On 2/10/2025 10:08 AM, Paul Koning wrote:
On Feb 10, 2025, at 3:58 AM, Jim Brain via cctalk <cctalk@classiccmp.org> wrote:
On 2/10/2025 1:14 AM, Steve Lewis via cctalk wrote:
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.
Not sure if it's been linked, but I found a list of baud->bps mappings at
Wikipedia:
https://en.wikipedia.org/wiki/Modem
For those who are OK using that resource to answer questions. I found it
interesting at 1200 bps had two options (1200baud * 2 tones or 600 baud * 4
tones)
Not 4 tones; 4 modulation states per signal element, that is what QPSK means.
Apologies, I was trying to use simpler terminology, given the writer's
attempt to understand the overall relationship.
The difference is that the 202 standard was designed to run half duplex over a
standard phone line, or full duplex if you had a 4 wire (leased line) circuit.
It's a very simple device that actually works at any speed up to 1200 bps (or a
hair more, as PLATO did). The 212 modem using QPSK is a clocked system, but it
can carry 1200 bps full duplex over a single phone line, with half the channel
bandwidth used for one direction and half for the other.
I realize it's extremely late and probably of no value except for
historical purposes, but having a way to visualize the various standards
in this space with respect to duplex, baud/bps rate, etc. would be of so
much value. Like the poster I replied to, how a modem worked always
seemed so oblique, especially as the speeds increased beyond 9600, even
without the added complexity of things like MNP and the negotiation
"dance" later modems held on the line. It was fascinating to hear about
and use, but I always felt I should know more about it. Yet, most
material in the day either waved a hand over the whole topic, or tried
to regurgitate the CCITT documentation. Specifically, in your above
statement, I'm still struggling to understand the duplex aspect of the
various standards. As a ham operator and having went through my EE
degree, I understand duplex, but since I always thought of the phone
line as a full duplex medium, how it would be used as a half duplex
channel eludes me. I'm OK with some terminology simplification, as
shown above, if it could help show how the bandwidth of the telephone
line was divided up in the various standards and how a 202 standard
managed to emulate a full duplex conversation (if it actually did this)
over the half duplex 2 wire telephone circuit. (And I use emulate in a
loose sense, I suppose. Back in the day, when IBM and LU.2 was a thing,
I worked at a company that created a general comms package that could
pass data over various protocols, including TCP/IP, LU6.2, NetBIOS, IPX,
and LU.2, which I believe was half duplex. But, the generic package
promised full duplex comms, so we (not me, but the team) had to build a
way to emulate a full duplex connection over that half duplex
technology. It worked, at least well enough to support the apps used
with it, but even it was "magic" to me, and I read all the source code)
Jim
--
Jim Brain
br...@jbrain.com
www.jbrain.com
