9 Feb
2025
9 Feb
'25
11:14 p.m.
Peter Corlett:
These easy-to-ask questions have very long answers.
But mostly, it was
*not*
a case of merely increasing the baud rate or the number of bits per baud
with a different modulation scheme, but multiple concurrent advances which
did those *plus* some other techniques which would maintain signal
integrity
despite the reduced SNR.
Apologies for the very open ended question. I found some old 1992
newsgroups thread that guided me towards the answer (including a long
excerpt from "Computer Select CD-ROM" with lots of CCITT referencing).
Interesting on comparing 1992 perspectives and contemporary confusion,
versus today's modern references on the topic.
If I'm understanding it right, a "sort of" answer to my own question is:
2400 baud (v.22bis) was an "amplification" (not the right word, but
"phase
magic") of 600 baud. While as has been mentioned, 9600 baud (v.32) was a
similar "amplification" of 2400 baud.
One thing I didn't realize was v.32bis (14400) was a dynamic speed (it
could slow down and speed up based on line quality). I don't recall
experiencing that, as far as varied transfer rates.
Someone here mentioned "Winmodem" and wow, I recall using one of those - I
think it was maybe one of the last modems I used, at around 28,800 baud? I
never got to experience the 33.6K or "57.6K" stuff. Even back then I
recall the v.42 "compression" stuff being confusing (to me), since I
didn't
know what the dial-up ISP at the time would support. And if I recall
correctly, the "WinModem" was somehow tied or dependent on Windows OS (or
they only build drivers for Windows) and it started to hit me how "DOS is
dead."
Neat review.
-Steve
On Sun, Feb 9, 2025 at 6:18 PM Peter Corlett via cctalk <
cctalk(a)classiccmp.org> wrote:
> On Sun, Feb 09, 2025 at 12:08:47PM -0600, Steve Lewis via cctalk wrote:
> [...]
> > So.. If you had a slow system that couldn't really take advantage of a
> > ~7MHz 16550 serial card (or I guess like a laptop that was stuck with an
> > older UART) That might be the use-case where this parallel v.fast might
> > help (by being able to "feed the modem" fast enough to actually take
> > advantage of the faster modem speed?) Or is there some other scenario
>
> Note that all but the dumbest modems reclock the data before transmission
> and the XON/XOFF or CTS/RTS flow control is handled locally, buffering in
> the modem as necessary. At faster speeds there's no longer a 1:1
> relationship between the signal level on the RS-232 cable and the screeches
> going down the phone line. Start and stop bits are not transmitted, giving
> a
> 25% speed boost from that alone.
>
> A parallel-connected modem is a bit pointless except in weird environments
> where one's serial port is broken or otherwise unusable. Information theory
> tells us that you can't get more than 64kb/s out of a dialup link, because
> that's the speed of the underlying digital channel used by the PSTN. Due to
> the reclocking, you actually need a serial port capable of 80kbaud to not
> drop data, and the next-highest standard baud rate is 115,200 baud, which
> any half-decent PC serial port can handle.
>
> To pre-empt the obvious retorts from the peanut gallery, sure, one may well
> have an original IBM XT or BBC Micro or whatever whose serial port drops
> bytes when driven faster than 9600 baud. Guess what: the machine's so slow
> that it can't handle the firehose of data even if its serial port wasn't a
> basket case.
>
> I don't know where you get "~7MHz 16550 serial card" from. The 16550
(and
> predecessors) sample the incoming serial signal with a clock 16x the baud
> rate. For 115,200 baud, that's 1.8432MHz, and it is no coincidence that
> this
> is a standard crystal frequency and also the maximum speed of a lot of
> UARTs.
>
> [...]
> > Is there any "natural rate" (Hz) of a modem? Meaning is 1200/2400
> > baud-equivalent modem an accelerated-by-enhanced-encoding version of 300
> > bps? and 9600 likewise an accelerated-by-encoding version of 2400? is
> > 300bps itself some kind of special accelerated-by-encoding? I see 1200
> > baud was also still sub 3KHz
>
These easy-to-ask questions have very long answers.
But mostly, it was
*not*
a case of merely increasing the baud rate or the number of bits per baud
with a different modulation scheme, but multiple concurrent advances which
did those *plus* some other techniques which would maintain signal
integrity
despite the reduced SNR.
>
> If you want the full gory detail, the relevant ITU standards are
> freely-available. Bring a good signal-processing textbook.
>
> > (did any modem protocol go above 3KHz?).
>
> V.90 used the full 4kHz analogue bandwidth for the downstream. Yes, even
> the
> frequency extremes which were heavily-attenuated by the line filters. It'd
> just listen much harder.
>
>