Telebit was doing the modulation in signal processing software in a dedicated big-grasshopper-sized Motorola 68000 chip inside the modem. These days we could do the same modulation in software in just about any chip. But as Brian said, the signal impairments in multi kilometer twisted pair telephone circuits with occasional loading coils, chopped into 56k digital bitstreams and reconstituted at the other end before transiting a different long twisted pair, are very different than the impairments typically experienced when passing a signal thru typical radio circuitry and kilometers of radio "ether" and interference. There's free software for messing with OFDM over narrowband radio (e.g. not WiFi); here are a few starting points: https://github.com/rwth-ti/gr-ofdm https://ro.uow.edu.au/cgi/viewcontent.cgi?article=4984&context=theses https://github.com/kit-cel/gr-lte John