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From: lstowell@pyrnova.pyramid.com (Lon Stowell)
Newsgroups: comp.dcom.modems
Subject: Re: V.32 vs. Telebit: Which should I buy?
Summary: V.32 Mythology
Message-ID: <156459@pyramid.pyramid.com>
Date: 23 May 91 18:14:58 GMT
Sender: daemon@pyramid.pyramid.com
Reply-To: lstowell@pyrnova.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 50

In article <48@mich-ns.Michigan.COM> tech@mich-ns.Michigan.COM (Mich. Network Sys. TECH SUPPORT) writes:
>"
>Thats whats nice about PEP. With V.32, if the line gets noisy, its drops
>back to half its normal speed (usually 4800bps). If the line gets even
>worse, it will drop the connection altogether. Even if the connection 
>doesn't drop, V.32 cannot re-train back up to a higher speed. 
>
>PEP can drop backwards in increments of as little as 100 baud AND can 
>speed up again, if line conditions get better. 
>
        This is a FQM...a Frequently Quoted Myth.   There is
	nothing in V.32 or V.32bis which precludes shifting from
	a lower to a higher speed.  The fact that few modem
	vendors have IMPLEMENTED the capability has nothing to
	do with the fact that a V.32 modem can request a
	retrain...and in that retrain, specify the baud
	rate...higher or lower.   

	If ONE of the modems can do a fall-forward..and asks for
	a retrain to 9600, a compliant modem will shift....although 
	it might request a drop back to 4800 if the "probability
	of error" is too high for 9600 or the train fails.  

	V.32 notes only the training and retraining
	sequences...which include the Rate Signal as part.  it
	does not specify either fallback or fallforward
	explicity....only that "a retrain may be initiated
	during data transmission if either modem incorporates a
	means of detecting unsatisfactory signal reception.
	Figure 5a/V.32 shows a retrain event initiated by the
	calling modem and Figure 5B/V.32 shows a retrain event
	initiated by the answering modem.....   (5.5 Retrain
	Procedure.)

	Some vendors have interpreted this to mean that
	excessive errors should cause a fallback...so they
	retrain and request a lower rate.   There is nothing to
	preclude interpretation that the error rate is to LOW
	and a higher rate would be possible.  A retrain with a
	baud rate is a retrain...period.  


	The V.32 training sequence is half duplex..and
	interrupts the line for several seconds.  Many vendors
	will stay at the lower rate once fallback has been done
	UNLESS the DTE requests the higher rate via Ckt 111.
	Others will fall forward.  

	Admittedly, gearshift modems are rare in V.32, but all
	the V.32bis modem's I've seen offer it standard.