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 a sig checking, the new one does.
AT 0x30 2 byte length n byte data 2 byte checksum sends A5 on success, 5A on failure
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
bootrom.h in gbootloader Archived 2008-07-25 at the Wayback Machine