Newsgroups: comp.dcom.modems
Path: utzoo!utgpu!cunews!hobbit.gandalf.ca!dcarr
From: dcarr@hobbit.gandalf.ca (Dave Carr)
Subject: Re: Uses of V.42 (bis?) data compression
Message-ID: <1991Apr23.133251.6947@hobbit.gandalf.ca>
Organization: Gandalf Data Ltd.
References: <10334@pitt.UUCP> <3908.280363d4@hayes.uucp> <1991Apr12.132116.11546@hobbit.gandalf.ca> <63840@bbn.BBN.COM>
Distribution: na
Date: Tue, 23 Apr 1991 13:32:51 GMT
Lines: 39

In <63840@bbn.BBN.COM> pplacewa@bbn.com (Paul Placeway) writes:

My original comment was:
	I agree.  So why do modems and PC get it together and get a decent
	interface worked out.  Why not Ethernet ?

>Probably because compression is not the job of the network.  Ethernet
>itself is PACKET based.  Even if you had the hardware to do the
>compression in real time, compressing 1K packets might not buy much.

Oh, I (and Telebit, Microcom,..) would say the opposite.  
It is the job of the network to appear
seamless.  Do you want the application layer to start saying "oh you want
to go to that destination, then you must compress the data first !".
Hardly.  Compression after all just saves money.  You could always buy
a more expensive link.

On a single 1K byte packet, you should get 2:1 assuming the tables are reset
between packets.  The learning time of LZW (for example Storers AP
variant) is extremely fast.
If you are working on a single connection (say a FTP
transfer) the compression approaches the compression figures of compressing
before transmission.  Never as good as precompressing, but transparent
to the application.

>Compressing E-net speed would require special hardware anyway.  You
>can spend your extra $$$ on getting ~30 Mbit/sec out of Ethernet, I'd
>rather spend my $$$ on a twisted-pair FDDI-oid interface at 100 Mbits/sec.

I wasn't talking about compressing the Ethernet.  Most PC's already have an
Ethernet connection.  What I meant was put an Ethernet connection on the 
modem to eliminate the 38.4Kbps restriction.  The modem would compress
over the Wide Area Link, an talk to the PC using IP over the Ethernet.

And for the record, compressing at E-net speeds wouldn't require special
hardware.  Check out Ross Williams paper "The World's Fastest Adaptive
Text Compression Algorithm".  It can compress 250 Kbytes/sec on a 
1 MIPS machine.  Take an 80960CA processor (or your average 680x0)
and you can get E-net speeds (1.25 MBytes/sec).