[Tfug] Switch problem

[Louis Taber]

On Sun, Mar 17, 2013 at 1:44 PM, Bexley Hall <bexley401 at yahoo.com> wrote:

> Hi Louis,
>
>
> On 3/17/2013 12:37 PM, Louis Taber wrote:
>
>> I set up a test bed in my shop with 600 ft of Cat-5.  It
>> quite happily auto-negotiated to 100MBit/sec, but would move no data at
>
>  all.  Put an old hub at one end to force it to 10MBit/sec and it worked
>> fine.  See: http://en.wikipedia.org/wiki/**Autonegotiation#Electrical_**
>> signals <http://en.wikipedia.org/wiki/Autonegotiation#Electrical_signals>
>>
>
> OK, so in each case, you get a "go" for the "Link" (LED) but the
> first case just leaves you with no *legitimate* signal at the far
> end. So, in Erich's case, the machine can think the link "up" and just

never see any "in band" data (e.g., DHCP traffic)  (?)
>
Yep.  That is easy to believe.


> In which case, forcing the interface to a specific configuration
> (my original suggestion) should work?  I.e., don't *let* the switch
> "think" the link (end to end) is capable of anything that it *isn't*!
>
That should work.

>
> [Though he stated that the slower/older NIC's seemed to be manifesting
> the most problems...  :-/ ]
>
>
>  A side note: I have contended for years that the SOHO market needed to
>> handle miss-wired cables and mid-x to reduce returns and customer service
>> costs.
>>
>
> I think AutoMDIX adds another set of issues.  E.g., I suspect that was
> the problem I was having with my two Gb switches (separated by 8 ft
> of cable).  Powering them up *singly* would work but if they both
> came up together, they just sat staring at each other...
>
> But I agree with your observation.  A wise manufacturer would design
> in a way that would discourage the user from even *suspecting* the
> kit (assuming it most usually is NOT the culprit!).  Or, at the very
> least, provide a reassuring means by which the user can convince
> himself that the kit *is* or is *not* the (part of) problem!
>
> Gb Ethernet is inherently AutoMDIX.  It uses all 4 pairs full duplex.  All
pairs have both a receiver and transmitter that do work at the same time.


>
>
>>
> I'd be more worried about its exposure to lightning strikes
> (not *direct* but, rather, *induced*) over such a long distance.
> The galvanic isolation afforded by the coupling transformers
> would be no match for an induced voltage spike from such a strike
> (No idea how far into the machine any potential damage would
> propagate).  I've lost (wired) phones from nearby strikes (the
> surge takes out the protection diodes across the line) -- and
> you *know* the phone company designs with this sort of thing
> in mind!
>
> But, I'm typically overly pessimistic about these things.  I
> don't like *fixing* things if I can figure out a way to prevent
> them from *breaking*, instead!
>
> Base-T network connections are ground isolated at both ends with a
coupling transformers.   Many of the older units also had a spark gap if
enough voltage is induced.   Not that lightning is not a problem.  At Pima
one strike caused $27,000 in damages when it hit a grounded metal building.
 (I did learn it is better to wind excess cable in figure-8s than just
round loops.)   Fortunately, in this case, Northern coastal California get
very few lighting strikes.
http://www.vaisala.com/VaisalaImages/Product%20and%20services/NLDN%20CG%20Flash%20Density%20Km%201997-2010.png

  - Louis
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