Was it 110, 135, then 300?

Bell 103 type modems used FSK, so they were rated for full duplex operation at any bit rate from 0 to 300 bps. In practice you could usually get 450 bps from them. I've heard talk of 600 bps, but never observed it to work reliably. 110 and 134.5 bps were common because of the use of Teletypes and IBM 2741 terminals. Bell 202 type modems used FSK for up to 1200 bps half duplex. There was provision for a much lower rate back channel, around 75 bps IIRC. At 1200 bps full duplex and higher, FSK is not usable. The modulation techniques got much more complex, starting with PSK (phase shift keying) then QAM (quadrature amplitude modulation). With PSK and QAM modems, you can't send arbitrary bit rates as with FSK. You pretty much *have* to send the bits when the modem wants them. The modulation is fundamentally synchronous, so what actually goes across the wire is async framing over a sync carrier, which is sometimes called isochronous. Don't get me started on the hairy details of speed matching, which involves stop bit shaving, and in some cases stop bit deletion and reinsertion. It's a mess. Be glad that your sub-$100 modem takes care of all this nastiness for you. At 1200 bps full duplex, there were two standards as well as a proprietary scheme. Bell 212 used PSK, CCITT V.22 used QAM (IIRC), and I have no idea what the Vadic 3400 used. PSK is not *too* difficult to implement in the analog domain, but QAM is pretty tricky. By the mid-1980s most QAM modems were implemented digitally (although many still used an analog bandsplit filter before the ADC). used QAM with increasingly complex constellations. The 56K modems use entirely different techniques. Eric