Re: [802.3_NGECDC] 4/16 NEA Call - Reminder
Hi Shawn and Denny,
I note the proposal within your presentation <http://www.ieee802.org/3/ad_hoc/ngrates/public/calls/19_0416/nicholl_nea_01_190416.pdf> to change the specification of the timestamp reference point in IEEE Std 802.3 to the first symbol after the SFD. I have a couple of comments in relation to the impact this would have on existing implementations.
This first comment is that PHYs consist of multiple MMDs (see IEEE Std 802.3-2018 Figure 45-1) and that path data delay registers are provided on a per-MMD basis (see IEEE Std 802.3-2018 subclause 90.6). It seems to me that this proposal could result in MMDs in the same PHY using different timestamp reference points for the values in their path data delay register values depending on which timestamp reference point the particular MMD uses.
An example of this would be a PHY with pluggable module interface, the onboard MMDs may use the beginning of the SFD as the timestamp reference point to set their path data delay register values as they are older, yet the MMDs on pluggable module might use the beginning of the first symbol after the SFD as the timestamp reference point to set their path data delay register values as they are new. I wondered if the impact of the combination of the MMDs using two different timestamp reference points to set their path data delay register values has been considered.
The second comment is that I don't believe we can just change the subclause 90.7 text as suggested as that would make all existing implementation non-complaint. Instead, we would need to find an approach that would grandfather in existing implementations.
Based on the approach that has been used in the past I would suggest that the existing timestamp reference point be redefined in Clause 90 as one option, and the new timestamp reference point be defined as another option in Clause 90. Finally, a statement would be added to Clause 90 to state that use of the new timestamp reference point is highly recommended due to its alignment with the timestamp reference point used in IEEE Std 1588-2008 and IEEE Std 802.1AS-2011.
We would then need to define a way for an MMD to identify which option it uses. I'd, therefore, suggest that some of the 14 currently reserved (Value always 0) bits in the TimeSync (MMD) capability registers be redefined as version bits. A value of 0 would indicate that the timestamp reference point was the beginning of the SFD, a value of 1 would indicate that the timestamp reference point was the beginning of the first symbol after the SFD.
Best regards,
David
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From: John DAmbrosia [mailto:jdambrosia@xxxxxxxxx]
Sent: 15 April 2019 23:09
To: STDS-802-3-NGECDC@xxxxxxxxxxxxxxxxx
Subject: [802.3_NGECDC] 4/16 NEA Call - Reminder
All,
Reminder of tomorrow's call on PTP Timestamping Clarifications. The call is from 10am to 11:30am EDT. Call info may be found at http://www.ieee802.org/3/ad_hoc/ngrates/email/msg00590.html.
Please note that i have received a request for an additional presentation from Shawn Nicholl. I will be updating the webpage and will send out a notification when completed.
Reminder - All ad hoc teleconference participants should review the following documents prior to participation in an ad hoc teleconference:
. IEEE Pre-PAR patent policy: https://development.standards.ieee.org/myproject/Public/mytools/mob/preparslides.pdf
. IEEE 802 participation policy: https://mentor.ieee.org/802-ec/dcn/16/ec-16-0180-05-00EC-ieee-802-participation-slide.pdf
Regards,
John D'Ambrosia
Chair, IEEE 802.3 NEA Ad hoc
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