Dear 802.3df Task Force Participants,
During yesterday's discussion, a number of misconceptions arose that need clearing up.
1. PMD Margin
In his presentation, Mike Sluyski pointed out that the 800G LR4 IMDD proposal link budget margin is zero, and suggested this is bad. Unfortunately; all IEEE PMDs ever (with
a couple of exceptions) have zero link budget margin, or more precisely unallocated loss. This may panic end users into shutting down their data centers, ripping out their optics, and replacing them with ones that have link budget margin. Fortunately, IEEE
methodology picks the worst case PMD application corner, and defines a link budget that exactly meets it. The margin comes in other ways. Most optics are not deployed in worst case conditions, for example 10km optics operate over less than 10km. In modern
manufacturing, IEEE spec. limits are several sigma out. There is margin for measurement uncertainty and other factors. So datacenter operators can be at ease. Despite all of their optics having zero link budget margin, the IEEE methodology is robust and their
optics will continue to work just fine.
Spoiler alert: a few optics fail in the field, which is a good thing, zero failures would be prohibitively costly.
2. Coherent Proposal
For months, we have been following the journey of exploration by the Coherent proponents, even though the 800G LR1 solution is obvious. It uses the ZR DSP and SiPIC, operates in C band, and replaces the tunable DWDM laser with a single lower performance
fixed source. Let's see how many more detours will be taken before the return home.
3. WDM Grid CD
The 800G LR4 proposal is on the LWDM grid, which is tightly grouped near ZDW, and gives roughly 1/4 of CWDM4 grid CD. This is offset by doubling of the rate from 100G to 200G per lane. To first order, 400G LR4 (10km) has similar CD penalty as 800G LR4
(10km).
4. Relative Coherent vs. IMDD cost.
The best way to compare cost is using existing shipments. During the meeting, Tom Williams cited >2x cost ratio of ZR to LR4 (10km), which for a variety of reasons is an excellent proxy and a reasonable ratio. Right now, ZR volume is more than a magnitude
higher than LR4 (10km) volume. We expect the ratio to fall over time. To first order, we can call the LR1 laser source savings washing out any cost ratio increase due to lower long-term volume ratio, giving a LR1/LR4 cost ratio projection of ~2x. All other
relative cost advantage calculations by the Coherent and IMDD proponents are convoluted and unconvincing.
Thank you
Chris
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