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Dear 802.3dj Optics Track Participants,
During the September interim, in multiple conversations, the question came up why Comment #399 was resolved in a joint electrical optical track meeting. After all this is a purely optical spec, which has no effect on any electrical link because the receiver
is fully retimed.
Today, during an informal off-line SMF consensus building discussion today, we learned the reason why. It was pointed out that transmitter jitter has an effect on DSP design, and DSP is electrical.
While this is absolutely true, it is not how we should determine which track specs belong in. This should be determined by what link they effect. Specs that effect only electrical or optical links should be resolved in separate electrical or optical tracks,
respectively. Specs that effect both should be resolved jointly.
Further, almost every spec in the Transmitter Characteristics table has electrical circuits associated with it, like Drivers, or effects electrical circuits in the receiver, like TIA. The only two specs that I found that could be argued not to effect electrical
circuits are Optical return loss tolerance and Transmitter reflectance. If we were to consistently apply the rule that if a spec has effect on an electrical circuit, even if only used for optical functions, then there is no need for an optical track, or optical
comment resolution. Everything should be considered in a joint electrical/optical track. Viewing this as unreasonable is not naive, as was characterized today, but rather common sense.
Hopefully during the November Plenary optical jitter will be correctly assigned to the optical track for resolution, since its only effect is on the optical link. If not, then hopefully a better and more consistent rationale will be given than the above.
Thank you
Chris
From: Chris Cole <chris.cole@xxxxxxxxxxxx>
Sent: Monday, September 22, 2025 1:28 PM To: STDS-802-3-B400G-OPTX@xxxxxxxxxxxxxxxxx <STDS-802-3-B400G-OPTX@xxxxxxxxxxxxxxxxx> Subject: [802.3_B400G_OPTX] Transmitter testing
Dear 802.3dj Optics Track Participants,
During last week's Interim meeting, Comment #399 against D2.1 was resolved by adding a jitter test to Transmitter compliance. This was unfortunate for a number of reasons, which a group of optics experts captured in a post deadline presentation.
The technical discussion around the presentation was cut short, so we have requested from the Optics Track Chair that we find time to continue the discussion.
There are many reasons why adding a jitter test in the manner in which it happened is problematic, and two stand out.
This issue has been discussed in multiple Optical Track calls over the past year and it has had no support from the optical community, as exemplified by lack of optics supporters on the comment. During comment #399 resolution, multiple optics experts spoke
against adding this test, and no optics experts spoke in favor.
802.3 is part of a greater ecosystem. We need to have mutual respect for each other's expertise and 802.3 forcing a test on a segment of the industry that does not want it, only undermines 802.3 credibility. Regardless of whether this test is part of the standard,
those that build and deploy optics are not going to use it. As it is, optics testing is expensive, and increasing the cost without a clear benefit is not going to happen.
Jitter problems are captured by the recently added functional test. In fact, comment #399 proposal showed this by using a BER test to illustrate jitter issues. If BER test is the ultimate arbiter of jitter problems, why bother with an additional indirect test?
During the brief discussion of the above presentation, concerns were raised about the ability of a functional test to catch jitter issues, for example the functional receiver having a pathological jitter tracking bandwidth. We need a forum to talk through
these concerns. Although in this case the answer is the same as a doctor gives a patient with headaches who bangs his head against the wall: don't do it.
Towards the Nov. the Plenary meeting, we will be working on a presentation to remove jitter testing from transmitter compliance and asking for broad optics industry support.
Thank you
Chris
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