It shouldn’t be too difficult to design a daughter board for the USRP to
sample phone line voltage (with an appropriate line interface circuit of
course). After that it should be rather easy in GNURadio to generate DTMF
tones for dialing, and modem tones for data communication.
DSL is a good example – it would be great for students, technicians,
and researchers to be able to study the performance of DSL modulation
techniques, and try making improvements.
You can probably add a telco block to the low frequency tx and rx
boards without trouble.
Don’t forget to support voice communication as well as DSL and modem!
(Hmm, anyone want to reimplement the Telebit modem for fun? It did
half-duplex communication on hundreds of ~2-bit-per-second carriers,
avoiding the frequencies that didn’t get through cleanly on your
particular phone network, and flipping back and forth from TX to RX
many times a second. There’s probably an acronym for it now.)
Hacking up an interface and code for 10 megabit Ethernet on coaxial
cables would be fun too. That could be done on standard TX and RX
boards, probably by just wiring them to the “transceiver cable” of an
Ethernet transceiver. Ethernet modulation is pretty straightforward.
For true incestuous pleasure, we could code up the signal modulation
and electrical interface for USB 1.1.