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Comment by _stephan | Hacker News Reader

Why would the baseband processor need access to anything but the RF and maybe the GPS hardware?

For one thing, putting the voice signal processing in the baseband means that it's not vulnerable to timing glitches from "noisy neighbor" apps running on the application processor. For another, as a practical matter, a lot of the baseband software started out as the entire software stack for the single processor in a dumb-phone/feature-phone, which necessarily included the voice processing. Simply leaving it there avoids the technical effort of doing a port.

Quite often the baseband has the only direct access to the handset mic and speaker, including things like the agc and speakerphone. That's why a room bug can be implemented this way.

The article hints at a way to democratize access to this capability by using the RIL commands to turn on auto-answer and turn off any indication it's happening.

Yep. I had no idea this was the case until I read an article that came out shortly after the original iPhone. The dialer app in iOS during those first few months was pretty buggy and had an occasional tendency to crash or lock up while on calls, especially when pressing the "hang up" button. Every time it locked up on me, it'd never impact a call in progress though (much to my annoyance when trying to hang up).

It made perfect sense once I read that the speaker and mic were wired straight to the baseband, and the state of the dialer application had zilch to do with what was going on in the baseband.

RIL commands being extended AT commands? There is a community of phone unlockers who know everything about these for Qualcomm chipsets but their tools have the best DRM I've ever seen.

Way back when a wrote a couple chapters for Android Application Development I wrote about Android's RIL daemon and the underlying device-specific RIL libraries. It's gotten a lot more complicated, but the source code is open and includes what appears to be a reference implementation: https://github.com/android/platform_hardware_ril

I have not looked at this part of Android source code in depth in a while, but from a quick look it still looks very edifying about how this part of a smartphone works.

Similarly, the open source implementation of the Firefox OS RIL daemon can be read here: https://mxr.mozilla.org/mozilla-central/source/dom/system/go...

(Sadly, it's not used in all production devices.)

That's a lot of code! I wonder if it has more feature coverage than the Android device-independent RIL layer.

Have you documented the RPCs to the modem as well?

It wouldn't. Though you could build it that way, I guess, if you felt so compelled.

http://en.wikipedia.org/wiki/Baseband_processor

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