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0 pointsuserbinator11y ago0 comments

The data represented by the signal is digital, but the signal over the wire is an analog waveform more complicated than simple high and low voltages.

I agree, but as long as the receiver interprets the values correctly (i.e. the 1 is above its threshold, and the 0 is below its threshold) it doesn't matter. Among other things I've watched what signals like USB 2.0 HS really look like on an oscilloscope, so I'm well aware that they don't look anything like the nicely-drawn diagrams in textbooks.

Poor cables can absolutely cause high BER and lower performance.

But was that the case here? They identified a cable that had sub-par signal characteristics, and without looking at the actual effects it had on network performance, made the premature conclusion that it was. We don't know whether or not it was really the cause of the customer's problem (imagine them getting back this nicely terminated cable and seeing that it has the same performance... I would not be surprised.)

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PhantomGremlin11y ago· 1 in thread

> without looking at the actual effects it had on network performance, made the premature conclusion

They didn't need to look at the "actual effects".

The very smart engineers who created the high speed Ethernet standards have already studied the "actual effects". They spent years designing, studying, analyzing, and characterizing.

The designers distilled their efforts into some simpler rules such as "near end crosstalk (NEXT) shall not exceed ...".

Those simpler rules allowed a company like Fluke to build a test instrument that checks for things like crosstalk and delay skew and report PASS and FAIL and even report how far off the cable was from meeting the requirements.

That's all there is to it. It's ludicrous to ask for an ab initio analysis of every network problem that could be possibly encountered.

It may eventually be necessary to study "actual effects" in a particular network, but only after dispensing with low hanging fruit such as poorly terminated cables.

VLM11y ago

There is also the long term labor costs of breaking what amounts to a standard API.

Perhaps this mfgr card will work on a almost-but-not-quite cat5 cable run, but that new one won't, or that other one only works half the time, and slowly, or that one won't work when that other fluorescent light is on. The labor cost of this kind of troubleshooting can be pretty spectacular compared to a cheap and simple "must conform to cat-5 minimum standards".

Its rather like AC line voltage. True, some stuff, sometimes, will work at 100 volts or 150 volts, but I'd call an electrician and get it fixed before you spend a lifetime troubleshooting individual things that won't work reliably and consistently at 100 or 150 volts.

simoncion11y ago

> [Blue Jeans Cable] identified a cable that had sub-par signal characteristics, and without looking at the actual effects it had on network performance made the premature conclusion that it was.

From the fine article:

"[Our customer is] dealing with network performance issues at a small company, and has come to suspect that bad patch cords, made by an electrician, account for some of the problems he's seeing."

Looks like the customer who sent them the cable had already done the troubleshooting required to determine that the cable was probably bad.

Also, you should probably read: http://www.bluejeanscable.com/articles/channel-certified-eth... Each part in an Ethernet network has well-specified tolerances for a reason. If one part of that network causes far too much signal degradation then all sections of the network that flow through that part can fail to function.

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