MDM6610

The Qualcomm MDM6610 is the baseband chip used in the iPhone 4S.

Baseband Firmware
The firmware for the baseband chip comes in Trek-X.X.XX.Release.bbfw files (with X being the version). The BBFW file is just a renamed standard ZIP archive and contains a couple of .mbn files which are all digitally signed with a certificate from Qualcomm. Qualcomm's Senior Engineer Shital Mehta is involved with the programming of the firmware. The amss.mbn file is an ELF file:

Mac OS X file command output
file Trek-1.0.11.Release/amss.mbn /Trek-1.0.11.Release/amss.mbn: ELF 32-bit LSB executable, ARM, version 1, statically linked, corrupted section header size

iPhone 4S
1.0.11   5.0     (Build 9A334) 1.0.12   5.0.1b1 (Build 9A402) 1.0.13   5.0.1b2 (Build 9A404) 5.0.1  (Build 9A405) 1.0.14   5.0.1   (Build 9A406) 1.2.01-1 5.1b1   (Build 9B5117b) 1.4.00   5.1b2   (Build 9B5127c) 1.5.04   5.1b3   (Build 9B5141a) 2.0.10   5.1     (Build 9B179) 2.0.12   5.1.1   (Build 9B206) 2.8.01   6.0b1   (Build 10A5316k) 2.8.04   6.0b2   (Build 10A5336d) 2.8.07   6.0b3   (Build 10A5355d) 3.0.00   6.0b4   (Build 10A5376e) 3.0.04   6.0GM   (Build 10A403) 6.0    (Build 10A403)

External Resources

 * Reverse-engineering a Qualcomm baseband - Video
 * Reverse-engineering a Qualcomm baseband - Slides