telecom@ucbvax.ARPA (06/05/85)
From: Jon Solomon (the Moderator) <Telecom-Request@BBNCCA>
TELECOM Digest Tue, 4 Jun 85 16:24:17 EDT Volume 4 : Issue 197
Today's Topics:
Racal-Vadic VA3451 Modem Problem
2400 bps modems can be non-standard
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Date: Fri, 31 May 85 13:39:20 pdt
From: ucdavis!bluebell!russell@Berkeley (Michael Russell)
To: CSNET-FORUM@CSNET-SH, INFO-CPM@AMSAA, INFO-MICRO@BRL-VGR, TELECOM@BBNCCA,
Subject: Racal-Vadic VA3451 Modem Problem
Cc: CIC@CSNET-SH, DDUSTMAN@CSNET-SH, LONG@CSNET-SH, MOOERS@CSNET-SH
In order to get connected to the CSNET PhoneNet, we bought a Racal-Vadic
VA3451 Auto-Dial modem. The modem was delivered in early 1985, and the
manual that came along with it was entitled "VA3451 Modem Installation/
Operation Manual", "Initial Issue", "November, 1984". There appears to
be an problem with the modem and an error in the manual.
After we put the modem into service, it occasionally got itself into a
funny state. The modem was off-hook even though there was no connection,
and the modem was generating a carrier. The lights that were lit were:
HS, DSR, and DTR. Further, disconnecting the modem from its DTE by
removing the RS232 plug (thereby dropping DTR) did not cause the modem to
go on-hook. Neither did unplugging the modem from the phone line. It
was possible to force the modem to go on-hook either by toggling power or
by flipping the DA/VO/MA switch on the front panel away from VO and then
back.
I telephoned the Racal-Vadic Service Hotline, and we tried a several
months worth of different things, but nothing would make the problem go
away.
On Wednesday, 5/29/85, Diane Dustman, CSNET-CIC Technical Staff, phoned
me to tell me that our modem was off-hook. She asked me to reset the
modem so that CSNET-RELAY could begin to call our host again. I told
her that I knew about the problem and that I was working on it. I asked
her if she often had to telephone site liaisons asking for a modem to be
reset. She said that she sometimes did, but that it didn't strike her
as a widespread problem. In any case, she mentioned my problem to Dan
Long, CSNET Technical Liaison, and it jogged his memory. He sent on to
me something he had received in early 1984.
It was an article, signed by "--Lauren--", that had been distributed to
the INFO-MICRO, INFO-CPM, UNIX-WIZARDS, and TELECOM interest groups.
Charles Lindahl at "ti-csl" (Texas Instruments Central Research Labs in
Dallas) had forwarded the article to Dan Long. Briefly, the article
described exactly the problem that we were experiencing with our modem.
The article said that the fix involved changing the strapping so that
the A1 switch was OFF. The setting of the A1 switch is ignored, and A1
is considered as ON, if the modem is set to its "Standard Configuration"
via the A6 switch. The article said that A6 should be set to ON in
order to turn off the "Standard Configuration", i.e., to enable all the
other switches.
I had set A6 to the non-"Standard Configuration" setting, but according
to my manual, that setting was achieved with A6 OFF, not ON as was
indicated in the article. The fact that the manual I have is wrong was
verified by a Racal-Vadic Field Service representative. I will forward
a copy of this to him, in order to be sure that the error in the manual
is corrected.
Unfortunately, I cannot tell you yet that, by setting A6 to ON and A1
to OFF, our problem has been solved. I had mailed the modem back to
Racal-Vadic for repair just before I got the information from Dan Long.
In any case, I would like to express my thanks to Diane, Dan, Charles,
and Lauren for getting this information to me.
Michael Russell
russell@ucd.csnet
...!ucbvax!ucdavis!bluebell!russell
ucdavis!bluebell!russell@berkeley.arpa
russell%bluebell%ucdavis.uucp@berkeley.arpa
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Date: Wednesday, 8 May 1985 16:25-MDT
From: "Robert P. Cunningham" <ihnp4!islenet!bob@Ucb-Vax>
Subject: 2400 bps modems can be non-standard
[These are my notes on some problems with U.S.-made 2400 bps modems,
using information from a variety of different articles and discussions
with various vendors. Clarifications and corrections welcome.]
2400 bps modems. Twice the throughput of 1200 bps modems for less
than twice the price. It sounds good, but many of the new 2400 bps
modems now on the market in the U.S. are not completely compatible
with similar models from other manufacturers.
