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[802.3_SPEP2P] Preemption and 10BASE-T1S half-duplex

Hi There,


Based on the straw poll that we had in the last meeting and other exchanges, I think it’s clear some people believe the changes needed to enable frame preemption (Clause 99 - MAC Merge sublayer) on half-duplex point-to-point 10BASE-T1S may be difficult/complex and/or require significant time from the group.


Based on my evaluation, I don’t agree, but I’m happy to be persuaded if there are things I missed. I’d ask people to look at my presentation on the topic from April 28th ( and let me know what, if anything, you think I missed. If I didn’t miss anything, I’d like to understand where we differ in our conclusions.


Our scope is 802.3, and we provide primitives or services that can be “consumed” by inside the MAC, or by MAC users (e.g., 802.1Q compliant bridges). As I showed in my presentation, half-duplex can be supported in the MAC Merge sublayer using the existing set of primitives that we already have defined and use to implement MAC Merge.


Just as with full-duplex, I propose that the transmitter can only preempt its own transmissions (not the link partners). Preemption is an optional feature and can only enabled if both link partners support it (see “99.4.2 Determining that the link partner supports preemption”).


Based on my analysis of the 802.1 specification for using frame preemption (802.1Qbu 2016 since included in 802.1Q 2018), 802.1 Frame preemption makes no distinction between full-duplex and half-duplex.


An example use case for preemption with half duplex is interrupting routine traffic with long Ethernet frames (e.g., uploading logs, or a downloading a firmware update) to reduce the delay to transmit more urgent control traffic (aka express traffic).


Making the simple and well-contained changes in my presentation should accomplish this, without, I believe, a long open-ended discussion.




Peter Jones               Cisco Systems           

Distinguished Engineer    170 West Tasman Dr.

Enterprise Networks       San Jose, CA, 95134, USA.

Wrk: +1 408 525 6952      Mob: +1 408 315 8024

Email:                    petejone at





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