Dear 802.3cy colleagues,
I am considering proposing a smaller limit on the maximum propagation delay of the channel. With this email, I outline my motivation and justification for this new limit and seek your opinion on this change.
Motivation:
A maximum on propagation delay also provides a limit on the round trip delay of the echo signal. This, in turn, provides a limit on the length of echo pulse response and the span of the echo canceller. A reduction in maximum propagation delay is beneficial as it results in reduced complexity of the echo canceller.
Justification:
The limit on maximum propagation delay was updated to 69 ns in the last review cycle. Since then, I have looked into a relatively large number of cable measurements and calculated the propagation delay. These measurements are from a diverse set of link-segments:
- a variety of cable lengths from very short to very long
- different segment lengths and various number of connectors
- cold/room/hot temperatures
- different cable manufacturers
- both 802.3ch and 802.3cy-grade cable types
My observation from this data set is that the propagation delay never exceeds 4.4 ns/m, and never exceeds 48 ns for all 11 m cable assemblies.
Proposal:
Given this, I think a limit of 55 ns for maximum propagation delay should be a reasonable number. It provides roughly 15% headroom and margin with respect to my data set. It also results in 20% reduction with respect to the current number in the draft which is a significant reduction in echo canceller span.
I have already contacted some of our colleagues representing cable and connector manufacturing. So far, I have not heard anybody objecting to this proposed number. I would like to open up this proposal to everybody in the group to check if there is any objection to this proposal.
Look forward to hearing your opinion on this matter.
Regards,
-Hossein
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