Mobius USSD Gateway Intro

Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network. 

USSD messages are up to 182 alphanumeric characters long (depends on encoding used, for example, in Unicode up to 70). Unlike short message service (SMS) messages, USSD messages create a real-time connection during a USSD session. The connection remains open, allowing a two-way exchange of a sequence of data. This makes USSD more responsive than services that use SMS. 

The advantage of USSD for message delivery and notification is that it is session-based and very efficient. Session-based communications are more reliable than services like email and SMS. This makes it useful for fast and simple real-time applications like balance checking, voice chat and advertising. Because USSD supports process supplementary service (PSSR) and unstructured supplementary service notify (USSN) requests and messages, real-time services like balance checking and voice chat work even when roaming. 

In addition, since USSD is so efficient, there is a huge advantage to using it when subscribers are roaming. All USSD messages are directed to the subscriber's home network but they can also be directed to a visiting network enabling a virtual home environment (VHE), which reduces operator costs and greatly improves user experience as all services work the same wherever they are. 

And, while modern 4G/LTE networks support voice chat applications using SIP, USSD-based voice chat applications can work seamlessly and well across older GPRS, 2G and 3G networks, providing the widest possible compatibility footprint for services important to the subscriber. 

A USSD gateway routes USSD messages from the signaling network to a service application and back. A 'USSD gateway' service is also called a 'USSD center'. 

How Mobius USSD Gateway Works 

The following diagram illustrates Mobius USSD Gateway server place in the mobile networking. 

There could be two types of USSD calls: pull or push. The pull call is initiated by mobile phone (mobile subscriber); the push call is instated by the network application. We (Mobius USSD Gateway) name them MS-initiated or Network-initiated, respectively. 

The diagram above displays the case with two mobile networks available: that’s because the mobile subscriber could be roaming. Mobius USSD Gateway uses “Send Routing Info for FM” command to detect the mobile subscriber whereabouts and what MSC (message service center) they use, so then we connect with that MSC during the user call. 

Push (Network-initiated) Call Flow 

The following diagram illustrates the network-initiated call flow with appropriate commands. 

On the push call the Mobius USSD Gateway requests the HLR (home location registry) if the user mobile device is connected to the mobile network and, if yes, what MSC it uses right now. Obtaining that MSC and the Global title, we (Mobius USSD Gateway) are connecting with the received MSC, and the whole call is proceeding via that MSC till the end. The USSD Configuration settings and USSD Destination settings hold the parameters required for such manipulations (note that SSNs are set up in USSD Configuration settings, and USSD Title in Destination settings). 

Pull (MS-Initiated) Call Flow 

The following diagram illustrates the MS-initiated call flow with appropriate commands. 

On the pull call the Mobius USSD Gateway receives the first data package from the MSC, so we know what MSC is to be used during the whole call, and we work with it until the call ends. Contacting HLR is not needed in this case. The USSD Configuration settings and USSD Destination settings hold the parameters required for such manipulations (note that SSNs are set up in USSD Configuration settings, and USSD Title in Destination settings).