RE: Estimating the magnitude of PMD
Vipul,
Point: don't wait too long before you include the clause 52 folk. We spent
some time talking about this on a teleconference today. We do be worried. To
the first order, I would assume that we would PMD directly to the DCD. This
would affect both the jitter budget and the power budget. In "z" we
discovered that 65 ps of DCD out of 800 made a huge difference. Imagine what
an additional 7 (or worse, 17) might do to "ae."
Question: why did you pick 10e-12 for your calculation? Because of the
assumption that PMD will add linearly to the jitter?
Something bothers me here, but I can't put my finger on it. I think you
might be mixing metaphors. Let me ask the question in a different way (no, I
do not know if this is the correct way to think about the problem):
statistically, what would the maximum PMD be for 99.99% of the population of
fibers (meeting some reference specification)?
jonathan
> -----Original Message-----
> From: Vipul Bhatt [mailto:vipul.bhatt@xxxxxxxxxxx]
> Sent: Tuesday, September 26, 2000 2:06 PM
> To: Equalization Ad-Hoc Reflector
> Cc: Mike Hackert; Steven E Swanson
> Subject: Estimating the magnitude of PMD
>
>
>
> Dear colleagues,
>
> The purpose of this note is to start a discussion: how significant is
> this PMD issue for us? (Yes, DMD for multimode is also on our agenda;
> it's next on my list.) We need to estimate its impact on
> equalization for
> singlemode 1550 nm link, and on the performance of an
> unequalized link.
> We need to start by quantifying the magnitude of PMD for the
> 1550 nm link
> under consideration by P802.3ae.
>
> I have done some back-of-the-envelope calculations, which I
> will describe
> here, then I will point you to a more rigorous document. To start, I
> needed to know what are the mean and variance of DGD
> (Differential Group
> Delay) PDF (Probability Density Function). I sent a note to TIA FO 2.2
> members, some of them responded, and subsequently I conducted
> an offline
> conversation with some of them. (PMD value is defined as DGD value
> averaged over all wavelengths present in a signal.)
>
> I learned that this PMD subject is still evolving, and many
> measurement
> methods are still being debated. But I needed to start somewhere, so I
> started with the following assumptions:
>
> - We can consider the PMD effect of our 1550 nm link as stochastic,
> dominated by cable. We can ignore PMD contribution of all other
> components, deterministic or random.
> - For such a link, assuming the wavelength averaging doesn't
> change the
> picture much, we can take the PMD value specified in a manufacturer's
> data sheet as the mean of the DGD PDF, which is Maxwell type.
> - In the PDF, parameter alpha can be assumed to be 1.
> - With a Maxwell PDF assumption, we can derive variance from the given
> value of mean.
> - With a desired upper limit on probability of catastrophe,
> we can derive
> the acceptable worst case value of DGD in picoseconds.
>
> Next, I confirmed that the worst case value of x (for which the
> normalized Maxwell_Probability reaches a value of 10^-12), is about 5
> times mean.
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%
> clear all;
> alpha = 1;
> mean = alpha*sqrt(8/pi)
> sigma = alpha*sqrt(3 - (8/pi))
> x = mean + 9.42*sigma
> Maxwell_Probability = sqrt(2/pi)*(x^2/alpha^3)*exp(-x^2/(2*alpha^2))
> %result: Maxwell_Probability = 10^-12
> %%%%%%%%%%%%%%%%%%%%%%%%%%
>
> The worst case value of x is 9.42 sigma away from the mean, or about 5
> times mean. So if our value of mean is 0.5 ps/sqrt(km), which is the
> value proposed by IEC E61282-3 draft, the DGD can reach 2.5
> ps/sqrt(km)
> in the worst case. For a 49 kilometer link, DGD can be 2.5*sqrt(49) =
> 17.5 ps. If I assume that DGD is approximately equal to pulse
> broadening,
> we conclude that the 10G pulse will broaden by 17.5 ps worst case.
>
> If I pick a link composite PMD value of 0.2 ps/sqrt(km) as
> given in some
> fiber data sheets, the pulse broadening will be 7 ps.
>
> My questions for the group:
>
> 1. Steve Swanson (Corning) has kindly given me a copy of the
> IEC E61282-3
> draft (Guidelines for the calculation of polarization mode
> dispersion in
> fiber optic systems). I have placed it on our website:
> http://www.ieee802.org/3/ae/public/adhoc/equal/
> If you feel up to the challenge, please wade through it, and
> tell me how
> far off my simple minded calculation is from the more accurate methods
> described in that document.
>
> 2. Once we agree on a number (pulse broadening in
> picoseconds), we need
> to decide if it is significant enough that we should alert
> our friends in
> 802.3ae who are working on defining the link specs for Clause 52,
> 10GBASE-E. And what do we recommend to them? Add this worst
> case value to
> horizontal eye closure (due to jitter, ISI, etc.)? How bad is the
> approximation that PMD horizontal eye closure will add
> linearly to that
> caused by chromatic dispersion?
>
> 3. The PMD value of 0.2 ps/sqrt(km) is from a recent data
> sheet. It has
> been suggested that fiber made prior to 1992 has higher
> values of PMD. We
> may have the option of dismissing this concern if we know that it is a
> negligible portion of the total fiber in the ground. Mike,
> can we request
> you to investigate that, and email that info to us?
>
> Thanks,
> Vipul
>
> vipul.bhatt@xxxxxxxxxxx
> (408)542-4113
>