Date   : Sat, 14 Jun 1986 19:09:33 GMT
From   : brian@AMSAA.ARPA
Subject: 2400 bps modem incompatabilities

Recently whilst trying to select 2400 baud modems for our dial-in lines 
on campus, I ran into an interesting problem. Some 2400 bps modems 
wouldn't connect to others at 2400 bps, but would connect by falling 
back to 1200.

After casting about with a certain degree of futility trying to find 
someone who could tell me WHY this was, I finally got ahold of an 
engineer at Anchor Automation who explained it to me. (By the way, I'm 
completely impressed with these people and their willingness to solve 
problems!)

[Brief flame to other modem companies: Yes, damnit, I know your modems
talk to each other. My problem is that I've got one hundred and seven
dial-in lines (at last count) and I have no control over what the
people out there are going to dial in with. Telling me to have them
all buy your modem to solve the problem is not the answer.]

So here's how I understand it (imperfect at best, but I couldn't find 
this information written anywhere, so here goes):

When you call a 1200/2400 bps modem, it answers in either of two ways. 
If it is a CCITT-V.22bis compliant modem, it answers with 3.3 seconds 
of 2100Hz tone, then 75 mS of silence, and then a burst of training 
signals to get the other modem to adjust to the line.

If the answering modem receives training signals in response, it 
assumes that it will be talking 2400 bps using 16-QAM and you have a 
2400 bps connection. If instead of the training signals, it receives 4-
PSK from the originating modem in response to its answering tones, it 
assumes that it will be a 1200 bps connection and switches off the 
training and (in the USA) uses 4-PSK (Bell 212 standard). [European 
modems use CCITT-V.22 (not V.22bis) for 1200 bps. Some modems claim to 
handle this as well as 212 for 1200 bps, but I've not been testing 
that!]

Here's the rub: some of the 1200/2400 modems don't answer using the 
CCITT V.22bis handshake. They instead answer with a different handshake 
(the engineer referred to it as the Bell 2400 bps handshake): After 
going off-hook, the "Bell handshake" answering modem sends 2125Hz 
(which is pretty close to the CCITT 2100Hz tone) and waits for the 
originating modem to respond either with 1200bps 4-PSK or with QAM 
training signals. It then switches to the appropriate mode, and either 
sends some training signals for 2400, or 4-PSK for 1200.

Some modems can handle both kinds of handshakes. I have, for example, 
no problem calling a Courier with a Courier, anything with a Racal-
Vadic or a MultiTech, etc. But my Courier can't call my Case-Rixon; an 
Anchor didn't connect to the Courier, etc. No, I don't have a chart of 
what talks to what, for reasons that I'll explain:

The point here is that I'm not interested really in what talks to what 
on a brand-name basis. I want instead to find some brands that "do the 
right thing" for both handshakes, and recommend those. So far I've 
found a couple (Racal-Vadic and Multitech come to mind) and I'll choose 
among those and others based on other factors, such as interface, 
reliability, mounting, etc. for the list of ones I recommend. (And then 
the purchasing people and telephone people get into the act, so who 
knows what we'll buy - or when....)

But the other manufacturers need to get on the stick and get it right. 
It seems to me that stating that a modem is CCITT-V.22bis compliant 
also means that it does the CCITT handshake, and yet I can call several 
of the modems out there and just by listening (no 75mS interruption, 
guys!) tell that they're using the "bell handshake".

And the ones that I've tested that answer with the "bell handshake" 
don't seem to accept the CCITT handshake when you dial out. Moral: It 
may be 2400 bps but they're not all compatable. C'mon, guys!

Modem manufacturers!  Heed my words!  Since there is a published
international standard and one other non-compliant "standard", you need
to accept both!  And you should probably default to the international
standard when you answer.

Listening test: I call the modem and listen. After it goes off hook, I 
hear in my telephone either

 1) a steady high-pitched tone, then a very brief interruption,
 then another slightly-different-pitch tone, then it
 disconnects.
-or-
 2) a steady high-pitched tone that lasts until it disconnects.

I surmise that #1 is the CCITT handshake. I just tried this with a 
Case-Rixon 1224 and a USR Courier. The Case-Rixon did #1, the Courier 
did #2. If I call both on a conference call, I hear what could be a 
25Hz beat note between them during the first tone on answer.

So, I'm not disparaging anybody's modems.  But I wish there was more 
standardisation so that I don't have to buy one of each to test them 
before we make some big mistake....

 Brian Kantor

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