On 26/11/2025 15:40, Joel Busch via NANOG wrote:
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.)
Just to add for more context, you are right about the tighter spacing of
the 400G-ER4 and ER4-Lite plugs being due to chromatic dispersion (CD).
Even in the O-band, CD and the dispersion slope start to matter for
high-baud PAM4 modulation.
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...
The LWDM (LAN-WDM) is an older spacing technique that was first used in
100G-LR4. nLWDM (narrow LWDM) still runs in the O-band, but with the
tighter wavelength spacing, primarily for 400G-ER4 (4x100G PAM4) to get
to between 30km - 40km on duplex single-mode fibre.
If the older LWDM was used for 400G-ER4 or ER4-Lite, the outer lanes
would sit too far away from the 1310nm zero-dispersion point, making it
easy for them to pick up noticeably different dispersion penalties and skew.
The narrower spacing keeps all lanes closer to 1310nm where the
dispersion is lowest and most uniform.
Mark.
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