[SI-LIST] Re: Do you really ship products at BER 10e-xx ?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: andyp@xxxxxxxxxxxxxxxxxxxxxx, Bradley.S.Henson@xxxxxxxxxx
- Date: Mon, 18 Apr 2005 04:31:26 -0700
Andy, three issues:
1) Know / control the environment. PLL's don't just fall out of lock by
themselves without help from the outside, power disturbance, being the
number one culprit. If you insure that power is clean and undisturbed by
things like digital return currents, a properly designed PLL will not lose
lock. Similarly, your cabling will be subejected to whatever ESD and EFT
is in the environment.
2) All data links eventually get errors. It is a matter of time. If bad
things will happen as a result of a communication link error or failure
then you need some sort of back-up.
3) Large interleaving ratios can help reduce the probability that a run of
errors will be uncorrectable.
Steve
At 03:35 PM 4/12/2005 -0700, Andy Pedler wrote:
>This is actually right-on topic with a design problem that I'm
>investigating. Here's what I require, and maybe someone can suggest
>something.
>
>I need a relatively high-speed serial link; let's say 1 Gbps, but if I
>can run 2.5 Gbps it will save me cost in another part of the design.
>I'd like to run over a backplane, but the design may simply be
>board-to-board connectors. It could also be 1-2 foot cables (perhaps
>Infiniband type cables). It's a theoretical exercise at this point.
>But I can certainly live with 1 Gbps. I can add forward error
>correction into my data that is traversing this link, so I can live with
>an occasional *single* bit error that comes along once in a blue moon.
>But my system will crash and burn if the receiver ever gets a continuous
>stream of errors. So I would be happy with a predictable BER of even
>1E-7 or 1E-9, so long as the errors are single bit and correctable. But
>even 1E-20 is bad if the errors show up in huge numbers all at once.
>
>When I've talked to serdes vendors about how they define BER, I've been
>told that these serial links typically operate error free, but every so
>often for whatever reason (Chris's cosmic ray), a PLL might get just out
>of sync and have to re-lock, and when that happens you get a ton of
>errors all at once. Obviously, that will kill my system.
>
>I've built chassis systems with 1 Gbps backplanes and run them for weeks
>at a time without recording any errors. But that still doesn't make me
>extremely confident that I would *never* see a problem. This system
>would have to run for months at a time, and a hiccup would cause a lot
>of problems.
>
>Any thoughts?
>
>Andy Pedler - Greenfield Networks
>
>
>
>
>
>Henson, Bradley S wrote:
>
> > This could make an interesting topic. I have to say that in general, I
> > have noticed the same trend: Links work so well the BER is hard to
> > determine (lots of test time or link-stress)-or- the links are totally
> > messed up. However, I did get called in to troubleshoot a Fibre channel
> > application that was just marginal on some of the links. By that I mean
> > they would almost make the spec 1E-12 BER sometimes, but usually fell
> > short. Some days they operated considerably poorer than 1E-12, but not
> > pure garbage.=20
> >
>
>
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