Solution for redsn0w "Usb communication problem" error
Unlike cryptic and non-informative iTunes Error Codes, jail-breaking utility redsn0w do offer useful information to diagnose possible issues. A common error encountered with redsn0w is "Usb connection problem", but despite the clear message and obvious solution to fix USB connection between the device and computer there could be other reasons for the error.
While the very first thing you need to check when encountering this error would be a good working cable and a properly functional USB port, this error can also be caused because of newer versions of redsn0w not compatible with Apple iPhone 3G. For me using older "redsn0w_win_0.9.12b1" fixed the issue, so go ahead and download redsn0w 0.9.12b1 from the links posted below if using iPhone 3G and facing this error.