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All – Thank you for the good discussion on the ‘duplex objective’ at today’s ISAAC meeting. From where I sit, the risk of changing the wording on the duplex objective is that we create more questions in peoples minds than we create clarity. It seems there is clarity in the study group that everyone involved considers an objective
“support full duplex only” to relate to the MAC. Mr. Law suggested that we would be well served to use the same wording unless we meant something different than in the past. I agree that this is a tried and true path to success. You might think a new wording
is clearer, but a change in wording that is well known will generally be taken by the community that you mean something different than what was made before – not that you are refining existing wording. Additionally, while some of the discussion was about the ‘definition’ of full duplex, inferred from descriptive text, it is interesting to note that in IEEE Std 802.3, the term “full-duplex” is actually defined. It can be found in the
definitions section of the standard (1.4.345), and it is defined relative to a network, DTE, or MAU (but not a PHY). The definition as used in 802.3 refers to simultaneous communication “between a pair of stations” (which means not necessarily at the MDI
or even the MII) “provided that the Physical Layer is capable of supporting simultaneous transmission and reception without interference.” No other reference. This would be consistent with a variety of duplexing techniques at the PMA/PMD used in 802.3, including
echo cancelling, FDD (including WDM) and using separate media (Spatial Division Duplexing).
As recently as 2021 there was a discussion confirming this in 802.3ct – that separate unidirectional WDM streams would comprise meeting the “support full-duplex only” objective, agreeing with the above. Based on David’s comment about using common-used wording, I went off on an exercise to review the language of duplexing objectives in all the 802.3 projects since the decision to go ‘full duplex only’ at 802.3ae (10G), a little over 20
years of history. I looked at PHY projects, new rate and new protocol projects. I found that maintenance projects (including corrigenda), power projects, and projects that did not address the physical layer generally had no duplexing objectives, as expected. What I found in the remainder was a great deal of consistency: - 32 projects with symmetric data rate PHYs (all but 2) either used the language “Support full duplex only” (28, sometimes with a hyphen sometimes not) , or referenced using existing PCS or other sublayers (4),
- Projects with more than one duplex mode used and more than one PHY (802.3cg, for example) called out the duplex mode separately on the PHY – these were all symmetric PHYs. - There probably should be one more “Support full duplex only” (802.3dg – I hadn’t realized I personally had forgotten this…) - One symmetric PHY project (10GBASE-CX4) did not mention duplex mode. - EEE (802.3az) called out the (MAC) duplex mode of phys referenced (full duplex), and refers to full-duplex 100BASE-TX, clearly the level of duplexing at the DTE.
- Asymmetric projects did NOT call out duplexing, except in the context of EFM’s variable rate PHYs, where it related to data rate capability: o There are 4 asymmetric projects, all EPON (802.3av, 802.3ca, 802.3bn, and 802.3cs) which separately call out objectives for different upstream and downstream rates, and none of these have a duplexing objective. o EFM (802.3ah) did not have a mac duplex objective, for EPON o EFM (802.3ah) did not have duplex objectives for its symmetric PHYs o EFM (802.3ah) had 2 asymmetric, variable-rate PHYs, this is the only project to call out “full-duplex” related to the PHY, and it was to specify the PHY’s rate-capability when used bidirectionally. (e.g., “PHY for single pair
non-loaded voice grade copper distance >=750m and speed >=10Mbps full-duplex) Again, looking at the history, the only place where the ‘full duplex only’ seems to not be used is with the EPON asymmetric data rate projects, where, I supposed, there could be confusion with whether the same rate was supported in both
directions. There are not variations in the wording, so I would suggest not introducing something new – because it is more likely to be interpreted not as a clarification, but as meaning something different altogether than what has been meant. Based on the above, I could support either: “Support full duplex only” – used by most projects to indicate use of the full duplex MAC, either clause 4 or annex 4a, Or No duplex objective at all, Both of these, I believe are consistent with prior practice. My support for no objective at all could be swayed if someone involved in EPON believes there was a reason other than asymmetry that they left duplexing out… I wouldn’t support additional words, as I think they would add confusion and require additional explanation. The consistency of 802.3 on this across more than 20 years and 60 amendments is pretty stunning. George Zimmerman, Ph.D. President & Principal CME Consulting, Inc. Experts in Advanced PHYsical Communications 310-920-3860 To unsubscribe from the STDS-802-3-ISAAC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=STDS-802-3-ISAAC&A=1 |