You need a USSD server / gateway in the environment of mobile network operators connected to the network of SS7 operators. USSD messages can be sent from a mobile phone to a USSD server or from a USSD server to a mobile phone. It is not possible to send a USSD message directly from one phone to another; you will need to send the message through the USSD server.
USSD messages are not standardized; they are specific to the mobile network and USSD compatible applications. However, there are some standard USSD messages that are understood by many HLRs to query and configure features such as call forwarding.
USSD is transported over SS7 using the MAP protocol. The business logic implemented on top of the MAP is related to the implementation of the actual protocol. On the server side, which can be hardware (interface cards), as well as operating system drivers or software, for example. SIGTRAN-Stack, depending on the supported physical layer (E1, SDH over fiber, ATM or Ethernet). The sample application depends on the protocol stack on which it runs. See sample applications from an interface hardware vendor or protocol stack. On the client side, USSD support is device dependent, for example. via J2ME JSR 120 "Wireless Messaging API".
USSD delivery is not guaranteed, especially when roaming. It only depends on the mobile network operator. If you do not have an agreement with the operator, it can disconnect your USSD messages from the firewall, especially if it detects large volumes that bypass its messaging services.
Bernd
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