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From: jeff@markets.amix.com (Jeff Crilly N6ZFX)
Newsgroups: comp.dcom.modems
Subject: Bell vs CCITT answer tone
Message-ID: <1991May29.223758.5121@markets.amix.com>
Date: 29 May 91 22:37:58 GMT
Reply-To: jeff@markets.amix.com (Jeff Crilly N6ZFX)
Distribution: usa
Organization: AMIX Corporation, Palo Alto, CA
Lines: 53

I've been having some problems with a 9600bps V.32 modem (lets call it 
brand-x, I don't want to disclose the mfr) when using it to connect to 
2400 bps modems (NEC and Avatex).  

When I use brand-x to connect to these modems I always get a connection
at 1200 bps.  I also tried turning off the auto-fallback feature in
brand-x and the modem would hang up as soon as it hears answer tone.

I have several other modems (Hayes personal modem and practical 
peripherals, to name two) that can connect fine to the NEC and avatex at
2400 bps.  Also, brand-x can connect to these modems at 2400 bps.

It turns out that the default answer tone for the NEC and Avatex is
Bell 212A.  I think the default for the hayes personal and practical
peripherals is CCITT V.22.  But I don't have the manuals so I can't
verify it.

So I changed the answer tone (also called protocol in the manual) on the
NEC and Avatex to V.22 (I think this is really V.22 + V.25 answer tone) 
and now brand-x can connect fine.  And so can the other modems.
Incidently, on the NEC I can select Bell212, CCITT V.22, or CCITT V.22+
V.25.  Only, CCITT V.22+V.25 works.

The question is: what is correct?  Should the modems be set to answer
with Bell 212A answer tone or CCITT V.22+V.25.  Here my understanding of
the standards, correct me if I'm wrong:

	1) Bell 212A -standard for 1200 bps modems.  Also includes a standard
		for answering the line (answering modem generates a 
		2225Hz tone for x ms.)

	2) V.22 -standard for 1200bps modems.  Includes answering with
		a some tone (which I'm not sure of.

	3) V.25 -answer tone of 2100Hz which can be used with V.22 (presumably
		to cancel echo supression equipment in the GSTN).

	4) V.22bis -standard for 2400bps modems that is based on V.22.
		(A V.22 connection is made first, then 2400bps is attempted.)

Is it correct that in the U.S., modems should use the Bell212 standard for
answering the line?  The modem should not determine the data rate from the
answer tone, correct?

Also, is the use of 212 answer tone for V.22bis connections incorrect (i.e. 
non-standard)?  It seems to work with some modems, but at least one modems 
(brand-x) and possibly others can't handle this.

Thanks for any info.

Jeff Crilly (N6ZFX)
AMIX Corporation  2345 Yale Street  Palo Alto, CA  94306
jeff@markets.amix.com, {uunet,sun}!markets!jeff, N6ZFX@N6IIU.#NOCAL.CA.USA