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MS MSC VLR HLRUSSDUSSDUSSDFigure 4 shows the message flow for USSD requests failed atVLR, MSC, MS for a single operation.HLRVLRUSSDReleaseUSSDUSSDReleaseUSSDReleaseUSSDUSSDReleaseUSSDReleaseActions at MSThe MS may at any time receive a USSD operation request/notification from MSC. The MS processes the operation if it is ina state in which it can handle the operation. After sending theresponse to a USSD operation, the MS waits for the network torelease the transaction. While awaiting this release, the MS willprocess any further USSD operation requests in the normal way.MSC initiatedUSSDUSSDAt times, MS may not be able to process the network-initiatedUSSD because of the following reasons:Release1 Feature not supported by the user (MS)Figure 3: Information flow for a USSD request (single operation)Figure 3 shows the message flow for a network-initiated (HLR,VLR, and MSC) USSD request for a single operation.In another case, when an application in the VLR needs to senda USSD request or notification to an MS, it sets up a transactionto the MSC where the subscriber is currently registered andsends the operation to the MSC. The MSC further interacts withthe MS as explained above. The VLR then awaits a response fromthe MSC. Because the VLR initiated the transaction, it is alsoresponsible for controlling the transaction.In a third case, when an application in the HLR needs to send aUSSD request or notification to an MS, it sets up a transactionto the VLR where the subscriber is currently registered andsends the operation to the VLR. The VLR further interacts withthe MSC which then interacts with the MS as stated above. TheHLR then awaits a response from the VLR. Because the HLRinitiated the transaction, it is also responsible for controlling thetransaction. The HLR normally releases the transaction afterreceiving a response from the VLR.VLR cannotcontactsubscriberMSCcannot contactMS MSC VLR HLRUSSDErrorUSSDUSSDErrorError2 Alphabet indicated in USSD is not supported by MS3 User is engaged in another USSD session (network- ormobile-initiated)4 A non-call related supplementary service transaction is inprogress. In all the above failure cases, an error indicatorwill be returned to the originator (MSC or VLR or HLR)4.3 MOBILE-INITIATED USSD OPERATIONSThe MS may initiate a USSD operation either during a call oroutside.Actions at the NetworkIf the serving network (MSC) does not recognize the USSD codein a mobile-initiated USSD operation, it sends the operation tothe next level (i.e., VLR). If VLR also does not recognize/decodethe operation, it forwards the same request to HLR. If even HLRis unable to decode it, an error message gets passed downwardand the session is terminated.If MSC, VLR, or HLR (in the same hierarchical order) is able todecode the operation/service requested, and if either of thenetwork nodes has the required data, then this information getspassed downward back to MS. However, if the network nodesare able to decode the operation/service request, but can’tsupport the required application, then a check is made with theUSSD platform. The decoded request is forwarded to USSDGateway and then to USSD applications to fetch the requiredinformation.If the mobile-initiated USSD transaction is found to beErrordetected at MSMS clearstransactionsUSSDErrorUSSDUSSDErrorUSSDUSSDErrorUSSDincompatible, the operation is rejected by a non-supportingnetwork and the attempt fails. Figure 5 shows the flow diagramfor a mobile-initiated USSD request. (The application at MSC/VLR may pass the request to another network element. Thatscenario is not shown here.)ReleaseReleaseReleaseFigure 4: Information flow for a failed USSD requestUSSD: A Communication Technology to Potentially Ouster SMS Dependency6
Requesthandled byMSCRequesthandled byVLRRequesthandled byHLRMS MSC VLR HLRUSSD requestUSSD responseUSSD requestUSSD responseUSSD requestUSSD responseFigure 6 shows the message flow for a mobile-initiated USSDrequest that failed at MSC, VLR, and HLR. It also depicts a casewhere an MS clears the transaction before it receives a responseto the initiated USSD request.USSD requestUSSD responseUSSD requestUSSD responseUSSD requestUSSD responseFigure 5: Information flow for a mobile-initiated USSD requestUSSD is an excellent choice for roaming with mobile prepaidservice, which utilizes the USSD connection to originate a callwhile roaming. USSD messages from handsets always route tothe home network. Thus, when roaming in another network,dialing a USSD string will always route the application on thehome network. This feature allows for the virtual homeenvironment concept.The MSC connects to HLR of the home network via VLR usingthe SS7 network. The HLR sends the request to the USSDGateway, which in turn passes the request to the prepaidapplication server. The application server ascertains the user’sbalance and provides instructions for handling the call via thesame path to the serving MSC in the visited network.Also, users accustomed to accessing a particular service in theirhome network are able to access that network from anothercountry. The processing happens in the same way as explainedabove, but the supporting USSD application server may bedifferent. Conversely, roaming subscribers from other networkscannot access USSD services on a host network.6 STRATEGICAL BUSINESS APLICATIONSUSING USSDErrordetected atMSCMS MSC VLR HLRUSSD requestErrorServices requiring menu-/session-based interaction betweenthe user and the application are ideal for offer via USSD. Somestrategic applications that can be developed or are still in theirnaive stages of development are discussed below in detail.Errordetected atVLRErrordetected atHLRMS clearstransactionbeforeresponsereceivedUSSD requestErrorUSSD requestErrorUSSD responseReleaseUSSD requestErrorUSSD requestErrorUSSD responseReleaseUSSD requestErrorUSSD responseReleaseFigure 6: Information flow for a failed mobile-initiated USSD request6.1 MOBILE BANKING VIA USSDNo other channels have the ability to reach the consumer asthoroughly as mobile phone. The coverage of cell phone networksin relation to fixed ATMs and branches helps reach morecustomers.Architectural view of banking systemA bank’s core banking system houses consumers’ accounts,related transaction management, and history. It is necessaryfor translating banking instructions received from consumersthrough bank channels such as ATMs, the internet, and mobiledevices into a format it can process.5 USSD USAGE AT HOME AND ROAM NETWORKUSSD can be accessed via two locations: Home Public Land MobileNetwork (HPLMN) and Visited Public Land Mobile Network (VPLMN).USSD messages can be handled either from the VLR or the HLR,depending on the current location of the user. When accessingUSSD at the home location (HPLMN), the user directly communicateswith the HLR in case of a mobile-initiated USSD operation. And whenaccessing USSD at a visitor location (VPLMN), the user communicatesto the HLR through the VLR.ATMCore Banking SystemEFT Channel SwitchMobile servicesBranchClient side Server sideJ2ME SMS USSD IVRFigure 7: The Banking System ArchitectureWebUSSD: A Communication Technology to Potentially Ouster SMS Dependency7
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