## [SI-LIST] Re: Fibre channel interconnect margins

• From: David Instone <dave.instone@xxxxxxxxxx>
• To: steve weir <weirsi@xxxxxxxxxx>
• Date: Tue, 04 Jul 2006 14:35:26 +0100

```Steve,
Firstly, my initial response was in support of Alan's posting not an
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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