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Path: utzoo!watmath!clyde!floyd!harpo!seismo!hao!hplabs!sdcrdcf!darrelj
From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.lan
Subject: Re: Re: Re: Ethernet cable length query - (nf)
Message-ID: <954@sdcrdcf.UUCP>
Date: Fri, 30-Mar-84 12:27:19 EST
Article-I.D.: sdcrdcf.954
Posted: Fri Mar 30 12:27:19 1984
Date-Received: Sun, 1-Apr-84 07:22:19 EST
References: <2897@fortune.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation, Santa Monica
Lines: 22

One important detail missed in the otherwise excelent article on TDR from
Fortune Systems:  Real ethernet coax is specified to use a foam dielectric,
resulting in a propagation speed of .78c (instead of .66c for a solid
dielectric).  As a result all the distances in the coax are 17% greater
than the figures they used.  The cables to the tranceiver, however do have
a speed of .66c.
1c   = 3E8    m/s = 15m/bit   round trip = 49.2 ft/bit round trip
.78c = 2.34E8 m/s = 11.7m/bit round trip = 38.4 ft/bit round trip
.66c = 2E8    m/s = 10m/bit   round trip = 32.5 ft/bit round trip

The propagation speed is supposed to be controlled withing about 1% in well
made cable.


-- 
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