From: jt@fuw.edu.pl (Jerzy Tarasiuk)
Subject: Re: Thinwire specs/problem?
Organization: Warsaw University Physics Dept.
Date: Fri, 22 Jan 1993 22:16:12 GMT

jt-> I mark what I write, leaving citations "as were"

Newsgroups: comp.dcom.lans.ethernet
Subject: Thinwire specs/problem?
From: longo@kodiak.sfpp.com
Date: 9 Jan 93 11:05:51 PST
Organization: Santa Fe Pacific Pipelines

Can someone please post the rules for a thinwire segment (length, number of
nodes, etc.).  We are getting several DECnet block check errors per day.  I
found one thinwire segment that is 909 feet long (which I believe is too long).

jt-> Some E. adapter manufacturers (e.g. 3Com) specify their cards allow
jt-> longer segments. They use their proprietary transceivers, possibly
jt-> having better specifications. The 500 ft limit seems to be over-
jt-> -safe, for a case of using poor quality transceivers.
jt-> Next, the distance meaningful here is distance between active
jt-> elements (transceivers, repeaters) rather than total cable length.
jt-> Can add any length to each end without causing trouble. This can
jt-> be used to simplify diagnosing: add "return cable" ended by
jt-> terminator, not connected anywhere more to end and put its end
jt-> near repeater; if some measurement is necessary can have both
jt-> ends in some place and can remove terminator for measurement
jt-> purposes without need to go to opposite end of building.

Our thinwire tester reports "anomalies" at two points on the segment.  Those
points are where two different types of cable are barrel connected.

jt-> Either too sensitive tester (however, for some maintenance even
jt-> more sensitive tester which can see reflection on every connection
jt-> is useful) or bad connections. Need measure reflection ratio, if
jt-> -20 dB surely no trouble, if -5 dB surely cable is bad.
jt-> (I don't know exactly what reflection is meaningful)

Could the segment length or junction of different cable types or both be the
cause of these errors.

jt-> Are you sure all cables are 50 Ohm ? Using cables of different
jt-> impedances (e.g. 75 Ohm TV antenna cable) E. can work, but with
jt-> a lot of troubles, especially when different impedances are used
jt-> on single segment. If you can see reflected wave on oscilloscope
jt-> can easily distinguish reflection on bad connection from r. on
jt-> cable impedance change: first makes differentiation.
jt-> Junction must be VERY poor to be source of trouble: I here have
jt-> cable of maximum allowed length, with few BNC-50 junctions, in
jt-> few places repaired by soldering and no problems reported!

Thanks for any information,
-Bob Longo (longo@sfpp.com)
Santa Fe Pacific Pipelines, Los Angeles, CA
========================================================================
From: fenton@iron.hq.aflc.af.mil (George I. Fenton)
Organization: U.S. Air Force Security Assistance Center (AFSAC)
Date: Sun, 10 Jan 1993 02:33:57 GMT

      Max coax segment:     185 meters (about 600 ft)
      Max number of link segments:   2
      Max stations per segment:  30
      Min distance between stations:  0.5 meters
      Max number of stations:  1,024

jt-> I was unable to find specs quickly, but surely minimum distance
jt-> should be bigger: to pass 0.5 meters signal needs 2.5 ns, since
jt-> it makes no difference if it is 0.5m or 5cm. Probably proper
jt-> distance is at least 2.5m => 12.5 ns (better 5m).

The different cables connected probably have different resistence causing
digital signal scrambling (e.g. garbage).  My recommendation is to check
all segments for distance and grounding problems, and don't mix cable
types by simply using barrel connectors (need appropriate transceivers
to go between 10Base2 and 10Base5, for instance).  I have had cases of PC
workstations plugged into a bad power outlet with bad ground, and the PCs
start to use the 10Base2 as a ground source.  Please also check the
ground potential variation between your power outlet ground and the
LAN connections (i.e. if your LAN backbone is 10Base5 {thick-wire}, you
need to ground one end, typically to the same ground rod as used for
the building's power).

jt-> It is VERY IMPORTANT the cable can be GROUNDED IN ONE PLACE ONLY!!!
jt-> On PC it is easy to make short-circuitry between BNC-50 T connector
jt-> and case. Such short-circuitry caused (at lest here) more problems
jt-> than bad terminators or connectors: note cannot have one ground,
jt-> every place in building has own "ground" potential and they always
jt-> are different; made a connection and current will flow; and the
jt-> current can be VERY harmful for E. communication. Trust me.

==========================================================================
From: mju@mudos.ann-arbor.mi.us (Marc Unangst)
Date: 10 Jan 1993 01:37:21 -0500
Organization: The Programmers' Pit Stop, Ann Arbor MI
NNTP-Posting-Host: mudos.ann-arbor.mi.us

>I have had cases of PC workstations plugged into a bad power outlet
>with bad ground, and the PCs start to use the 10Base2 as a ground
>source.

Huh?  The coax shield on a 10Base2 network should only have one ground
connection, and doesn't *need* any.  Most PC cards I've seen have a
plastic insulator between the BNC connector and the card bracket, to
prevent the shield from being grounded at the PC.

jt-> I suppose every card has it. But the T connector can reach
jt-> something connected to case (which is usually grounded).
jt-> ALWAYS make sure it don't reach enything or put isolator.

