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From: David Tamkin <0004261818@mcimail.com>
Newsgroups: comp.dcom.telecom
Subject: Re: Data Access Lines
Message-ID: <8450@accuvax.nwu.edu>
Date: 30 May 90 21:35:00 GMT
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X-Telecom-Digest: Volume 10, Issue 399, Message 2 of 9


Mike Riddle wrote in volume 10, issue 391:

|Jeremy [Grodberg] wants to use a Telebit 9600 bps modem, and his version
|of Ma Bell said that only < 2400 {actually, <= 2400 --DWT} was guaranteed
|on a voice line.

|My understanding is that a 9600 bps modem actually operates at 2400 baud
|with four levels, creating a 9600 bps signal.  This method was used
|precisely because of the inherent bandwidth of a "normal" voice line.  It
|seems to me that whoever told him 9600 wouldn't work on a "normal" line
|either didn't understand 9600 bps methodology or was trying to sell up.

John Higdon commented in volume 10, issue 394:

:1200 and 2400 bps modems don't operate at 1200 and 2400 baud
:respectively, but rather at a slower baud rate and carry four or eight
:bits per baud.  This is accomplished by introducing a phase (and in the
:case of 2400, amplitude) component.

1200 bps and 2400 bps modems operate at 600 baud with two or four bits
of information in every baud.

In volume 10, issue 395, Rob Warnock quoted an official description of
PEP and observed:

+So the rate never exceeds 88.26 baud. Your local telco ought to be able
+to do *that* at least.

And I think that's the problem: Jeremy's telco promises that ordinary
lines will support 600 baud (regardless of bps counts attained through
artifice or cunning) but not the 2400 baud possibly required for 9600
bps.  {I won't venture a guess whether he needs 2400 baud modulation
for 9600 bps as Mike said or only 88.26 baud as Rob quoted.} The reps
are told that voice lines can handle 2400 bps (the presumed speed
limit for 600 baud) but reliability at [the higher baud rates possibly
needed for] higher data rates requires premium service.

If PEP is modulated only at 7.35 or 88.26 baud, it should be no
difficulty for the local lines to carry it, unless shoving so many
bits into so few bauds requires so many carrier pitches that local
telco lines might not be reliably able to discriminate that fine.


David Tamkin  P. O. Box 7002  Des Plaines IL  60018-7002  +1 708 518 6769
MCI Mail: 426-1818   CIS: 73720,1570   GEnie: D.W.TAMKIN  +1 312 693 0591