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From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.dcom.modems
Subject: PEP "baud rate changes" (was Re: PEP turnaround time)
Summary: No such thing without a retrain.
Message-ID: <1991May25.055346.11454@netcom.COM>
Date: 25 May 91 05:53:46 GMT
References: <9105210700.AA14518@ucbvax.Berkeley.EDU> <1991May24.060836.7247@netcom.COM> <106126@sgi.sgi.com>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 43

In article <106126@sgi.sgi.com> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
>In article <1991May24.060836.7247@netcom.COM>, gandrews@netcom.COM (Greg Andrews) writes:
>> 
>> Yes, an uncompressed implementation of SLIP will make PEP thrash between
>> different sized packets.  It's not a "baud" rate switch at all.  It's a
>> simple matter of packet size.
>
>I think it really is a "baud change."  I understand from Telebit that while
>running micropackets they use something like 18.3 baud, but doing big
>packets they use about 7 baud.  That is, the signal transmitted by the
>modem changes either 7+ or 18+ times/second.  Of course, the instantaneous
>bit rate at the DTE-DCE interface never changes.
>(Yes, the old lecture that "buad" != "bps")
>

Take it from Telebit support technician (me).  This is not, repeat NOT a
"baud rate" change in any sense of the term.  The modems do not alter the
baud rate, nor the modulation, on any of the carriers without retraining.
Since the symbol rate and carrier structure don't change without a retrain,
neither does the raw bit rate between the modems (as reported in the S70 and
S72 registers).

What you are hearing is primarily the different packet lengths.  You are also
hearing a difference in the number of carriers used for transmitting.  Out of
the total number of carriers available to the modem, long packets will use
all of them, short packets use half of them, and micro packets use around
1/8th of them.  The characteristics of the "hash" will change between the
different types of packets because of the different combinations of carriers
used.

Once again, this is NOT a baud rate change.  No carriers send symbols faster
or slower than they did previously.  The total *number* of symbols sent
simultaneously will change as the number of carriers changes, but all of the
carriers still send symbols at the same rate.  


>Vernon Schryver,   vjs@sgi.com

-- 
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 |  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
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