Hello,
Our objective has been to get everything we can out of one waveform acquisition, which was why I made the comment against 3.3 to allow the SSPRQ pattern to be allowed for the transition time tests. We acquire the entire SSPRQ data pattern
and from that can extract Outer OMA and Outer extinction ratio from the specific bit sequences these measurements require (7 threes and 6 zeroes). We can now do something similar with the transition time tests, but for different sequences in the SSPRQ (33333000000
and 00000333333). For the TDECQ metric, the same waveform is re-used, with the entire sequence effectively composed as an eye diagram. So if an eye mask is required, it could be applied to this one acquisition, meaning there would be virtually no increase
in test time. The mask could be applied to either the equalized or unequalized eye. This is based on all tests being done with the TDECQ reference receiver bandwidth of 0.5 FBaud (13.28 and 26.56 GHz). If a different bandwidth is required for any measurement,
a separate waveform acquisition is required along with a significant increase in test time, so for the sake of test efficiency it is desirable to define all the measurements and actual spec values using the same instrument bandwidth.
-Greg
From: Ali Ghiasi [mailto:aghiasi@xxxxxxxxx]
Sent: Monday, July 16, 2018 6:00 PM
To: STDS-802-3-50G@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_50G] .3cd updates and plan for Tuesday
Hello David
Given that there is no need to apply the eye mask to region 1, 2, and 3 ( PAM eyes 1, 2, 3) and all we need is overshoot into region 5 and undershoot into region 4 the positive and negative edges used for rise time measurement
could be used also to measure overshoot and undershoot. This will not require separate eye mask alignment and test!
We may also want to consider filter BW of 0.74xBaudrate for overshoot/undershoot for better representation.
Since we already added a transition time measurement at particular points (one for the positive edge and one for the negative) in the SSPRQ pattern, I think it would be easier to combine an overshoot/undershoot mask with that test. This
avoids the need to capture a separate eye diagram. Do any of the scope vendors have opinions on that idea?
I do agree with your overall assessment! If the intention is to limit overshoot from nasty transmitter the most direct method would be an eye mask.
In case of PAM4 we could define eye mask regions as:
Region 1 - lowest PAM eye
Region 2 - middle PAM eye
Region 4 - region below lowest PAM eye
Region 5 - region above upper PAM eye
Overshoot in region 1, 2, and 3 already covered by TDECQ, given that eye mask test only need to apply to region 4 and 5 the test is straight forward!
Thanks,
Ali Ghiasi
Ghiasi Quantum LLC
I received several questions last week as to how upper and lower limits of an eye mask limit overshoot.
Enclosed is a 10G eye, mask and mask margins pulled off the web (apologies Pavel; it’s the first one that came up in the search). By closely squinting, notice that the the middle mask is numbered 1, upper mask is numbered 2, and lower mask
is numbered 3. The solid gray is the mask and the light gray is the mask margin. The 2 and 3 mask margin keeps the eye nearly flat. The 2 and 3 normative mask allows considerably overshoot. This can be superimposed on the
problem eye presented to the TD this week, and it’s evident how masks 2 and 3 determine the amount of overshoot. If only 2 and 3 masks are used, either a NRZ or PAM4 eye can be used. Mask 1 can be used
to limit slow edges of an NRZ eye.
Mark’s expectation in his latest email is that any further changes to the cd draft will be restricted to refinements. As we start deploying 100G per lane optics, we will learn a lot more and substantive changes are sure to come. Since volume
deployment is several years away, we have the time to get it right. If we are too late for cd, we can consider other forums or future IEEE activities.
We will discuss with scope makers a couple of mask implementation questions. What are the best 2 and 3 masks for overshoot, and what’s best for limiting edges: 1) direct rise/fall time of step transition or 2) mask 1 hits of at-speed PRBS.
If there is interest, we can present this in cd, otherwise in other forums.
Since we adopted PAM4 modulation for 50Gb/s and 100Gb/s wavelengths, the 802.3 TFs had to develop a new set of interoperable specifications, handicapped by lack of experience with PAM4 optics. We have made a lot of progress and the current
set of specifications is converging. We have refined these specifications as we have gained experience with 50G PAM4 optics, and very recently with first 100G PAM4 optics.
