USSD is one of the hidden gems of 2G. It is based on MAP signalling protocol and is used by the mobile device, MSC/VLR, HLR / HSS and an assigned USSD Gateway. Good example of a use case is mobile prepaid top-up and balance enquiry. USSD and SMS are examples of the first mobile Internet services.
Generally you can have one spc for a MSC.
VBR/ Wallis Dudhnath