Baseband Bootrom Protocol
This is the protocol used to talk to the old, and probably the new baseband, at the bootrom level. The old bootrom didn't have an sig checking, the new one does.
AT 0x30 2 byte length n byte data 2 byte checksum sends A5 on success, 5A on failure
Correct me if I am wrong, but on the iPhone 3G bootrom, the "protocol" section is pretty much identical, besides the last line, which is instead this:
sends 01 on success, FF on failure