Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!apple!motcsd!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Why Ethernet-over-twisted-pair, anyway?
Message-ID: <2230040@hprnd.HP.COM>
Date: 5 Dec 89 20:05:01 GMT
References: <4645@blake.acs.washington.edu>
Organization: HP Roseville Networks Division
Lines: 93

david@ms.uky.edu (David Herron -- One of the vertebrae) writes:

> As Henry pointed out, UTP has an advantage when there's already that
> sort of wiring in place.  A lot of places were wired with "extra" phone
> wiring under the assumption that something computerish would be able
> to make use of it.  It turned out to be a bit of a self fulfilling
Historically, the origional reason for putting in a lot of wire was
that multi-line phones required multiple physical lines.  I remember
when the secretary's phone had a 25 pair bundle feeding it.  More 
sophisticated PBXs came about that did the switching so that the multi-
line phone only needed one or two pairs.
> prophecy -- the vendors saw all this extra wiring (and possibly were
> a source of advice for some people that the "extra" phone wiring
> would be useful Real Soon Now) and did networking on it.
> 
> 
> Now, why is a wiring scheme which was designed for the unique problems
> of analog circuitry going to be useful for digital circuitry?  I don't
> know, someone want to tell me?
The line between digital and analog is not so distinct.  Any signal
that goes over more than a couple of meters of wire has some change
to its waveshape.  When data and clock are encoded onto the signal
and it is sent over 100 m of wire, you need to do some analog
processing (eg filtering, wave shaping, clock recovery) to recover
the digital data.  You analyze the characteristics of the wire and
from that you determine if what you want is feasible and what kind
of processing you do.  In the case of 10BASE-T, there were some
givens: we wanted to use the same encoding and the same encode/decode
IC's as existing 802.3 10Mbit implementations.
> 
> As I see it -- without the shielding the cabling is going to affect
> more of the wiring around it and will in turn be affected more by the
> wiring around it.  There will be crosstalk between adjacent wires.

Yes, there is crosstalk to adjacent wires.  The amount is controlled by
filtering.  The crosstalk is one of the primary reasons why the 
distance is limited to 100 m.

> There will be more radio interference in the air -- something which
> people (pregnant women especially) have been up in arms over in recent years.

RFI from 10BASE-T is no more than from 10BASE2.  At any distance from the
pair, fields caused by the two wires cancel and emmissions are negligable.
> 
> Are the power levels low enough that the signals aren't measurable
> beyond the cabling?  If so then how is the signal strong enough for the
> components on the network to deal with them?  (that is, make out the
> signal in the noise from other sources (radio stations and the like)...)

Because, the receiving device is looking at the differential signal
sent over a balanced media.  If the media was perfectly balanced and 
the transmitter emitted no common mode signal onto the media, there
would be no emmissions regardless of signal level.  Similarly for
susceptablity.  Of course, balance is not quite perfect, so we do
things to limit emmissions and susceptability.  We limit the signal
strength, filter the transmitted signal at about 15 MHz, filter the
received signal to remove most of the noise, implement a squelch,
etc.

> Heck, how does the power emmissions from the unshielded cable compare
> to those of the 100,000 watt FM station a couple miles away or the
> high tension power wiring ?
> 
Emmissions from properly designed 10BASE-T equipment (including the
attached wiring) can meet regulatory requirements and are comperable
to that of any other modern computer equipment.  That means that they
are a lot lower than emmissions from older equipment since the FCC
regulatory requirements are fairly recent.  
> 
> These are the things I'm worried about with it..  If those concerns,
> and other related concerns, can be answered adequately then I think
> there's not much room for complaint.
> 
> 
> In my old job @ ms.uky.edu we used a combination of thin wire and thick
> wire coax with appropriate repeaters between segments (DEBET's
> and DEMPR's).  There weren't *any* problems, not even the one that
> the campus communications guy claimed you have with thin coax pulling
> out of the systems and bringing down the network.
> 
> In our case when the building was designed an extra set of conduits
> was run which was intended for some sort of intercom system which
> was never installed.  So we used that for both our serial and ether
> cabling.
> 
> In my new job all the wires look like phone wires.  But then that's
> one of the side effects of working at a phone company (AT&T)... :-)
> -- 
> <- David Herron; an MMDF guy                              <david@ms.uky.edu>
> <- ska: David le casse\*'      {rutgers,uunet}!ukma!david, david@UKMA.BITNET
> <- 
> <- New official address:  attmail!sparsdev!dsh@attunix.att.com
> ----------