What is the info you are looking in USSD?.
Usually it’s used in developing un-supported mobile services!!, and MAP supports few messages too!!!.
Like USSDArg, USSDRes, USSDNotify………
And usually there will be a USSD-gateway which will host the service.
When user sends a USSDRequest *111#xyz….., the the VMSC will formward the USSD Request to HLR which routes the USSDRequest to specific USSDGW according to info on HLR which is configured.
In response if the USSDGW wants to send a message, then USSDRes will be sent either directly to VMSC or through HLR.
In case of HLR, the USSDRes will be forwarded to VMSC which intern sends the USSDRes to MS.
-Hope this helps………..Let me know in case you need more info on this!!!!!!!!!!!