[SI-LIST] Re: Fibre channel interconnect margins

  • From: Hal Murray <hmurray@xxxxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sat, 01 Jul 2006 16:43:54 -0700

> Because the statistical  theory behind those errors is that random/
> Gaussian noise is, by  definition, unbounded - errors are a fact of
> life, even if the error  rate is very low. Eventually you have to an
> edge that is outside the  jitter spec. A single unrepeatable error out
> of billions of bits simply  has to be expected.

By that line of reasoning, your CPU would make occasional errors.

I think the Gaussian noise assumption is reasonable on fiber systems.  If the 
error rate is low, it is exponential in signal/noise ratio.

On copper systems, errors are often associated with crosstalk and/or 
reflections.  Those are deterministic, not Gaussian.  Even EMI and power 
supply noise are probably more deterministic than Gaussian.

A well designed CPU is essentially error free because it's reasonable to 
engineer a system with a signal/noise ratio that turns into less than one 
error per age of the universe.

Years ago, when I worked with fibers, consensus was that if you could measure 
the error rate it was too high.  The context was long links - telcom.  Their 
engineering may be less conservative these days.

The error budget included quite a bit of reserve for the laser aging and 
additional splices that get added when backhoes find the fibers.  The systems 
were designed to have a low error rate under worst case conditions.  Most 
systems are far from worst case.  That's especially true just after 
installation when you are trying to verify that everything is working 
correctly.  The extra/reserve signal/noise ratio makes the error rate so low 
you can't measure it.

One trick useful for fiber systems is to add artificial attenuation.  
Reducing the signal reduces the signal/noise ratio which increases the error 
rate.  With the right attenuation you can measure the error rate within a 
reasonable amount of time and then calculate the error rate you would get 
without the extra attenuation.  That works well because the noise on fibers 
really is Gaussian.

This thread started when somebody awked about margins in a fibre channel 
link.  Since pre-emphasis and equalization were mentioned, I assume it's a 
copper link rather than fiber.

Has anybody tried inserting attenuators in high speed coax links?  Can you 
learn anything interesting that way?

Is there some way to estimate the deterministic vs Gaussian components of the 
system noise?

If nothing else, increasing the error rate should speed up things like tuning 

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