We are now about to add an entirely new transmitter penalty; TDECQ - 10Log10(Ceq) ≤ TDECQ (max) (http://www.ieee802.org/3/cd/public/July18/mazzini_3cd_01d_0718.pdf#page=12)
with which the industry has zero experience, based on an entirely new abstraction, with no body of measurements, and a vague understanding of actual limitations on transmitters. TDECQ was also a novel TX penalty, the difference being that we have been living
with TDECQ for several years and have gained insight based on great deal of analysis and measurements.
It is worth taking a step back to briefly review history of TX specifications. The general idea is simple. We want the transmitter eye open, with no or low errors when detected by expected range of receivers. The approach has been two types
of specifications: 1) time domain waveform limitations, and 2) BER limitations. Examples of waveform limitations are ER, noise (like Rin) and most importantly eye mask, all into a defined receiver. The BER limitation has been expressed as a penalty against
a good (ideal) transmitter into a specified receiver.
In succeeding optics specifications we have reacted to the limitations of previous generations. Zero hit eye mask was replaced by stat. eye, based on the realization that sitting for many minutes (hours) in front of a DCA, waiting for as
single hit in an eye mask is not the best use of time. BTB and over fiber loopback penalty was replaced by penalty into a reference receiver realizing that tuning a transmitter and receiver to each other does not guarantee interoperability with other vendors
transmitters and receivers.
Which brings us to PAM4. A traditional eye mask was found not useful because a closed eye can be opened by receiver equalizer. We attempted something new, a transmitter specification based primarily on a TX penalty, with only auxiliary
waveform limitations including ER and Rin. Over time we found this was not enough. In our lab, our system test engineer tasked with verifying 50G PAM4 optics (LR8) would periodically triumphantly announce that he found a TX waveform that met TDECQ but broke
the receiver complying with SRS.
Multiple 802.3cd contributors started reporting limitations of TDECQ in guaranteeing interoperability and an understanding developed that we need to supplement TDECQ. Broadly what was required is a limit on TX being too slow, too fast,
and too noisy. In past specifications this was done by the eye mask. Very simplistically, the inner eye mask limited excessively slow or noisy transmitters, and the upper and lower eye limits limited overshoot
or excessively fast transmitters.
I proposed rise/fall time lower limits to restrict slow transmitters, and overshoot limits to restrict fast transmitters. The rise/fall time limit was adopted, but the overshoot limit was recognized as problematic. In hindsight, an eye
mask for a simplified waveform, like NRZ, or half rate should have been proposed. There was no need to reinvent the wheel, and I wish I came to this realization sooner. The eye mask is just fine, and all that is needed is a method to get around the PAM4 eye
closure.
Pierce was one of those who identified the problem with TDECQ as the sole TX limitation. As part of illustrating the problem, he used the concept of TDECQ map which plotted TX penalty variants against each other. As a result this led him
to propose various complex TX penalty variants as the fix. The second TX penalty about to be adopted by the TF derives directly from viewing the problem through the prism of the TDECQ map.
While the TDECQ map is a way to look at transmitter performance, its applicability is limited. It’s not what would routinely be used to characterize or compare transmitters in the lab.
The most glaring example of why this is not broadly applicable is the example Pierce has used in support of adopting the second TX penalty spec.
In reverse order, a waveform is presented which is problematic:
Everyone can understand this. This waveform is too peaky. It would likely have hits in the upper and lower limits of an eye mask.
This simple problem has been transformed into an abstraction, TDECQ map, which few in the TF seem to understand, at least based on the discussion in today’s TF meeting:
Given this reformulation of a simple problem into a complex abstraction, it is understandable why an equally complex and abstract solution is offered. If adopted, we will be left with an unprecedented standard with two TX penalty limits.
And this complex, novel approach is being done to solve a problem which has been solved in all previous optical specifications with time domain limits, like eye mask.
Before embarking on this unchartered journey we should invest a lot more effort to confirm that what has worked in the past, will not work now. One approach is to replace the rise/fall time limit with eye mask for NRZ waveform or half rate
waveform. There may be other time domain limits which work equally well. But a second TX penalty is not one of them.
A reminder that we will start @ 8am on Tuesday
First order of business will be to review the two liaison letters (now posted) and then continue the comment resolution
The latest version of Matt Brown’s working document is posted for your review.
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