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The effect of secondary reflections generally shows up as the wiggle in the insertion loss, and is called ILD. The high-speed groups (for example, 802.3dj) use a reference receiver model with a lot of detail – called COMS. It is transitioning to an open source development model. I don’t think we need all that detail. COM is described in Annex
93A in detail. Code implementations have been kept up to date and examples of the latest use of COMS can be found at
https://www.ieee802.org/3/dj/public/tools/index.html ILD is specified in the backplane channels and is used in SERDES because it is important in controlling secondary reflections. It can be found in IEEE Std 802.3 at: 85.10.3 (for 40GBASE-CR4/100GBASE-CR10, which use 10 Gbps serdes lanes), and if you look around there in the 85.10.x sections, you’ll see how the insertion loss and return loss relate to it. The most recent version of ILD is in annex 93A, specifically at 93.A.4, where they compute a figure of merit for ILD. I’ve heard from many that this isn’t necessarily the greatest approach or architecture independent, and that Figure 85-11
might be better. Regardless, the use of these generally precludes those situations where return loss is a flat line hugging the mask. Masks get used, but its understood the peaks just touch it.
These are important specifications for SERDES where the equalizer complexity is limited. You can have short lines (not really short, but not maximum length) where the ILD doesn’t cause violation of the insertion loss limit, but the secondary
reflections would otherwise require a very long DFE or FFE to handle. 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 |