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From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.lan
Subject: Re: Ethernet cable length query
Message-ID: <928@sdcrdcf.UUCP>
Date: Wed, 21-Mar-84 11:24:25 EST
Article-I.D.: sdcrdcf.928
Posted: Wed Mar 21 11:24:25 1984
Date-Received: Fri, 23-Mar-84 21:01:16 EST
References: <1115@cbosgd.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation, Santa Monica
Lines: 18

The Ethernet specification requires random pieces of Ethernet cable to be
1 or 3 or 5 times 23.4 meters (76.8 feet) in length.  The rules are less
stringent for a single piece of cable or cable entirely from a single lot.
The rationale is that different lots of cable have slightly different
impedences, resulting in reflections at each splice.  23.4 meters is one
bit time, so this keeps reflections on bit boundaries, or even provide
destructive interference with the reflections.  (By the way, these
guidelines are not guaranteed; if you want to be SURE of a given cable
assembly, sweep testing of SWR is recommended)

-- 
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA