"Using the BER as the benchmark for acceptance (something like 10e-12)" I started the whole discussion based on the above statement. And I agree with what you say that no company in its right mind would = ever reveal their BERT. But have you ever consider what those sigma will = look like at PCIe Gen II or 10G ethernet (pick a BERT between 10e-12 to = -19 as you pointed out). At 1 or 2G rate, you can live with a few ps of = non-existing Rj but not at 10G, it will eat your margin alive. -----Original Message----- From: Paul Levin [mailto:levinpa@xxxxxxxxxxxxx] Sent: Wednesday, July 05, 2006 1:50 PM To: Chris Cheng Cc: weirsi@xxxxxxxxxx; David Instone; si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Fibre channel interconnect margins Dear Chris, You've asked this question before and not received what you consider a=20 satisfactory answer. Clearly it has to be better than 1e-12; almost as=20 clearly, it has to be worse than 1e-19 (1 error after thirty years at 1=20 Gbps if my math serves me correctly.) Have you ever considered that (in my humble opinion) no company in its=20 right mind would ever reveal what their number was since its competitors = would then try to use that information against it? Regards, Paul Levin Xyratex _____________________ Chris Cheng wrote: > Steve, > I start off by asking the same question before, "does your company = ship =3D > products with 10e-12 error rate ?" and it seems it comes down to the = =3D > following answers : >=20 > a) "Its a fact of life, I'll try to keep it to 10e-x13,14,15 or = better." >=20 > Well, I still want to know what that number is ? And when you have a = =3D > customer that has a peta byte of installation, how does that translate = =3D > into actual errors per second in your FCAL ?=3D20 >=20 > b) It may happen in some part of the link but overall the link can =3D > tolerate it >=20 > In particular, >=20 >>If over any finite period of time the RJ causes the frequency=3D20 >>as seen by the PLL to change then the PLL will move the VCO, thus=3D20 >>creating a limit to the max observed RJ. If the RJ is = distributed=3D20 >>so that the frequency does not have to change then the 'single=3D20 >>incremental interval' effect will apply. >> Have we not then got a jitter distribution that is Gaussian in=3D20 >>form but with limits to the maximum deviations? >=20 >=20 > I don't know about FCAL jitter tolerance spec but let's take the OC-xx = =3D > spec as a reference, what happen when the jitter spectrum passes ft ? = Is =3D > the jitter still bounded ? > To take it out to a non-ideal world, how does one trade off jitter =3D > tolerance (for example, by Rj and Dj) and jitter accumulation (for =3D > example, by PLL supply noise) ? You only have one choice of loop =3D > dynamics in your PLL and you can not help one without hurting the = other. =3D > And I would bet jitter accumulation from PLLVDD dwarfs the concern = over =3D > jitter tolerance. Just look at how elaborate those PLLVDD distribute = =3D > schemes we have (isolation traces, filters etc). Can you still say = your =3D > Rj is unconditionally bounded by such PLL's ? >=20 > c) 10e-12 happens only in methodology, you are really running a BERT = =3D > much much lower, possibly beyond the life of the system or the = component =3D > or reasonable measurement technique >=20 > Which, I think, is essentially what you are saying in the last = sentence. =3D > Do we really claim a 10e-12 simply because we got an erroneous Rj from = =3D > an instrument in a Dj dominated system, multiply the sigma to 14 or 15 = =3D > times per the BERT spec and scare ourselves to such error rate ?=3D20 > And before the instrument guys jump on me again, let me be clear that = I =3D > don't think the instrument is at fault but rather the problem lies on = =3D > the application (Dj dominate system measurement) that force the =3D > instrument to report back a number that it cranked out based on some = =3D > fixed assumptions not suited for such situation. >=20 > For those who claim a), my hats off to you because you are a braver = man =3D > than me to publicly say that. >=20 > For those who claim b), I sure would like to know what kind of PLL you = =3D > have >=20 > And my money will be on c). >=20 > -----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 >=20 >=20 > David, I did not regard it as an attack just an opinion that is=3D20 > different and worth discussing. >=20 > The basis of our disagreement appears to be in the definition of=3D20 > bound. I look at things from the standpoint of electrical=3D20 > noise. Time interval in a timing circuit is the result of the=3D20 > magnitude of some electrical quantity, and is always causal, each=3D20 > event defining a new interval follows the previous. This means = that=3D20 > noise effectively multiplies the interval by some factor 1/oo <=3D3D = K=3D20 > <=3D3D oo. Jitter is still unbounded, but every incremental interval = has=3D20 > a positive duration. >=20 > So far we have been talking about noise in the oscillator itself. >=20 > Now, let's see what a PLL does to this quagmire. If noise = hammers=3D20 > the VCO then the PLL feedback loop applies gain to divide the = effect=3D20 > of the noise. If we still believe in infinity, infinity divided = by=3D20 > anything is still infinity. In practice will the oscillator stop = for=3D20 > an unlimited time? It will only when it fails. On the other = end,=3D20 > two successive pulses can occur essentially on top of each other. >=20 > A receiver PLL will take a finite amount of time to realign within = a=3D20 > fixed amount of phase to the jittered stream for the case of the=3D20 > oscillator event, and will take a different, much longer amount = of=3D20 > time to align to the short term frequency offset that noise in = the=3D20 > PLL error amplifier causes. The phase error between the source=3D20 > stream and the recovered clock in the latter case generally follows = a=3D20 > classic 2nd order step response. The golden PLL is a PLL with=3D20 > specific frequency response and damping. Even if we have a PLL = that=3D20 > uses N=3D3D1, the PLL only starts correcting after a timing error = is=3D20 > already apparent. For a timing error of sufficient magnitude = data=3D20 > moves outside the timing window, a data recovery error is = guaranteed,=3D20 > and no PLL is going to prevent that. A nasty little problem that=3D20 > gets into systems is power supply noise coupled into the VCO = and/or=3D20 > error amplifier by one means or another. For systems with high Ns = it=3D20 > can get really ugly. >=20 > On a slightly different tack, for a PLL using a PFD, the unit=3D20 > interval is that at the phase comparator input which is VCO/N or=3D20 > Fref. Noise whacking the error amplifier will push the VCO off=3D20 > frequency until new information arrives to get it back. If the = noise=3D20 > jumps the VCO up it can take up to VCO/N cycles before we start=3D20 > correcting. If noise slows the VCO down, it will take at least = one=3D20 > cycle of Fref to get it back. >=20 > So, I think the only place that we are having any semantic trouble = is=3D20 > on the notion of unbounded noise. While we likely will never see=3D20 > such a thing, the math really does tell us that an interval can = go=3D20 > virtually to zero 1/oo, or last forever. I think the important = part=3D20 > of this concept is that it says that random noise ( jitter ) will=3D20 > create data errors sooner or later. And I think doubt about that = is=3D20 > where the discussion began. The tough issue is finding the = actual=3D20 > random jitter. The value is often way overestimated because=3D20 > deterministic jittter that we have difficulty correlating gets=3D20 > incorrectly classified as RJ. People turn the crank on the math = and=3D20 > conclude that their links are 10E-12 or 10E-14 when they are = really=3D20 > more like 10E-20 from an RJ standpoint. >=20 > Regards, >=20 >=20 > Steve. > At 06:35 AM 7/4/2006, David Instone wrote: >=20 >>Steve, >> Firstly, my initial response was in support of Alan's posting = not=3D20 >>an attack on your reply to him. Your definition follows that of = FC=3D20 >>and other serial standards. FC defines random jitter in FC-PI-3 as >> >>>jitter, random (RJ): Jitter that is characterized by a Gaussian=3D20 >>>distribution. Random jitter is >>>defined to be the peak-to-peak value for a BER of 10-12, taken = to=3D20 >>>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=3D20 >>>jitter of more than -(1UI-epsilon). >> >>If that jitter is all Gaussian then hasn't it