NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/9185

Internet Message Format | 1996-08-05 | 3.2 KB

Path: inforamp.net!ts31-16
From: [e-mail] (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: Re: HS/Link and Thoroput speeds
Date: 26 Mar 1996 03:21:33 GMT
Organization: InfoRamp Inc., Toronto, Ontario [phone]
Message-ID: <4j7nrt$[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: ts31-16.tor.inforamp.net
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In article <[e-mail]>,
   "ENRIQUE S. IGNARRA" <[e-mail]> wrote:
>2)  Does anyone know where I could get a list of what my thoroughput
>should be for various speeds?
>i.e. At 14.4 with compression to 57.6 text should run at 3400
>ZIP at 1600, database at 4600, and Graphics at 2900.
>Anyone have a list for other speeds? i.e. 9600 19.2 24.0k etc?

It's easy enough to calculate in two steps: first, the raw bit rate and then
the data throughput.

The maximum raw CPS rate of an asynchronous modem with no error correction
protocols (like we had in the days of 300, 1200, and 2400 bps modems) is one
tenth the bit rate (every data byte was preceded by a start bit and followed
by a stop bit for ten bits per byte, unless you went out of your way to choose
unusual settings).  With error correction, the maximum raw CPS rate is reduced
because start and stop bits are no longer necessary, but the error correction
protocol imposes a little overhead itself; the ballpark figure is that the
maximum raw CPS rate will be about the bit rate divided by 8.4.  That's about
what you'll get for uncompressible data; the maximum raw throughput for
compressible data is the serial port speed over ten (because, at the serial
port level, you're still sending start and stopbits to and from the modem).

OK, so why don't you get 1714 CPS when transferring .ZIP files at 14.4?
Because the file transfer protocol itself imposes some overhead.  In the case
of streaming protocols like plain ZMODEM, that overhead is about five per
cent (with MobyTurbo, it's less than half that).  With nonstreaming protocols
like XMODEM, the overhead grows as the line speed increases and as the
response delay of the two computers increases.  Of course, whether a
particular text file's throughput approaches 95% of your serial port speed or
not depends not only on the data itself, but the details of the implementation
of the data compression in both modems, such as the dictionary size and string
search algorithm.  For every pair of modems (models or even revisions), a
given file may transfer at a different speed.

You now know how to estimate expected figures for compressed files for _any_
speed of modem, and hopefully you realize how difficult it is to give figures
for compressible files.

--
            Geoffrey Welsh, Developer, InSystems Technologies Inc.
        Temporary: [e-mail]; At work: [e-mail]
  At home: [e-mail] or [xenitec.on.ca|m2xenix.psg.com]!zswamp!geoff
  Capitalism is a cold-hearted system which guards the interests of whoever's
  at the top, yet hypocritically claims that it offers everyone a fair shot.
              So is every other system ever put in place by man.