400G ER4 wavelengths - two grids

[Joel Busch via NANOG <nanog () lists nanog org>]

Dear colleagues

I have come across a weird mishmash of offers for QSFP56-DD 400G ER4 and 
ER4 lite transceivers in the market, and now I'm looking to compare notes.

It seems to me that there were two separate development trains going on 
at once. Resulting in two incompatible grids of four wavelengths being 
used (and sold, see below):

- 1295.56 nm 1300.05 nm 1304.58 nm 1309.14 nm
- 1304.58 nm 1306.85 nm 1309.14 nm 1311.43 nm

Why you would go tighter seems obvious to me: To stay close to the zero 
point of dispersion with all four lanes, and perhaps to make all four 
experience more similar levels of dispersion, to make dispersion 
compensation in the receiver easier.

It seems that the wider spaced set is based on IEEE802.3 clause 122 
(added in IEEE802.3bs), more specifically in Table 122–6 "200GBASE-LR4 
and 200GBASE-ER4 wavelength-division-multiplexed lane assignments". 
(Note that it doesn't officially specify 400GBASE-ER4, this was 
apparently added in the 50G-per-lambda-time.)

The tighter spaced seems to be based on the 100G Lambda MSA 
specification called "400G-ER4-30 Technical Specification 1.0" 
specifically Table 2-1 "Wavelength-division-multiplexed lane 
assignments" [0]

FS.com and Fibermall sell transceivers with the wide spacing [1][2] 
Flexoptix sells transceivers with the narrow spacing [3]

According to a contact at one of the shops the wide spacing is also 
called LWDM and the tight one is called nLWDM. Both now exist and are 
out there, so caveat emptor, I guess...

Does anyone have any parts sourced directly from a router manufacturer? 
Could you share what they went with optically?

Or any other pertinent insider knowledge? :-)

Best regards,
Joel


[0] https://100glambda.com/specifications
[1] https://resource.fs.com/mall/resource/qdd-er4l-400g-data-sheet.pdf
[2] 
https://www.fibermall.com/sale-461143-qsfp-dd-400g-er4-lite-lwdm4-30km.htm
[3] https://www.flexoptix.net/en/d-164hg-30-e.html?option875=1


--
Joel Busch,
Network Engineer
Switch, AS559


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