Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!oliveb!amdahl!netcom!gandrews
From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.dcom.modems
Subject: Re: What all does S120=16 do?
Summary: It turns off the fiberoptic mods.
Keywords: Trailblazer, PEP, S120
Message-ID: <28581@netcom.COM>
Date: 17 Mar 91 01:30:09 GMT
References: <356@alderan.uucp> <9908@discus.technion.ac.il> <1991Mar16.160708.152892@ramona.Cary.NC.US>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 46

In article <1991Mar16.160708.152892@ramona.Cary.NC.US> andrew@ramona.Cary.NC.US (Andrew Ernest) writes:
>In article <9908@discus.technion.ac.il> devil@techunix.BITNET (Gil Tene) writes:
>>To detail : S120=12 means no use only LONG packets, no micro
>
>What are the side-effects of S120=16?  This is the value Telebit
>says turns off the short burst of V.25 tone at the beginning of
>each PEP answer tone.  This tone, which appeared in the BC5.10A
>firmware upgrade for my TB+, caused my neighbors with older firmware
>to no longer be able to connect to my modem.  S120=16 solved the
>problem but I am now curious if that register value (16) has any other
>effect I should know about (like packet size, etc).
>

It turns off all of the mods put into the PEP modulation to combat trouble
with echo cancellation devices on fiberoptic long distance carriers.
One of the changes was a 3 second V.25 answer tone when the modem first
answers the phone, even if PEP tones are first.  Previous versions of
firmware didn't put out the V.25 answer tone before the PEP tones.

>
>Can anyone guess why the V.25 tone was added?  Any reason why turning it
>off is a bad thing?  (Not that I have any intention of turning it back
>on...compatibility with my PEP neighbors is of the most importance to me.)
>

It was added to better conform to the V.25 recommendation, which says
an automatically answering modem transmits a 2100 Hz tone when it goes
off hook.  Part of the reason for the V.25 answer tone is linked to the
reason PEP was modified for the fiberoptic carriers - the tone helps to
disable the telco's echo cancelling and/or echo suppressing equipment.

PEP was having trouble over those types of lines, so it was modified to
compensate.  The V.25 answer tone was only part of those mods.  Setting
S120=16 disables ALL the mods - not just the V.25 answer tone.

The V.25 answer tone probably isn't the problem.  It's more likely to
be due to how the mods work when you have PEP tones last.  If PEP tones
are first, only the oldest TrailBlazers have trouble with the V.25 answer
tone before PEP, and many of those can set S50=255 to overcome it.  When
PEP tones are last, it's a different story.


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| Greg Andrews      |   gandrews@netcom.COM |
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