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Hi Scott, Thanks for the response. Several key technical questions remain unanswered, and the Task Force needs closure on them before a 90ns delay ceiling can be considered.
i. Turn around – channel must settle before valid data can be interpreted
ii. Guard interval – Quite period is inserted between TX and RX bursts to prevent collisions, intersymbol overlap, and signal residuals –
https://www.ieee802.org/3/dm/public/adhoc/062625/jonsson_3dm_01_06_26_25.pdf
iii. Clock uncertainty due to noise – possible causes: CDR wander, jitter, AFE gain settling, etc.
iv. AFE/DSP switching – possible causes: TX filters, RX gain and filters, DSP must relock EQ task, etc.
i. Insertion Loss must drive link length – not delay
ii. IL budget of -23.08dB and cable IL ~0.8dB/m achieves 28.9m reach
iii. At 4-5ns/m this naturally results in
100-150ns propagation delay
iv. Therefore, delay should be derived from length and IL, not imposed first
Requests
Thanks, TJ Houck From: Scott.Muma@xxxxxxxxxxxxx <Scott.Muma@xxxxxxxxxxxxx>
Hi TJ, Thanks for your thoughtful comments, I appreciate your time and effort providing this additional information.
The presentation did not claim that higher link segment delays are “out of scope” but made the point that the data presented and available so far support that the vast majority of automotive link segments (including within commercial vehicles)
are <=15m. The PAR does not state “up to at least 15 m”, but through consensus the 802.3dm task force approved the objective “up to at least 15m reach on at least one type of automotive cabling”. I would agree that this means greater than or equal
to 15m on at least one type of automotive cabling in automotive operating conditions is the minimum. I believe the 15m objective was established through robust discussion, consensus building, and data sharing, including some on the reflector which is consistent
with some of the points you make below. I get the sense that similar information to what you provide below was available when establishing the 15m objective, but if that was not the case or the group wishes to update the consensus objectives that is welcome.
You argue below for using a minimum of 32m of 5ns/m cable to establish the maximum propagation delay, and going well beyond (>2x) the objective is great in the case that it doesn’t lead to suboptimal results for the vast majority.
As discussed in the presentation a link delay of 90ns allows >15m for cables with 5ns/m propagation delay and >20m for cables with 4.3ns/m propagation delay. So 90ns does not put a ceiling at 15m. Margin for TDD turnaround, EMC filtering,
PoC filter delay are on top of the link segment delay, so do not need to be included in the 90ns (not that you said they are, just to be clear).
I expect the ACT proposed protocol can support links >15m with a propagation delay of 90ns, so a propagation delay of 90ns doesn’t impose any changes on ACT. The current TDD protocol proposal would not have overlap between the HS and LS
bursts with a 90ns delay because of the chosen parameters, while a much larger delay would lead to overlap. So the proposed 90ns link segment delay works for both ACT and TDD and allows both to meet the approved objectives. If the ACT proposal specifies
a much higher link segment delay it would still meet the objectives, but I wouldn’t be able to say what that limit could be. If consensus can be reached that higher link segment propagation delays or longer cables is an objective of the task force, or a necessary specification in the draft text, then new TDD parameters could be proposed and studied.
Thanks for clarifying your goal. My goal is to propose specifications that will allow both ACT and TDD to be optimized and meet the approved objectives. If the approved objectives are inadequate vs. the physical reality or scope definition,
or no longer the consensus, I’m happy to see that amended so we can once again have a common goal. Best Regards, Scott From: TJ Houck <TJ.Houck@xxxxxxxxxxxx>
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Hi Scott, Thanks for the presentation today. I want to address the recurring claim that longer-reach or higher-delay channels are “out of scope” or represent a market too small to justify inclusion. I think this characterization is inaccurate.
The OICA commercial-vehicle segment—which explicitly includes light-commercial vans, heavy trucks, and buses—represents tens of millions of vehicles globally each year. These platforms use camera and sensor links with harness lengths that
can exceed 15meters and one-way delays of >90 ns, even in baseline configurations. These are in-scope automotive applications, not niche prototypes. The IEEE 802.3dm PAR states “up to at least 15 m,” establishing a minimum, not a ceiling. Nothing in the PAR prevents support for longer channels when the insertion-loss and EMC budgets remain valid. Designing for only 15m assumes away
real, measurable vehicle topologies already in production. 1. The technical issue remains unresolved As Ragnar noted
Relationship between TDD IBG , TDD proponents have not addressed the fundamental problem. Instead of reconciling this with actual vehicle geometries, the conversation keeps shifting to “market adequacy” (“few vehicles need longer runs”). That framing doesn’t
eliminate the electrical risk. It only changes the topic. 2. The relationship between insertion loss and delay is being reversed. This is not a “delay-limit” problem but a channel-design problem. The standard should not constrain insertion loss because of link delay — it should define insertion-loss first, and let the resulting delay follow from the physical channel. When delay is capped at 90 ns, we are effectively forcing an arbitrary length ceiling, reducing the permissible IL margin for longer-body vehicles. The spec should instead allow full IL cable lengths. 3. Why 160 ns is the technically defensible number? Automotive coax/STP VF ≈ 0.66 – 0.78 c (≈ 5 – 4.3 ns/m). Some cabling can be <0.66 velocity 160 ns ÷ 5 ns/m ≈ 32 m of cable reach or shorter cable with a lower Velocity factor cable. This aligns with realistic routing in the upper end of OICA commercial classes (e.g., articulated buses ≈ 18 m, tractor + trailer ≈ 16 m + coupling ≈ 24–28 m effective path). A 160 ns budget therefore covers the complete OICA “commercial vehicle” range — LCV, heavy truck, and bus — while preserving adequate guard margin for TDD turnaround, EMC filtering, and PoC filter delay. 4. Public data contradicts “15 m is enough” Passenger cars already reach 12 – 13 m: Krieger (VW Group) presented Typical Automotive Harness Topologies to 802.3ch with 0.5 – 12.5 m total channel lengths for sedans/compacts. As stated earlier 15meters is the
minimum, not the ceiling. Long-body LCVs: adding roof-rail, high-roof A-pillar, cross-dash, service loops, and inlines yields ≈ 16 – 19 m which could lead to delays >90ns Breakdown
Total additional over Sedan = 4-6meters = 16.5 to 18.5meters Trailers/attachments:
Public references (all open):
https://grouper.ieee.org/groups/802/3/ch/public/mar18/krieger_3ch_01a_0318.pdf
https://www.indexbox.io/blog/trailer-and-semi-trailer-world-market-overview-2024-1/
https://www.mbvans.com/en/upfitter/tech-info/bulletins My goal in raising this is to ensure that the Task Force bases its delay assumptions on physical reality and scope definition, not on an unverified estimate of “market adequacy.” Best Regards, TJ Houck Infineon Technologies Americas Corp. – Detroit
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