I’ve spoken about it a few times, but admittedly haven’t done much in a while.
I’m working on a very widely featured user mode ISDN stack, primarily in Erlang, with bits of C and Java sprinkled in. In the end it will support DAHDI compatible interfaces, osmo-e1d interfaces, generic terminal adapters, that one ISDN chipset that was sided in certain AlphaStations (when i get my hands on one) and is designed so anyone can expand it to support their hardware interface. It also will support a wide variety of protocols as the the standard LAPD, Frame Relay, and X.25 in the D-channel.
Currently most of my design problems are around hardware.
For example, if I want to do H.320 video conferencing I require channel binding. I’m not sure if any of the hardware interfaces I have support that natively, and have not done enough research to see if I can “simulate” it in software. Also BRI uses a special hardware interface for connecting to the local loop known as the U-interface, which is common in Terminal adapters, where both my current BRI hardware and most of terminal equipment(phones, etc.) tend the use the S/T interface.
I had a number of other project ideas and come up with new ones every day, but outside the above idea, I’m giving up programming as a hobby until I can give it up as a career.