From: vthrc@brolga.cc.uq.oz.au (Danny Thomas)
Subject: wiring for 10baseT/CDDI
Keywords: UTP 10baseT CDDI wiring crosstalk interference cable quality
Organization: Prentice Centre, University of Queensland
Date: Wed, 7 Oct 1992 07:10:11 GMT


Hello people,
             we're just in the middle of designing the cabling of a new 
(small) building wing. A fibre link bridged from our current net will be 
the entry point. While all equipment shifting to the new wing has AUI/coax 
interfaces and thinwire is the primary cabling for the short-term, the 
question is in laying out UTP for 10baseT with the possibility of using it 
for faster services in the longer term, eg CDDI or whatever becomes 
popular. If we were really committed to a faster network, we would have 
run fibre as well, but given budgetary constraints it seemed better 
spending a rather small increment on the UTP which at least will produce a 
higher-quality 10baseT net that will definitely be used in the medium 
term.
    I guess there aren't many people yet with real-world experience at 
laying out CDDI and as should become obvious, I'm operating from a 
position of real ignorance so would be grateful of any comments and 
pointers to documents that may be of assistance (preferably FTPable).
    The plan is to use Belden 1583A level 5 UTP. The only locally 
available RJ45's rated to 100M are Suttle 110. There will be twin runs 
from each office's dual RJ45 panel direct to RJ45s in patch panel. The 
longest run is 33m (100' for the non-metric countries, last I heard 
Yemen/USA 8-)) with only 2 connectors (plus 2 more on the interfaces) so 
it is well within the 10baseT budget.
    I'm worried about crosstalk/interaction between the cable runs along 
with noise pickup from parallel mains cables in adjacent cable trays (6" 
apart, though with a grounded metal tray between). The longest run close 
to mains would be around 12m (40').
    The easiest way of taking cables from one side of the building to the 
other is through a 40mm (1.5") conduit - but is it a good thing running 
ten cables so close together for 13m? Even for 10baseT, let alone CDDI? 
Parallel cables would bunch up as they approach this one conduit too.
    As a means of identification the pair of cables from each office are 
run in opposite directions, but I'm not sure whether that will have any 
effect on crosstalk between them. When there are several cables running in 
a tray, is it a good idea to randomly redistribute along the length of the 
run, ie not having the same cables close together for the whole length?
    My guess is that higher quality UTP offers tighter and more consistant 
twisting to maximize crosstalk between the pairs at different twist rates, 
more uniform and perhaps lower impedance. Something I wonder about though, 
is the effect on impedance when the cable comes off the grounded cable 
tray (which it is intimately tied down to) into a longish run through 
plastic conduit. Presumably that would represent a reasonable impedance 
discontinuity that might be quite significant for the sharper edges of 
CDDI, even if they are shaped as nicely as posssible.


In terms of 10baseT, a quick perusal of back issues of the BIG-LAN mailing 
list showed some different experiences in regard to interference from 
adjacent mains. In this part of the world we use 240V AC so presumably the 
smaller current flows result in less magnetic interaction?

FROM ISSUE #57
>From: Robert Lummis <rcl@aecom.yu.edu>
>Subject:      Re: 10Base-T Noise and Crosstalk
>
>I've wired a whole (small) campus recently and worried a lot about
>10Base-T noise immunity but I have not had a single problem in that
>regard.  In an experiment at the beginning of installation we jury-rigged
>to most noise-inducing awful mess of wiring we could think of using the
>cable designated for standard wiring and actual electronics at both ends.
>We ran up 12 stories though the 55-foot diameter utility shaft of a
>large research building and intentionally draped the wire over and around
>electrical equipment -- DC elevator motors, compressors, DC relay panel,
>flourescent lights, some magnetic resonance equipment, etc. etc.  
Anything
>that anybody could think of that might cause trouble we wrapped or draped 
with
>the cable.  We tested it over several days with actual ethernet traffic
>and also with a Hewlett Packard 10BAse-T cable tester.  The result: not a
>single incident of noise interference.  (tested in various ways with a
>sniffer, the HP cable tester, and a Lanalyzer)  Mind you-- this is new
>AT&T 10Base-T cable, not in-place telephone wiring.

and from Bruce Nielsen (Nielsen@act-act4-po.act.org)
>  In my opinion (and experience) the unshielded twisted pair (UTP) LAN
>  cable running alongside the power cable will likely cause you problems.
>  I'm not certain about the crosstalk, but the signal-to-noise ratio 
(SNR)
>  is possibly where you'll have problems.  In any event, I always require
>  our contractors to "certify" the cable by documenting the following for
>  each and every cable run:
>
>    1) Signal-to-noise ration >10 dB for Ethernet & Token Ring
>    2) Near-end crosstalk (NEXT) >23 dB for Ethernet, >30.25 TRing
>    3) Total distance (must be >329 feet)
>    4) Attenuation <11.5 dB for Ethernet and <14.5 TRing
I think that 3) should have been "<329 feet"

Finally, will a cheapish TDR testing unit be much help in predicting cable 
quality at CDDI speeds? I would guess that most units employ a tunnel-
diode to generate a sufficiently sharp edge, but what about the rest of 
the electronics. Perhaps the line S/N figure would still be useful but 
maybe crosstalk less so.

PS for ATM's 200Mbps raw rate, is it likely that all four UTP pairs will 
be used?

an email copy of any netnews reply would be muchly appreciated - will 
summarize if sufficient interest.

cheers,
X500: @c=AU [I'm about to update VTHRC entries, soon...]
       @o=University of Queensland
        @ou=Physiology and Pharmacology Department
         @ou=Vision Touch and Hearing
          @cn=Danny Thomas
           @rfc822Mailbox=vthrc@cc.uq.oz.au

Newsgroups: comp.dcom.lans.ethernet