[SI-LIST] Re: Do you really ship products at BER 10e-xx ?
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Chris.Cheng@xxxxxxxxxxxx
- Date: Tue, 12 Apr 2005 19:36:47 -0400
Chris,
I agree that your observation has a lot of truth in it. One of the
reasons behind it is that with electrical signaling, almost all of
our noise sources are eventually deterministic, the noise
contributors tend to have truncated distributions, so apart of a
very narrow active region, the BER slope is like a flip-flop;
either works or doesnt work.
The truly Gaussian white noise is surely there, but our signal
levels are so much above this noise floor that most of the failures
are traceable to deterministic events in the system. The
distributions of our disturbances are mostly truncated, we just
often times tend to extrapolate to more sigmas down the curve.
And if we dont extrapolate, but take the time to
collect the number of samples to get the BER, we may not be sure
that our 'noise' contributors stayed stationary during the data
collection window.
To answer the original question, yes, I have seen several systems
to fail with frequent enough bit errors so that it was obviously worse
than 1E-12, but not so bad that the system would stop working.
In these systems, with some digging, usually one can pinpoint a
set of circumstances and/or data patterns that eventually will force
the system fail repetitively. But in a complex system, where 'noise'
contributions may also come from remote sub systems, it is close to
impossible to catch those same exact circumstances AND data patterns
just by excercising the high-speed serial link with any practical length of
pseudorandom data. The conclusion to me is: the forgiving coded
differential signaling does not substitute for a proper worst-case
design check of the link.
Regards,
Istvan Novak
SUN Microsystems
Chris Cheng wrote:
>I've been shipping Gb/s serial products for a while and have my share of
>fail parts. However, I have yet to see a physical channel that is not either
>working like a charm or just fall on its face and barfing errors like crazy.
>Sure, chips or disk can fail and generates errors but no flaky channels that
>spits an error every other hour or days. To me, the channel is either have a
>BER that is near 1 (barfing errors like crazy) or near 0 (never fail, or at
>least approaching the life of the product it is attached to).
>Are we just kidding ourselves with these fancy BER analyzers or jitter
>instruments ? Do you really let a machine runs at say BER 10e-12 and say "ah
>ha, it only fails once a day and let's ship it" ? Is BER really meant for
>IEEE spec committees and not for real engineers who actually have to ship a
>product ?
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