On 28/11/2025 19:07, Bryan Fields via NANOG wrote:
The other major advantage to the PAM4 is that it all has Forward Error
Correction (FEC).
Well, FEC is a necessity for PAM4 to work, as it is more susceptible to
noise than NRZ.
Here's the fun thing about the 100g standards, IEEE made -DR (500m),
then the 100gLambda trade group made the -FR (2km) and -LR (10km)
standards. DR and FR is exactly the same in terms of link budget, so
DR is FR and FR is DR. From FR to LR is 2.3 dB difference; there's no
optic-electronic component (Laser/PIN Diode/etc) that has such a small
delta. This means the DR and FR are built with the same underlying
parts, and DR will have no issues pushing 10km.
https://100glambda.com/specifications/send/2-specifications/9-100g-fr-and-100g-lr-technical-specs-rev2-0
If you look at the 2.1.3 table, you'll see how much signal loss is
given to cable, connector, and splice losses. In most cases, this is
5-7 times more than what is achievable in the real world. Unless you
have a circuit going through 10 patch panels with saggy fiber, it's a
non issue. This does mean you should demand each cross connect is
scoped and cleaned with wet/dry process if needed.
tl;dr - Use 100g-DR for everything and monitor your pre-FEC error rates.
This is not really good advice.
Optically, yes, you can mix-and-match DR1, FR1 and LR1 on the same link
in any combination, because they all use a single optical wavelength.
However, there is no guarantee that DR1 and FR1 will have the same
performance as LR1, especially when the link is longer than 500m (DR)
and 2km (FR) and/or has varying levels of OSNR quality.
The fundamental building blocks for modern pluggables generally tend to
be the same. Because of this, including DR and FR specs. alongside LR
makes sense for manufacturers and suppliers because it increases their
revenue yield.
In production, these are the link budget differences between all 3
variants:
* DR @ 3.0dB
* FR @ 4.0dB
* LR @ 6.3dB
If you also consider the minimum Rx power for each variant, you will see
the cheaper specs. are also the least sensitive:
* DR @ -5.9dB
* FR @ -6.4dB
* LR @ -7.7dB
Yes, each of them can Tx at between +4.0dB - +4.5dB, but the Rx
sensitivity will come into question if the link is too long and/or has
high loss.
Like I mentioned before, we are aggregating 100G-LR1 EoDWDM customers
into 400G-FR4 plugs with no problem, inside the data centre. I would not
necessarily try doing this between LR1 <=> DR4, because you seriously
limit your optical margin. It may work, but you can't have your glass of
wine at the bar completely stress-free, ever.
Mark.
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