USSD differ from store and forward SMS?
USSD is session oriented, unlike SMS, which is a store-and-forward, transaction-oriented
technology.A text message is first sent to a sender’s Short Message Service Center (SMSC) before the SMSC tries to deliver the text message to the recipient.
The sender receives an indication of whether the text was successfully received by the SMSC; however, the text message is not guaranteed to reach the recipient instantly and can be dropped if the recipient cannot be reached for a period of set time.
Turnaround response times for interactive applications are shorter for USSD than SMS because of the session-based feature of USSD, and because it is NOT a store and forward service.
A USSD message is 182 bytes (characters) long whereas that of a message is only 160 bytes. You would notice that when a USSD textual menu is too big, the last option uses the phrase “9. More…”, so. More often, you would respond with a GSM predefined number, to fetch more menu content. that more textual menu is fetched, which would not fit in the 182 characters limit.
This is just a tip of the iceberg, more to come, feel free to comment.