Path: utzoo!utgpu!watmath!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.dcom.modems
Subject: Re: Courier HST/IX
Keywords: 30,000 bps, USR
Message-ID: <16744@onfcanim.UUCP>
Date: 18 Nov 88 18:40:08 GMT
References: <1414@nmtsun.nmt.edu>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 30

In article <1414@nmtsun.nmt.edu> dansnsr@nmtsun.nmt.edu (Daniel A. Church) writes:
>
>"Data compression and error control (MNP class 5 and software-based 
>protocols) deliver up to 30,000 bps error-free performance, so the Courier
>HST/IX quicjly pays for itself in reduced phone charges.

This is a pretty meaningless figure, since they don't quote the
compression ratio.  The real measure of how fast the modem is is the
throughput on truly random data, where compression does no good.  Since
most news feeds have already been compressed on the host, and the
resulting byte stream is a pretty good approximation to random data,
this determines how much you can actually push through the modem.

For example, USENET news articles get about a 2:1 compression ratio,
formatted text does somewhat better because of all the white space.  If
USR is quoting their figure based on something that compresses 2:1,
then the real modem bandwidth is 1500 cps, only slightly faster than a
Trailblazer.  But I can get a 4.8:1 compression ratio on uucico
LOGFILEs; if USR used something like this as their test case, then the
real throughput of the modem is only 625 cps.  And if you allow a
continuous stream of blanks to be the test case, then any modem with a
decent compression algorithm will give you a "throughput" that is
limited by its maximum RS232 bit rate.  (Like 30 kbps through a 38.4
kbps serial port).

For the Trailblazer, the real throughput figure is 1400 8-bit
characters per second of useful data, or 14000 bps if you count the
start and stop bits.  It has good built-in compression, that will allow
it to send most data at a speed limited by its maximum 19.2 Kbps serial
rate, but that doesn't matter for sending news.