Was it 110, 135, then 300?

From: Eric Smith <eric_at_brouhaha.com>
Date: Fri Oct 27 13:19:33 2000

> What was the progression in modems? Was it 110, 135, then 300, or were the
> first two speeds pre modem technology?

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).

>From 1200 bps to V.34bis (or do they have V.34ter now), almost all modems
used QAM with increasingly complex constellations. The 56K modems use
entirely different techniques.

Eric
Received on Fri Oct 27 2000 - 13:19:33 BST