10 Feb
2025
10 Feb
'25
7:50 p.m.
On Feb 10, 2025, at 11:46 AM, Jim Brain
<brain(a)jbrain.com> wrote:
On 2/10/2025 10:08 AM, Paul Koning wrote:
Yes, a phone line is full duplex, to humans. What actually happens is that the line
carries both directions, superimposed -- you hear both sides in the headphone. That works
well for us because we know how to sort the two signals.
A telephone line modem can be done three ways, roughly:
1. Half duplex: the channel is only active in one direction at a time. (202 modem)
2. Full duplex band split: one direction uses the lower half of the bandwidth, the other
the upper half. (212 modem)
3. Full duplex full bandwidth: each direction uses the full bandwidth but carefully
subtracts the local signal from what is seen on the line to arrive at the signal from the
other side. (V.32 etc., I believe)
The "negotiation" you're referring to may include things like equalization
and calibrating the local side suppression for case (3), since those things vary from one
connection to the next (and, possibly, over time even for one continuing connection).
A 202 modem, on a POTS line, did not emulate full duplex. It could only run half duplex,
with the usual RS-232 modem control signal handshakes to do line turnaround. Some
(perhaps most or all, I don't know) of 202 modems can be connected to a 4-wire
channel, essentially two phone lines in parallel where each pair carries one direction,
the two pairs in opposite directions. You could get that sort of line from Ma Bell but it
would be a permanent circuit, not a dialed phone line.
As for datacomm protocols like TCP/IP and such: IP sits above the data link layer so
it's not really in the picture. Looking at DECnet, the original data link (DDCMP) has
support for both half and full duplex. Interestingly enough, multipoint involves one
station (the "master") polling the others ("tributaries") for traffic,
so it seems an obvious half duplex setup, but DDCMP 4.0 supports multipoint both on half
and full duplex lines.
Above the data link you'd typically see what looks like a full duplex system, but if
the data link is half duplex then at that layer the "both directions at the same
time" property disappears. Since it's packet switching with indeterminate timing
that is fine. If you want hard-synchronous transmisson it isn't, but that isn't
in the domain of IP.
The distinction doesn't just appear in modems: DDCMP over "integral modem"
coax links can be half duplex (one coax) or full duplex (two coax cables). Original
coaxial cable Ethernet is half duplex, while the introduction of twisted pair enabled full
duplex but at least in the lower speeds doesn't require it. The various token schemes
(802.5, 802.4, FDDI) are all inherently half duplex.
paul
...
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)