Path: utzoo!mnetor!uunet!ncc!alberta!edson!tic!dragos!work From: work@dragos.UUCP (Dragos Ruiu) Newsgroups: comp.dcom.modems Subject: Re: Merits of compression (was Re: hayes 9600 vs. trailblazer) Message-ID: <405@dragos.UUCP> Date: 19 Apr 88 05:14:04 GMT References: <8804110136.AA16978@ucbvax.Berkeley.EDU> <15612@onfcanim.UUCP> <1441@pt.cs.cmu.edu> Organization: UNIX Device, Canada Lines: 39 Summary: Trailblazer should kill LZW if it sees negative compression. In article <1441@pt.cs.cmu.edu>, tgl@ZOG.CS.CMU.EDU (Tom Lane) writes: [Discussion on whether compression in the modem is a win, mostly with news feeds] > > Note that this implies that compression in the modem isn't a win; in fact, > it's a major loss if the modem tries to compress already-compressed data. > (At least, it is if both are using the same compression method. In the > case of LZW, you are likely to see as much as 50% expansion if you try > compressing compressed data.) > > -- > tom lane > Internet: tgl@zog.cs.cmu.edu > UUCP: !zog.cs.cmu.edu!tgl > BITNET: tgl%zog.cs.cmu.edu@cmuccvma It is very simple to keep track of your on the fly compression rate with LZW. I would assume that as soon as it sees negative compression for X packets, the 'Blazer would shut off compression for a while or send the next packet in plain text, only resuming the compression when a packet that gives a positive compression ratio hits. This would seem straightforward to implement. Can anyone enlighten us as to whether Telebit does this or not ? BTW the UNIX compress utility will not replace files that give negative compression ratios. It might also be interesting to hear how Telebit chose to implement LZW, i.e. when they flush their code table and if they use variable length bit-strings when compression is turned on. How about it ? Does someone know ? Are you allowed to tell ? Someone out there with a 'Blazer might test the compression shut off by timing the compressed transfer of two files, one compressed and one in its original format. -- Dragos Ruiu ruiu@dragos.UUCP ...alberta!dragos!ruiu "cat ansi.c | grep -v noalias >proper.c"