On 20/01/2025 17:09, ron minnich wrote:
I'm getting more and more bothered by the mountain of things going on in
my phones that I can never understand.
So I got curious and found this:
https://www.instructables.com/Build-Your-Own-Smartphone/ <https://
www.instructables.com/Build-Your-Own-Smartphone/>
and now I can't get a cellphone that runs plan 9 out of my head.
Am I out of my head?
The problem with 2G and 3G is that they're either gone or soon to be
gone in a lot of places. With 4G and 5G things get more complex, since
there is no circuit switched voice; you either fall back to 2G/3G for
voice and SMS, or need an IP Multimedia System (IMS) client to support
native voice and (VoLTE/VoNR a.k.a. "HD Voice") and text messaging.
IMS is a curious mix of SIP and IPsec and I don't think there is as yet
a fully working open source IMS client. Those in COTS handsets are
proprietary and for reasons to do with integration with baseband
features IIRC, are supplied by the baseband silicon vendor and then
integrated into Android and iOS etc.
I know of one LTE modem which supports VoLTE directly, Quectel EC25.
Which I believe it achieves by embedding a cut down version of Android,
complete with IMS client. So then you have the usual AT interface over
UART for dialing, plus audio input and output connections on the module.
Andrew
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