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Date:	Sun, 16 Sep 2001 21:25:20 PDT
From:	Peter Skye <pskye@peterskye.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: Cable modem data transfer info

Content Type: text/plain

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Ray Davison wrote:
>
> How do [the ipspeed] numbers relate to advertised
> transfer rates which are generally expressed in
> tens or hundreds of Kilo something or other.

The IPSpeed display is in "net characters per second", which is pretty
darn hard to correlate to your cable modem ISP's advertised transfer
rate which is probably in "gross bits per second".

-- 1. There's "space" between packets, and during that time you don't
get any data. The advertised transfer rate assumes there's no space.
Thus, the smaller the packets, the more "space" you'll have each second
during which there's no data.

-- 2. There are headers on the packets which use up a noticeable amount
of the advertised transfer rate. As far as I know IPSpeed sees the data
after the headers are removed, so IPSpeed's count doesn't include all
the bits in the headers.

I've occasionally gotten an 80K cps reading from IPSpeed on an ftp
connection with my 768Kbps (bits per second) DSL line, which means it
was running full tilt boogie (80K chars/sec * 8 bits/char => 640K
chars/sec). Usually, however, I see readings of about 1/4 of that --
around 20K cps or so, even with several connections downloading
simultaneously. Downloads from IBM and Hobbes are usually pretty speedy
(at least here), so if you want to test the speed go download Java from
IBM or some big file from Hobbes.

Cable modems are (as far as I know) plain old Ethernet connected to
every house on your cable loop. If everybody in the neighborhood is
downloading simultaneously, the "neighborhood" is getting the full
advertised bandwidth but there's less bandwidth available for you
because you're sharing with the neighbors.

Also remember that if you see a speed constriction it's not necessarily
at your end -- the other guy's server might be heavily loaded, or there
might be congestion on the 'net.

- Peter

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