Path: utzoo!mnetor!uunet!vixie!paul
From: paul@vixie.UUCP (Paul Vixie Esq)
Newsgroups: comp.dcom.modems
Subject: Re: hayes 9600 vs. trailblazer
Message-ID: <863@vixie.UUCP>
Date: 14 Apr 88 06:19:41 GMT
References: <8804110136.AA16978@ucbvax.Berkeley.EDU> <15612@onfcanim.UUCP> <494@edsews.EDS.COM>
Reply-To: paul@vixie.UUCP (Paul Vixie Esq)
Organization: Vixie Enterprises, San Francisco
Lines: 35

In article <494@edsews.EDS.COM> roberts@edsews.EDS.COM (Ted Roberts) writes:
>I realize that news is normally compressed by the system before transmission
>partly due to the modems that don't do data compression in the hardware.
>However, I'm curious if in a this is actually the best method for the
>Trailblazer.

It depends.  On a Vax 750 with DZ-11's, an interrupt is generated for almost
every incoming character ("almost" because BSD4.3's driver, at least, can go
into a polling mode of sorts when it detects high-speed input).  On the 750,
I get 6000 baud maximum on incoming, and the Pyramid 98x on the other end is
twiddling its metaphorical thumbs waiting for a clear output buffer.  *Many*
machines and/or serial ports and/or device drivers fail in this way --- they
were optimized for high-speed OUTPUT, and the designers just didn't have the
Trailblazer in mind when they were thinking about input.

In such a situation (i.e., PEP isn't saturated), you want to cut down on the
amount of data passing between the CPU and the modem.  Compressing it is one
very good way to do this.

In the ideal configuration, the receiving host would eat data up as fast  as
it  came in, and the sending host would send so fast that the modem would be
the slowest part of the link.  In *that* sitation, it would  make  excellent
sense to have the modem do the compression -- it's like an extremely conven-
ient  (transparent,  even) data-compression co-processor.  Most hosts around
these days just can't get the data in (or out) of  the  "co-processor"  fast
enough for it to be useful.

I wonder if a host CPU quick enough to saturate a PEP channel would automat-
ically be exempt from even _needing_ help  from  a  modem-based  compression
scheme?  :-), sort of.
-- 
Paul A Vixie Esq
paul%vixie@uunet.uu.net
{uunet,ptsfa,hoptoad}!vixie!paul
San Francisco, (415) 647-7023