Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!batcomputer!cornell!uw-beaver!mit-eddie!bbn.com From: pplacewa@bbn.com (Paul Placeway) Newsgroups: comp.dcom.modems Subject: Re: Uses of V.42 (bis?) data compression Message-ID: <63840@bbn.BBN.COM> Date: 22 Apr 91 17:57:21 GMT References: <10334@pitt.UUCP> <3908.280363d4@hayes.uucp> <1991Apr12.132116.11546@hobbit.gandalf.ca> Sender: news@bbn.com Distribution: na Lines: 40 dcarr@hobbit.gandalf.ca (Dave Carr) writes: < In <3908.280363d4@hayes.uucp> tnixon@hayes.uucp writes: < >I always advise people to compress before sending if possible. An < >offline compression program can make multiple passes over the data < >and really optimize the compression, while a modem only has one shot < >at it. < CAN make multiple passes over the data. In reality, most commercial < or shareware compression programs make only a single pass. I was < suprised to find out my favorite (PKZIP) does only one pass. < LZW based algorithm are usually one-pass. LHarc is one-pass. < Unix Compress is one-pass. Unix compress(1) is (sorta) two-pass: if the resulting compressed file would be bigger than the original, it doesn't compress it. Of course this is only true of comressing a *file*, compressing pipe input is, of course, one pass. Several of the Macintosh compressor/archiver programs are multi-pass. StuffIt is a notable example -- StuffIt 1.5.1 tries nothing, Huffman and LZW and picks whichever is smallest. It almost allways uses LZW, though, since (order-0) Huffman coding is pretty weak. < >Also, the compression acheivable in the modem is limited by < >the ratio between the DTE-DCE speed .. < 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. 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. -- Paul Placeway