[SI-LIST] Re: Fibre channel interconnect margins

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>, "David Instone" <dave.instone@xxxxxxxxxx>
  • Date: Wed, 05 Jul 2006 22:43:20 -0700

Chris yes c) is the running bet for terrestrial links.  Accurate 
assessment of Rj is tricky stuff.  Our experts Ransom and Al spend 
quite a bit of time on it.

Best Regards,


Steve.


At 01:16 PM 7/5/2006, Chris Cheng wrote:
>Steve,
>I start off by asking the same question before, "does your company 
>ship products with 10e-12 error rate ?" and it seems it comes down 
>to the following answers :
>
>a) "Its a fact of life, I'll try to keep it to 10e-x13,14,15 or better."
>
>Well, I still want to know what that number is ? And when you have a 
>customer that has a peta byte of installation, how does that 
>translate into actual errors per second in your FCAL ?
>
>b) It may happen in some part of the link but overall the link can tolerate it
>
>In particular,
> >If over any finite period of time the RJ causes the frequency
> >as seen by the PLL to change then the PLL will move the VCO, thus
> >creating a limit to the max observed RJ.  If the RJ is distributed
> >so that the frequency does not have to change then the 'single
> >incremental interval' effect will apply.
> >  Have we not then got a jitter distribution that is Gaussian in
> > form but with limits to the maximum deviations?
>
>I don't know about FCAL jitter tolerance spec but let's take the 
>OC-xx spec as a reference, what happen when the jitter spectrum 
>passes ft ? Is the jitter still bounded ?
>To take it out to a non-ideal world, how does one trade off jitter 
>tolerance (for example, by Rj and Dj) and jitter accumulation (for 
>example, by PLL supply noise) ? You only have one choice of loop 
>dynamics in your PLL and you can not help one without hurting the 
>other. And I would bet jitter accumulation from PLLVDD dwarfs the 
>concern over jitter tolerance. Just look at how elaborate those 
>PLLVDD distribute schemes we have (isolation traces, filters etc). 
>Can you still say your Rj is unconditionally bounded by such PLL's ?
>
>c) 10e-12 happens only in methodology, you are really running a BERT 
>much much lower, possibly beyond the life of the system or the 
>component or reasonable measurement technique
>
>Which, I think, is essentially what you are saying in the last 
>sentence. Do we really claim a 10e-12 simply because we got an 
>erroneous Rj from an instrument in a Dj dominated system, multiply 
>the sigma to 14 or 15 times per the BERT spec and scare ourselves to 
>such error rate ?
>And before the instrument guys jump on me again, let me be clear 
>that I don't think the instrument is at fault but rather the problem 
>lies on the application (Dj dominate system measurement) that force 
>the instrument to report back a number that it cranked out based on 
>some fixed assumptions not suited for such situation.
>
>For those who claim a), my hats off to you because you are a braver 
>man than me to publicly say that.
>
>For those who claim b), I sure would like to know what kind of PLL you have
>
>And my money will be on c).
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
>Sent: Tuesday, July 04, 2006 3:40 PM
>To: David Instone
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Fibre channel interconnect margins
>
>
>David, I did not regard it as an attack just an opinion that is
>different and worth discussing.
>
>The basis of our disagreement appears to be in the definition of
>bound.  I look at things from the standpoint of electrical
>noise.  Time interval in a timing circuit is the result of the
>magnitude of some electrical quantity, and is always causal, each
>event defining a new interval follows the previous.  This means that
>noise effectively multiplies the interval by some factor   1/oo <= K
><= oo.  Jitter is still unbounded, but every incremental interval has
>a positive duration.
>
>So far we have been talking about noise in the oscillator itself.
>
>Now, let's see what a PLL does to this quagmire.  If noise hammers
>the VCO then the PLL feedback loop applies gain to divide the effect
>of the noise.  If we still believe in infinity, infinity divided by
>anything is still infinity.  In practice will the oscillator stop for
>an unlimited time?  It will only when it fails.  On the other end,
>two successive pulses can occur essentially on top of each other.
>
>A receiver PLL will take a finite amount of time to realign within a
>fixed amount of phase to the jittered stream for the case of the
>oscillator event, and will take a different, much longer amount of
>time to align to the short term frequency offset that noise in the
>PLL error amplifier causes.  The phase error between the source
>stream and the recovered clock in the latter case generally follows a
>classic 2nd order step response.  The golden PLL is a PLL with
>specific frequency response and damping.  Even if we have a PLL that
>uses N=1, the PLL only starts correcting after a timing error is
>already apparent.  For a timing error of sufficient magnitude data
>moves outside the timing window, a data recovery error is guaranteed,
>and no PLL is going to prevent that.  A nasty little problem that
>gets into systems is power supply noise coupled into the VCO and/or
>error amplifier by one means or another.  For systems with high Ns it
>can get really ugly.
>
>On a slightly different tack, for a PLL using a PFD, the unit
>interval is that at the phase comparator input which is VCO/N or
>Fref.  Noise whacking the error amplifier will push the VCO off
>frequency until new information arrives to get it back.  If the noise
>jumps the VCO up it can take up to VCO/N cycles before we start
>correcting.  If noise slows the VCO down, it will take at least one
>cycle of Fref to get it back.
>
>So, I think the only place that we are having any semantic trouble is
>on the notion of unbounded noise.  While we likely will never see
