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- From: dcarr@gandalf.ca (Dave Carr)
- Subject: Re: Microcom B-Routers (TCP/IP)
- Message-ID: <1992Jul27.153253.2425@gandalf.ca>
- Organization: Gandalf Data Ltd.
- References: <1992Jul21.222325.27762@greco-prog.fr> <1992Jul23.124525.917@gandalf.ca> <1992Jul23.172932.5072@acc.com>
- Distribution: na
- Date: Mon, 27 Jul 1992 15:32:53 GMT
- Lines: 62
-
- In <1992Jul23.172932.5072@acc.com> art@acc.com (Art Berggreen) writes:
-
- >In article <1992Jul23.124525.917@gandalf.ca> dcarr@gandalf.ca (Dave Carr) writes:
- >>We make a COMPRESSION BRIDGE. We don't max out at 4:1. That's the
- >>typical performance however. Dual 64K links, and soon 256K or 384K.
- >>Currently requires our bridge at remote, but PPP interoperability
- >>in the works.
-
- >I just want to point out to folks not familiar with compression technology
- >that "typical" compression ratios are highly dependent on the data stream.
-
- No kidding. I wasn't trying to snow anyone. I was merely trying to point
- out that even at 12:1, we have the horsepower to keep the link full.
-
- In fact, some algorithms such as LZW are not linear. As the compression
- ratio increases, the CPU time per byte compressed decreases. The same
- is true for most LZ77 variants. Arithmetic encoding is fairly linear.
-
- Typical is what you get when you run in many customer sites. Not everyone
- spends all day sending binary data.
-
- >When we added compression to our bridge/router we tested several compression
- >algorithms and many different types of data.
-
- You make it sound that we did not. Our product has been in the field for
- over a year.
-
- >Data that is already
- >compressed often expands unless the algorithm can quickly switch into
- >a "transparent" mode and back.
-
- Or unless the algorithms expansion is so low that it doesn't make a
- difference. The 50% expansion of LZW is clearly not acceptable.
- However, there are LZW variants that expand at most by 6% and V.42 bis.
- We use neither however, and our worst case expansion is 1% (on 64
- byte packets), and .05% on 1500 bytes.
-
- >Binary computer data often only gets
- >about 2:1 compression. Ascii text can range from 2:1 to 4:1 or better
- >depending on the nature of the text. Contrived sample data that can
- >really exploit the compression algorithm can give 25:1, 50:1, or better
- >(depending on several factors). I don't want to say that 4:1 compression
- >won't be typical for some networks, just don't expect it until you try
- >it on your net or have some idea about the compressibility of your network
- >data.
-
- Exactly. The marketting types really have a hard time putting the compression
- specs on paper. You need a "Calgary Corpus" set of benchmarks, or everyone
- will be quoting there own favourite numbers. Note, the Calgary Corpus is a
- set of benchmarking files for measuring archivers such as ZIP.
-
- >Also, interoperability requires compatible algorithms at both ends.
- >Due to the rapid evolution of compression technology and the state of
- >standarization, it might be a while before you can expect general
- >interoperability.
-
- PPP with compression should be available shortly after INTEROP. We just
- need to standardize on the compression algorithm.
- --
- Dave Carr | dcarr@gandalf.ca | It's what you learn,
- Principal Designer | TEL (613) 723-6500 | after you know it all,
- Gandalf Data Limited | FAX (613) 226-1717 | that counts.
-