iPhone 3GS have to communicate with Apple’s servers before allowing the installation of any version of firmware (restore or update ) .With the release of Firmware 3.1, Apple’s server has stopped certifying all previous firmware and this makes it impossible to restore your terminal to version 3.0 or 3.0.1. The certification is done through 3 files called IBSS, IBEC and ECID that, once signed, are a kind of “green light” to install the firmware through iTunes.