[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
equalization.
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