==========================================================================
Newsgroups: comp.dcom.lans.ethernet
From: fenton@iron.hq.aflc.af.mil (George I. Fenton)
Date: Mon, 11 Jan 1993 00:27:11 GMT

Yes, there is a plastic shield, but it has been my experience (particular
in old PCs) that the T-connectors can sometimes touch the metal chassis.
If the PC does not have a properly grounded outlet, the PC will find a
ground and if the T-connector is touching the chassis, the 10Base2 cable
becomes the ground for the PC.

jt-> I see I am not the only who noticed it.
jt-> One more: must do not connections between different parts of cable
jt-> shield (although they are connected). We call the problem "ground
jt-> loop" and know it can cause serious troubles. For example 80%
jt-> of short packets (and almost 100% of long) to become a garbage.

==========================================================================
Newsgroups: comp.dcom.lans.ethernet
From: vhalkka@klaava.Helsinki.FI (Vesa Halkka)
Organization: University of Helsinki
Date: Sun, 10 Jan 1993 22:17:51 GMT

>      Max coax segment:     185 meters (about 600 ft)
This one comes from RG58 attenuation.. better cables are available.
Belden 9907, for example.

jt-> or from transmitter/receiver parameters: for largest possible
jt-> distance signal must be large enough to make possibility to
jt-> receiver to distinguish signal from own transmitter only from
jt-> collision. Fortunately, low frequencies (MUCH BELOW 5MHz) are
jt-> most meaningful for collision detection, since the cable doesn't
jt-> make great difference (unless it has too big DC resistance),
jt-> more important are transmitter/receiver parameters (especially
jt-> their variations in time or as result of temperature change).

==========================================================================
From: matt@wardsgi.med.yale.edu (Matt Healy)
Date: 13 Jan 93 22:31:17 GMT
Organization: Yale U. - Genetics

> >      Min distance between stations:  0.5 meters
> But where does this come from ?

I believe the minimum distance rule is intended to
reduce the risk of major standing wave patterns
forming.  Any radio hams out there who can explain
more about Standing Wave Ratios?  I only remember
that *any* transmission line will have some level of
standing waves, but you want to make as low as you
can...

jt-> More simply: wave reflection caused by capacitance of
jt-> many connectors put in almost same place. Need distance at
jt-> least 2.5 m (not 0.5) to make significant phase difference
jt-> between reflected waves to reduce reflection.

"I pretend to be a network administrator; the lab net pretends to work"

==========================================================================
From: seifert@netcom.com (Rich Seifert)
Organization: Networks & Communications Consulting
Date: Sun, 17 Jan 1993 22:07:50 GMT

> >      Max stations per segment:  30
> This comes from BNC attenuation/reflection. It is actually 60 BNC
> connections.. if you have any kind of boxes with connectors at
> the wall this number is smaller.. BNC connections reduce it to 15,
> some non-breaking systems give only 10 or 12 stations.

Actually, it is more than just BNC connectors. The limit is a function of
transcevier loading capacitance, shunt resistance, and input bias currents,
PLUS the BNC connectors.
 
> >      Min distance between stations:  0.5 meters
> 
> But where does this come from ?
>

This prevents "lumped" capacitive loads.
 
> >      Max number of stations:  1,024
> 
> And this.. something with collision recovery algorithms ?
>

Yes, the backoff algorithm assumes that there are never more than 1024
stations offering load to the network at any time.
 
Rich Seifert                    Networks and Communications Consulting
"... specialists in Local Area Networks and Data Communications systems"

jt-> Seems all info is OK, thanks for clear explanation.

==========================================================================
From: gdmr@dcs.ed.ac.uk (George Ross)
Organization: Department of Computer Science, Edinburgh University
jt-> (I deleted it, nothing new there. Except E. card in PC can be faulty)

==========================================================================
Newsgroups: comp.dcom.lans.ethernet
From: stevey@cpc450.Columbia.NCR.COM (steve yackey)
Subject: Re: Thinwire specs/problem?
Organization: NCR Corp, E&M-Columbia, Columbia, SC
Date: Thu, 14 Jan 93 14:05:41 GMT

Here's a post from some time ago, maybe this will help ???????

Newsgroups: comp.dcom.lans
Subject: 10 Base 2 ThinWire Ethernet Cable
Keywords: IEEE 802.3 ThinWire Ethernet
Date: 19 Aug 89 06:32:48 GMT
Organization: Total Support Computer Systems, Tampa, Florida

In article <442@qtc.UUCP> (2 Aug 89 15:07:25 GMT), Jim Battan writes:

> My questions:
> Are the technical advantages of fully IEEE-compliant cable (e.g. Belden
> 9907) worth the extra cost (cable, new connectors, new crimper)?
> (We're not up against any of the 10BASE2 limits (#nodes, length) yet.)

jt-> I have read the specs from Belden. Avoid the cable!!!
jt-> Catalysed corrosion of the Al foil can cause problems.
jt-> Also, avoid polyvinyl - recrystalization after few years
jt-> can occur (why ask for trouble? :-).

 3.  Shields of foil are an aluminum/poly/aluminum sandwich material of
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     0.002"/0.001"/0.00035" thickness.  Braid sheilds are made of a
     tinned copper (TC) briad (e.g. 16x6x36 is a 16 by 6 weave of 36 gauge
     tinned copper wire).

 4.  Jacket material is polyvinyl chloride (PVC) with optional National
                        ^^^^^^^^^^^^^^^^^^
     Electrical Code rating of CL2 (NEC CL2).