>such a thing, the math really does tell us that an interval can go
>virtually to zero 1/oo, or last forever.  I think the important part
>of this concept is that it says that random noise ( jitter ) will
>create data errors sooner or later.  And I think doubt about that is
>where the discussion began.  The tough issue is finding the actual
>random jitter.  The value is often way overestimated because
>deterministic jittter that we have difficulty correlating gets
>incorrectly classified as RJ.  People turn the crank on the math and
>conclude that their links are 10E-12 or 10E-14 when they are really
>more like 10E-20 from an RJ standpoint.
>
>Regards,
>
>
>Steve.
>At 06:35 AM 7/4/2006, David Instone wrote:
> >Steve,
> >  Firstly, my initial response was in support of Alan's posting not
> > an attack on your reply to him.  Your definition follows that of FC
> > and other serial standards. FC defines random jitter in FC-PI-3 as
> >>jitter, random (RJ): Jitter that is characterized by a Gaussian
> >>distribution. Random jitter is
> >>defined to be the peak-to-peak value for a BER of 10-12, taken to
> >>be approximately 14 times
> >>the standard deviation of the Gaussian distribution.
> >
> >
> >  So lets look at it both ways
> >>That means that any single incremental interval can never have
> >>jitter of more than -(1UI-epsilon).
> >If that jitter is all Gaussian then hasn't it been truncated, or do
> >we have to say that it's not RJ because it's bounded?
> >
> >
> >>If on the other hand we want to integrate phase compared to some
> >>distant fixed timing reference, then a stream can theoretically
> >>precess total time interval error by an unbounded amount.
> >FC measures jitter against a timing reference derived from a golden
> >PLL.   If over any finite period of time the RJ causes the frequency
> >as seen by the PLL to change then the PLL will move the VCO, thus
> >creating a limit to the max observed RJ.  If the RJ is distributed
> >so that the frequency does not have to change then the 'single
> >incremental interval' effect will apply.
> >  Have we not then got a jitter distribution that is Gaussian in
> > form but with limits to the maximum deviations?
> >
> >Regards
> >Dave
> >
> >steve weir wrote:
> >>David,
> >>
> >>I would just like to make certain that we are talking along the
> >>same lines here.  The operation of the oscillator, no matter what
> >>its construction is causal.  So the closest that any two events can
> >>occur is epsilon.  That means that any single incremental interval
> >>can never have jitter of more than -(1UI-epsilon).
> >>
> >>If on the other hand we want to integrate phase compared to some
> >>distant fixed timing reference, then a stream can theoretically
> >>precess total time interval error by an unbounded amount.
> >>
> >>Regards,
> >>
> >>
> >>Steve.
> >>At 03:10 AM 7/4/2006, David Instone wrote:
> >>>Steve,
> >>>   I didn't disallow an infinite time between events.  I allow for
> >>> the time between events to be between 0 and infinity, but not negative.
> >>>Thus if I'm measuring the time between edges and my reference I
> >>>can measure an infinite time between my reference and a following
> >>>edge but never more than 1 UI between the last edge and my reference.
> >>>That last edge could of course be from a edge that should have
> >>>occurred an infinite amount of time in the future, but from the
> >>>point of view of the measurement it's only 1 UI early.
> >>>Regards
> >>>Dave
> >>>
> >>>
> >>>steve weir wrote:
> >>>>David, I disagree.  It does not change causality.  It changes the
> >>>>incremental delay between two events.  Imagine for a moment that
> >>>>we have a simple relaxation oscillator as the basis of our
> >>>>VCO.  In the presence of an infinitely large noise pulse, which
> >>>>is the limit for random noise, it takes an infinite amount of
> >>>>time for the ramp to reach the threshold.  The next cycle will
> >>>>not begin untilt he current cycle completes.  It may sound like
> >>>>something from Douglas Adams, but it really is mathematically and
> >>>>physically sound.
> >>>>
> >>>>Regards,
> >>>>
> >>>>Steve.
> >>>>At 01:50 AM 7/4/2006, David Instone wrote:
> >>>>>Because it makes for a nice simple clean definition.  However, I
> >>>>>believe you have to take the real world into consideration.
> >>>>>Allowing the RJ to be really unbounded means that each edge in a
> >>>>>bit stream could be advanced or delayed by an infinite amount.
> >>>>>Taken to extremes this means that  the order of  edges  could 
> be reversed.
> >>>>>This is obviously absurd, the measured time between edges can
> >>>>>reduce until it is zero, it cannot go negative.  The time
> >>>>>between edges can of course go to +ve infinity, but that isn't a
> >>>>>bit error, the system has failed or been switched off.
> >>>>>steve weir wrote:
> >>>>>>RJ really is unbounded by definition.
> >>>>>>
> >>>>>>Steve.
> >>>>>>At 09:46 AM 7/3/2006, Steven Kan wrote:
> >>>>>>
> >>>>>>>>Date: Fri, 30 Jun 2006 21:48:56 -0700
> >>>>>>>>From: Alan.Hiltonnickel@xxxxxxx
> >>>>>>>>Subject: [SI-LIST] Re: Fibre channel interconnect margins
> >>>>>>>>
> >>>>>>>>In fact, I think that companies DO ship products that toss a random
> >>>>>>>>error approximately every 10e-xx or so. Why? 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.
> >>>>>>>I suppose we're way off in the weeds, here, but is the noise actually
> >>>>>>>unbounded? Or does it just behave in a Gaussian-like manner
> >>>>>>>within the realm
> >>>>>>>of times/rates that matter for shipping product? I suppose if
> >>>>>>>I sat in my
> >>>>>>>chair for long enough, a truly unbounded system might cause a
> >>>>>>>gold bar to
> >>>>>>>pop into existence on my desk, but my empirical GBR (gold-bar rate) is
> >>>>>>>currently 0.
> >>>>>>>
> >>>>>>>------------------------------------------------------------------
> >
> >

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