Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!amdcad!ames!nrl-cmf!ukma!rutgers!topaz.rutgers.edu!ron
From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.dcom.lans
Subject: Re: Quest: Ethernet cable restrictions?
Message-ID: <17985@topaz.rutgers.edu>
Date: 11 Feb 88 16:48:51 GMT
References: <3880004@nucsrl.UUCP>
Organization: The Office of Mismanagement and Bean Counting
Lines: 67

>	I have a question for the ethernet experts.  I have heard some
> people mention that the restrictions on segment lengths and transceiver
> placement (on the black lines), and I really do not see the rational
> behind it.

The restriction on Ethernet transcievers (IEEE 802.3 MAU's) are that they
be located 2.5 meters from each other (minimum).  The black marks on the
cable are located 2.5 meters to be used as a guide.  This is especially
handy when the cable is in the cieling and you can't see 2.5 meters in
each direction on the cable, so always putting them on the black marks
is a safe way of handling it.

The 2.5 meter marks have NOR relationship to the end of the cables.  The
standard (IEEE 802.3, the only one I have handy) specifically says that
the mark spacing is discontinuous at the connectors in the cable.  The
main reason the marks are there is that the standard for the cable properties
lists insulation with these markings.  Note another useful marking is cable
"serialization."  These are numbers that are marked every so often down the
cable with constant spacing.  Knowing the number at the end of a segment
will allow you to compute the distance to a number further down the cable.
This makes TDR work possible when you have random cable in the cieling.

>	I have a question for the ethernet experts.  I have heard some
> people mention that the restrictions on segment lengths and transceiver
> placement (on the black lines), and I really do not see the rational
> behind it.

If you look at a frequecy domain plot of it, you'll find it at 10MHz
(plus harmonics, square waves are full of 'em).  The frequency is always
10 MHz, the Manchester encoding changes the phase of the square waves by
180 degrees.  If we didn't use manchester encoding, then the frequency
could become 5MHz often, and even DC for a while, which would be bad.

>	Now certainly we will have reflections off couplers, transceivers
> and any other discontinuity, but whether these reflections add or cancel
> depends on the bit pattern, and thus is unknown.  The ONLY way to 
> insure that these reflections have no adverse effect is to insure that
> they are small (not by hoping they cancel).  Thus it seems to me that
> there is no benefit in the cabling restrictions.

The major place reflections occur is at impedence mismatches.  The main
reason for impedence mismatch on Ethernet is cable joining, not the MAU's
themselve (unless they are inline, which effectively adds two connectors
to the system).  This is why there are some guidelines (restrictions?) for
wire lengths in the system.  They go, in decreasing order of preference.

1.  Make the Ethernet out of one piece of wire (no discontinuities).
2.  Make the Ethernet out of pieces of wire from the same manufacturer
    and lot (i.e, from the same spool).  This has no cable to cable
    impedence changes, any problems will be those of connectors.
3.  If you must mix random cable pieces, use lengths that will cause
    the reflections not to add in phase.  The standard points out that
    if you use half-wavelenths of 5MHz for all your cable pieces, you
    won't have a problem before using up your 500m of cable.  These
    lengths are the familiar ones (23.4, 70.2, 117, ... meter) that
    your mama told you about.
4.  There is actually a measurement test that can be done to verify
    the allowable configuration with trully random lengths and parts.
    I don't have my IEEE 802.3 handy (it's at home).  But most people
    don't have the equipment to do that anyway.

Generally, I usually refer people to the DEC "Networks and Communications
Buyers Guide" which explains many of the Ethernet design criteria.  They
leave out a lot, but the are conservative, so if you put together a network
based on their specs it will work.

-Ron