USSD is a stand for Unstructured Supplementary Service Data what is a capability of GSM mobile phone much like the Short Message Service (SMS). But there is a difference between USSD and SMS handling.
- SMS uses "store and forward" method of message delivery. Short Message is delivered first to Sender's Short Message Service Center (SMSc) which will try to deliver the message to recepient. So SMS does not guarantee that message will be delivered instantly.
- USSD information is sent from mobile handset directly to application platform handling service. So USSD suppose to establish a real time session between mobile handset and application handling the service. The concept of real time session is very usefull for constructing an interactive menu driven application.
Network architecture for USSD service
A user who is dialing USSD service number initiates dialog with USSD handling application deployed on the Mobicents Platform as depicted of the Figure 1. The "Network Node" depcited could be MSC, HLR or VLR. The Mobicents Platforms integrates with "Network Node" using MAP protocol.
The detailed description of the allowed MMIs or phone number which uses can dial is prsented in 3GPP TS 22.090. In the user's home network the following number range is defined for USSD services:
1, 2 or 3 digits from the set (*, #) followed by 1X(Y), where X=any number 0-4, Y=any number 0-9, then, optionally "*" followed by any number of any characters, and concluding with # SEND
For example user can dial *#122# to reach a specific USSD service which is deployed in the home network and. The application in its order can reply with menu. One of the biggest benefits is that this service is always alailable even when user is currently in roaming.
USSD Signaling
Figure 2 shows typical MAP message flow for implementing data transfer between "Network Node" and Mobicents platform to implement menu driven application.
For more information on mobile- (and network-) initiated USSD operations and the use of MAP USSD services, refer to [3GPPTS 24.090] in the References section.
Figure 2
Mobile initiated USSD service starts when user dials USSD string *#122#.
- The Network sends TCAP Begin message with Component MAP_PROCESS_UNSTRUCTURED_SS_REQUEST to the Mobicents platform. The Mobicents platform invokes USSD application logic .
- Application request additional information from user (action one or action two) via MAP_UNSTRUCTURED_SS_REQUEST incapsulated in TCAP Continue message. At this time TCAP Dialogue starts.
- Application receives user's selection of the action.
- Application performs its logic and sends a response back to the user. At this time application do not want to get additional information from the user and it sends response using MAP_PROCESS_UNSTRUCTURED_SS_REQUEST and terminates TCAP dialogue.
USSD gateway
Existing MSC, VLR, and HLR network elements are proprietary and run on non-standard operating environments located in trusted operator's zones that make it difficult to build and deploy new applications. Also, these network elements do not provide the tools and interfaces needed to access and retrieve data from content providers over Internet. The USSD Gateway connects to the MSC, VLR, or HLR and enables the flow of USSD messages to be extended to an open, standards-based application server located in the IP network. The AS also provides the tools and interfaces to enable access to the content providers through the Internet.
MAP to SIP mapping
The following diagram explain USSD gateway mapping between MAP and SIP. The original MAP message is processed by USSD gateway connected to GSM network and forwarded over Intenet to provider's Application server.
1. Original TCAP BEGIN message is translated to SIP INVITE.
2. Application server sends message using SIP OK message which is translated to TCAP CONTINUE with response Dialogue portion. TCAP dialogue is created with SIP dialog.
3. Message exhange is transclated using SIP INFO message (TCAP CONTINUE)
4. SIP BYE message is used to terminate TCAP dialogue (TCAP END) alongside with SIP dialog termination.
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