When the MS makes originate the USSD operation according to the input of the subscriber, the MS first sets up a connection to the network, and then sends a request to the MSC and waits for the response. Once the MS receives the response from the network side, the MS displays the information in the response message on its screen to the subscriber. During the waiting process, the MS may receive the USSD REQUEST or USSD NOTIFY message originated by the network. At this time, the MS must handle these messages immediately and continuously wait for the response of the original USSD operation from the MSC.
After the MSC/VLR receives the MS originated USSD operation:
- If the Service Code in this operation indicates that this operation must be handled by the HPLMN, the MSC/VLR tries to set up the conversation with the network. If such a conversation cannot be set up, the MSC/VLR returns error information to the MS. Otherwise, the MSC/VLR forwards this operation to the network side. The MSC/VLR transfers the subsequent USSD operations transparently between the MS and network.
- If the Service Code in this operation does not indicate that this operation must be handled by the HPLMN, the MSC/VLR decides how to handle this USSD operation according to the configuration of the USSD Control table, that is, handles this USSD operation by itself or forwards it to other network element.
- If the MSC/VLR does not support the characters in the MS originated USSD operation, the MSC/VLR must forwards this USSD operation to the network side. If the network side does not support the characters in the USSD operation, the network side must notify the MS and release the conversation.
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