USSD Call FLow

There are two types of USSD
--

1. Network Initiated - Push Mode 
2. User Initiated - Pull Mode

USSD Message Types for each types

• Network Initiated
• USSR - Unstructured Supplementary Service Request - First message initiated by the network to the subscriber
• USSR ACK - Unstructured Supplementary Service Request ACK - Contains the subscribers response to the USSR
• USSN - Unstructured Supplementary Service Notify - This message is sent by the gateway to the subscriber.
• User Initiated
• PSSR - Process Unstructured Supplementary Service Request - First message from the subscriber to the network. The response for this message is a USSR from the network.
• PSSR ACK - Process Unstructured Supplementary Service Request ACK - This message is sent by GW to the subscriber

User Initiated USSD Call Flow

MS                    VLR                    HLR                    USSD GW

------ *123------->

                              ------->PSSR----->

                                                            ------PSSR---------->

                                                            <------USSN---------

                              <------USSN------->

<------Text-------

                              ------USSN ACK-->

                                                             ----- USSN ACK--->

                                                             <--- PSSN ACK -----

                              <---- PSSN ACK----