[SI-LIST] Re: Fibre channel interconnect margins

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: David Instone <dave.instone@xxxxxxxxxx>
  • Date: Tue, 04 Jul 2006 15:40:03 -0700

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.


At 06:35 AM 7/4/2006, David Instone wrote:
>  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?
>steve weir wrote:
>>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.
>>At 03:10 AM 7/4/2006, David Instone wrote:
>>>   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.
>>>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.
>>>>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.
>>>>>>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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