Path: utzoo!mnetor!uunet!oddjob!hao!gatech!mcnc!rutgers!topaz.rutgers.edu!ron
From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.dcom.lans
Subject: Re: barrelled ethernet (yellow) coax
Message-ID: <17792@topaz.rutgers.edu>
Date: 4 Feb 88 18:05:12 GMT
References: <3559@batcomputer.tn.cornell.edu>
Distribution: comp
Organization: The Office of Mismanagement and Bean Counting
Lines: 24
Keywords: cable length restriction

The following is for IEEE 802.3 10BASE5 cable.  This is perhaps the
most up to date specification for Ethernet (the yellow kind).

The sum length of the cable may not exceed 500m and may
be made up of sections joined with two male plugs and a
barrel (double female).   802.3 recommends:
1.  that you try to make up the entire distance from a
  single cable piece,
2.  that you use the same lot/manufacturer on multiple pieces
  to minimize impedence mismatches,
3.  that if you are going to use mixed lengths that you mimize
  reflections by using odd integral multiples of a half wavelenth
  of the cable at 5 MHz to avoid the reflections adding in phase
  (these yield your 23.4, 70.2, and 117 meter lengths +/- .5m) and that
  if you use only these lengths you should be safe.
4.  If you use random lenghts measure the worst-case signal reflection
  due to the discontinuities does not exceed 7% of the incident wave when
  driven by a MAU.

The 2.5m rings are for transciever placement and not for cable cutting.
Discontinuities in the ring spacing at the ends of the cable are explicitly
allowed by the spec.

-Ron