NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/4813

Text File | 1996-08-05 | 3.5 KB | 68 lines

Newsgroups: comp.dcom.modems
Path: onramp.ca!geac!itcyyz!xrtll!bokonon!stephen
From: [e-mail] (Stephen M. Dunn)
Subject: Re: To V42Bis or not to V42Bis??
Organization: United System Solutions Inc.
Date: Sat, 10 Feb 1996 00:38:28 GMT
Message-ID: <[e-mail]>
References: <4etjse$[e-mail]> <[e-mail]>

In article <[e-mail]> [e-mail] (Joseph Waldvogel) writes:
$Unlike MNP-5, V.42BIS is quite SMART!  You can always just leave it on, I'll
$only commpress files that arn't already commpressed.  If there commpressed,
$V.42bis is Smart enough to leave them as they are.

   Technically, a V.42bis compressor does _not_ have to be this smart.
However, most (if not all) modem manufacturers did build this logic
into their V.42bis compressors, so in real life, yes, V.42bis modems
will realize when they're transmitting uncompressible data.  However,
a V.42bis compressor can, if it wishes, continue sending the output of
its compression routine, even if that output is larger than the
input, and still comply with V.42bis, because V.42bis defines a way
of signalling when the data is compressed or uncompressed but does not
force the compressor to use this (the decompressor, however, must
be able to handle it).

   V.42bis also doesn't specify how the compressor should determine
when to send uncompressed data; that, too, is left up to the
implementor.

$                                                    MNP-5  Data commpression
$is pretty stuppied Your better off to leave THAT off!

   It depends on what you're doing.  If you're transferring data which
is not compressible by MNP5, then yes, you should leave MNP5 disabled.
If you're transferring text files, then MNP5 is a big win.

   This is becoming a less important point all the time, fortunately.
Over the last several years, most modems sold with MNP5 have also had
V.42bis, I'm sure; I don't recall the last time I saw a new MNP5 modem
which did _not_ support V.42bis.  Yes, there are still some legacy
modems out there which do MNP5 but not V.42bis, but I'd imagine that
as a percentage of all modems in use, that number is relatively
small and shrinking.

$                                  Leaving V.42bis OFF will NOT speed up
$Downloading of Commpressed files,

   Actually, it should, by a factor of about 1/256 (or roughly 0.4%).
Imagine you're a modem with V.42bis, and you're sending some data
which you can't compress.  At some point, the data becomes compressible
again.  You need a way of signalling to the modem at the other end
that you're starting to compress the data again.  This is done by
inserting a token into the data stream; for the sake of argument,
let's say it's the ASCII code for the letter a.  If the letter a
happens to appear in the uncompressible data stream, you need
some way of indicating to the other side that it's just a letter
a, and not a signal that you're compressing the data again.  Therefore,
this character must be escaped.  On average, in uncompressible 8-bit
data, the letter a should appear once out of every 256 characters,
and so for every 256 characters of data, you will need to transmit
an average of 257 characters (255 characters, plus the one letter
a and one escape character).
--
[e-mail]               ...!{xrtll,gts.org}!bokonon!stephen
----------------------------------------------------------------------------
Stephen M. Dunn, CNE, ACE, Sr. Systems Analyst, United System Solutions Inc.
104 Carnforth Road, Toronto, ON, Canada M4A 2K7          [phone] x251