Re: Compression/Tokens/Big Files/Small Files/and rubber ducks

Brian Behlendorf ([email protected])
Wed, 31 May 1995 20:40:06 -0700 (PDT)


On Wed, 31 May 1995, Neophytos Iacovou wrote:
> 3) Both compression and tokenization of VRML came about because people
> don't want to see 1meg VRML files fly across the net, the people on
> the end of a SLIP link are going to suffer. So lots and lots of charts
> have been made that show how well VRML might compress under various
> criteria. It wasn't until tonight that something struck me as odd
> about those charts. Here are some questions in this regard:
>
> a) a 1 meg VRML file is roughly the same as the average HTML page,
> about 3 gifs is the average on an HTML page, and equal about 1 meg.
> How many people suffer through HTML pages over SLIP? lots - with
> inline images on?

No, sorry, a "page" (HTML + images) is considered extremely heavy if it
has more than about 75K in it. 14.4Kbits/sec = 1.8kbytes/sec, which over
slip probably results in 1.5K/sec at best. 1 meg takes 11 minutes at
this rate.

> b) do most SLIP users turn on compression, ALWAYS, NEVER or AUTOMATIC?

Compression isn't that widespread yet, and the most that has ever claimed
to accomplish is a 3x speedup.

> c) if a user's modem is already compressing data, and you send a
> precompressed VRML file to him, and then he needs to uncompress it
> by his parser, does that take longer or less time than if the user
> was using modem compression, got a uncompressed VRML file, and did
> not have to ask his parser to uncompress it? How much time is wasted
> by the modem trying to figure out that it can't compress the
> compressed file? Is that a factor?

Not all compressions are created equal :) For most applications I don't
think this is significant compared to the baud rate - as long as the
application could decompress more than 1.8K of compressed text per
second it shouldn't matter.

Sending compressed VRML over a modem using compression *may* result in
a performance decrease, but that's more a quality of implementation issue
for the modem - this is true for other compressed media as well (mpeg,
jpeg's, etc).

Brian

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