USSD Gateway -- some insight

What is a USSD Gateway?

A USSD Gateway routes USSD messages from the signaling network to service applications and back. "USSD gateway" and "USSD center" are synonyms.

USSD Gateway is based upon the ability of the delivery agent or the source to send and receive USSD messages. As USSD is a session-based protocol unlike its siblings (SMS and MMS), therefore, the session needs to be allocated to each and every interaction.

[edit] Difference between USSD and other Gateways

The difference between USSD Gateways and other kinds of gateways is that USSD Gateways require to know what session is going on. Also, put in the locator (it locates where the MS (Mobile Station) is currently), and you have a highly dynamic session – based protocol.

[edit] Modular Operation

1. Session Module : As per directions from the 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.
2. MAP layer: Mobile Application Part is present both on the server and on the MS. Details can be found in other articles on Wikipedia and other popular internet and hard sources.
3. Gateway: A Gateway will wait for messages from the MAP layer and work to route these messages into SMPP protocol which is then delivered to the server applications. This is the most important operation, and this is the reason why USSDs are primarily used, as it helps to directly connect users to applications like bill checking and others.
4. Locator: This tries to find out the current cell and relays it to the Gateway. Then the messages are routed using Routing Numbers.
5. Home Location Register: This is the home zone where the given cell phone’s number is registered in the database. This is different from the Visitor Location Register which is where the user is roaming.

Typical Applications:

• Balance Check: The user can send a Process unStructured Supplementary request (PSSR) to the home zone which will forward this, under guidance from the Gateway, to the correct application. Then, the application sends an acknowledgement via USSD Gateway, HLR etc, known as PSSR response back to the user. Balance Notification at the end of charged call can also be given using Unstructured Supplementary Service Notify (USSN) message.
• Voice Chat: Using the same process as above, one can use voice chat. This is highly useful when VoIP enabled phones are not available.
• Advertising: The application can advertise their product using USSD which is more non-invasive than telemarketing.
• Roaming: This has huge advantages while 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).

Apart from PSSR and USSN, there is another method called Unstructured Supplementary Service Request (USSR) message that initiates a session by USSD Gateway to a Mobile User. This message can be used in conjunction with USSR initiated session to provide session based services like Menu services through USSD. Also, in the earlier phases of MAP (Mobile Application Part), PSSR message was called PSSD (PSS Data).