been truncated, or = do=3D20 >>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=3D20 >>>distant fixed timing reference, then a stream can theoretically=3D20 >>>precess total time interval error by an unbounded amount. >> >>FC measures jitter against a timing reference derived from a = golden=3D20 >>PLL. If over any finite period of time the RJ causes the = frequency=3D20 >>as seen by the PLL to change then the PLL will move the VCO, thus=3D20 >>creating a limit to the max observed RJ. If the RJ is = distributed=3D20 >>so that the frequency does not have to change then the 'single=3D20 >>incremental interval' effect will apply. >> Have we not then got a jitter distribution that is Gaussian in=3D20 >>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=3D20 >>>same lines here. The operation of the oscillator, no matter = what=3D20 >>>its construction is causal. So the closest that any two events = can=3D20 >>>occur is epsilon. That means that any single incremental = interval=3D20 >>>can never have jitter of more than -(1UI-epsilon). >>> >>>If on the other hand we want to integrate phase compared to some=3D20 >>>distant fixed timing reference, then a stream can theoretically=3D20 >>>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=3D20 >>>>the time between events to be between 0 and infinity, but not =3D >=20 > negative. >=20 >>>>Thus if I'm measuring the time between edges and my reference I=3D20 >>>>can measure an infinite time between my reference and a = following=3D20 >>>>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=3D20 >>>>occurred an infinite amount of time in the future, but from the=3D20 >>>>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=3D20 >>>>>incremental delay between two events. Imagine for a moment = that=3D20 >>>>>we have a simple relaxation oscillator as the basis of our=3D20 >>>>>VCO. In the presence of an infinitely large noise pulse, = which=3D20 >>>>>is the limit for random noise, it takes an infinite amount of=3D20 >>>>>time for the ramp to reach the threshold. The next cycle will=3D20 >>>>>not begin untilt he current cycle completes. It may sound = like=3D20 >>>>>something from Douglas Adams, but it really is mathematically = and=3D20 >>>>>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=3D20 >>>>>>believe you have to take the real world into consideration. >>>>>>Allowing the RJ to be really unbounded means that each edge in = a=3D20 >>>>>>bit stream could be advanced or delayed by an infinite amount. >>>>>>Taken to extremes this means that the order of edges could be = =3D >=20 > reversed. >=20 >>>>>>This is obviously absurd, the measured time between edges can=3D20 >>>>>>reduce until it is zero, it cannot go negative. The time=3D20 >>>>>>between edges can of course go to +ve infinity, but that isn't = a=3D20 >>>>>>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 = =3D >=20 > random >=20 >>>>>>>>>error approximately every 10e-xx or so. Why? Because the =3D >=20 > statistical >=20 >>>>>>>>>theory behind those errors is that random/Gaussian noise is, by >>>>>>>>>definition, unbounded - errors are a fact of life, even if the = =3D >=20 > error >=20 >>>>>>>>>rate is very low. >>>>>>>> >>>>>>>>I suppose we're way off in the weeds, here, but is the noise =3D >=20 > actually >=20 >>>>>>>>unbounded? Or does it just behave in a Gaussian-like manner=3D20 >>>>>>>>within the realm >>>>>>>>of times/rates that matter for shipping product? I suppose = if=3D20 >>>>>>>>I sat in my >>>>>>>>chair for long enough, a truly unbounded system might cause = a=3D20 >>>>>>>>gold bar to >>>>>>>>pop into existence on my desk, but my empirical GBR (gold-bar = =3D >=20 > rate) is >=20 >>>>>>>>currently 0. >>>>>>>> >>>>>>>>-----------------------------------------------------------------= -=3D >=20 >=20 >> >=20 > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >=20 > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list >=20 > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >=20 > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ >=20 > List technical documents are available at: > http://www.si-list.org >=20 > List archives are viewable at: =20 > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 >=20 >=20 >=20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu