9 Feb
2025
9 Feb
'25
4:09 p.m.
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.