There is no U.S. asynchronous dial-up 2400 bps standard in the same
sense that the Bell 212 modem set the standard for 1200 asynchronous
modems.
There are two European standards: "CCITT V.22 bis" and "CCIT V.26 ter".
These are written standards, while the Bell 212 was a complete working
product, with very well known operating characteristics. A minor
difference in principle, but a tremendous difference in practice.
Not only are there loopholes in the CCITT standards that give each
manufacturer considerable room to be creatively different, but there
are some modifications that U.S. manufactures tend to make in order to
maintain some compatiblity with existing U.S. equipment.
The result is that many of the 2400 async dial-up modems are
incompatible with each other in various ways.
Most of the new U.S. made 2400 bps async dial-up modems follow the
V.22 bis standard. They transmit and receive simultaneously by
splitting the available bandwidth in half, using half to receive and
the other to transmit, with a 16 point Quadrature Amplitude Modulation
(QAM) technique at 600 baud [precisely speaking, the baud rate is the
rate of change of the signal ... QAM and most other techniques provide
a way of encoding several bits into each change of the signal].
A few use an alternate CCITT specification -- V.26 ter. Signal
cancelling (the receiver cancelling out the echo of its own
transmitted signal) allows the whole bandwith of a phone line to be
used. V.26 ter uses a Differential Phase Shift Keying technique to
handle 2400 bps at 1200 baud.
V.22 2400 bps is probably less reliable than 212-type 1200 bps over
long-distance lines, V.26 ter is probably better than 212.
Of course V.22 bis and V.26 ter are completely incompatible.
While V.22 bis seems to be the preferred standard now, there is a good
chance it may eventually be superseeded by V.26 ter. [Then again,
maybe not; the Vadic 1200 bps technique is -- in some ways -- more
effective than 212, but it's never really caught on.]
Now, about those loopholes in V.22 bis ...
The standard designates a fall-back speed, if the originate and answer
modems can't handle a 2400 bps connection. However, the standard
doesn't specify how the connected DTE equipment (computer or terminal)
is to be notified of the fall-back. The RS232C standard doesn't cover
it.
With V.22 bis, each U.S. manufacturer seems to have chosen a DIFFERENT
way of indicating a speed change when the modem falls back, using
various of the seldom-used secondary control pins on the RS232C
connector.
Chances are that the typical DTE device you hook up your 2400 bps
modem to will ignore the speed change signal. Then, when you obtain a
dial-up connection that's a bit noisy, the modem falls back. It sets
up and maintains the connection nicely, but not at the baud rate your
computer or terminal expects. This can tie up the equipment at each
end indefinitely.
V.22 bis specifies the CCITT V.22 format for 1200 bps fallback.
Unfortunately, that's incompatible with Bell 212. To allow V.22 bis
modems to be used together with regular 212 modems, many (but not all)
of the U.S. manufacturers have chosen to make 212 rather than V.22
the fallback.
As a convenience, some (but, again not all) of the U.S. makers who
provide 212-type 1200 bps fallback also provide a further 103-type 300
bps fallback from 1200 bps. Nice feature, but definitely not in V.22
bis.
There's still another common "Americanization" that U.S. manufacturers
have adopted. V.22 bis assumes that the European standard 2,100 Hz
answer tone be sent by the answering modem during initial connection
handshaking. Many U.S. manufacturers have instead adopted the regular
U.S. 2,225 Hz answer tone -- again for 212-type compatibility.
Unfortunately, this means that many U.S.-made V.22 bis modems won't
handshake at all with a European V.22 bis modems.
V.22 bis specifies V.25 (or V.25 bis) autodialing. U.S. makers prefer
their own variation of the Hayes autodialing commands (or the Concord
technique, or the AT&T technique, or Cermatek ... there's definitely
no effective U.S. standard for autodialing commands).
Summary & recommendations:
If you want a 2400 modem that will talk to European-made modems make
sure it uses the 2.1 kHz answering tone, and has V.22 (not 212)
fallback. Find out whether the other end uses V.22 bis or V.26 ter.
If you need point-to-point 2400 bps dialup in the U.S., choose your
favorite manufacturer, but you'll have more consistent results of you
have the same model from the same company at the other end.
Otherwise, don't be surprised when your modem "hangs". In any case,
may expect to see more "phone line hits" -- especially over
long-distance lines -- than you get with your 1200 bps modem.
--
Bob Cunningham ..{dual,ihnp4,vortex}!islenet!bob
Honolulu, Hawaii
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End of TELECOM Digest
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