Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!accuvax.nwu.edu!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.dcom.modems
Subject: Re: Why are dial-up modems improving faster than direct-wire ones?
Message-ID: <19363@mimsy.UUCP>
Date: 1 Sep 89 07:42:37 GMT
References: <3971@phri.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 29

In article <3971@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>... 30 times the analog bandwidth of a dialup connection, yet I can
>only coax about 6 times the bit rate through it. ... I've seen a modem
>which claims to be able to do 230 kbps over our line, but it doesn't
>work.  On the other hand, I've heard of V.32 modems which can do 38.4
>kbps over dialup lines.

They do not work (that way) either.  V.32 modems get 9600 bps in each
direction, no more.  (Sometimes less, when they fall back to 4800 bps,
etc.)  Modems that claim higher rates really shove fancy compression
protocols on the front.  ASCII text can often be compressed to 30% of
its original size, so the apparent bit rate can become 9600/.3=32000
bps (not 38400 bps: that speed is merely required by the doubling rule:
either you run at 19200 bps or you run at 38400 bps, so if you want to
get 32000 bps, you had better run at 38400 bps.)  Trivial text can be
compressed even more, but trivial text does not occur often.

Anyway:

>	So, my question is why do modern dialup modems get so much better
>performance (compared to the channel they have to work with) than leased
>line modems?

Mainly marketing.  It is easier to do fancy compression and DSP at low
rates like 38400 b/s than at 100000 b/s, but the biggest reason is that
there are more telephones out there than leased lines.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris