On 11/29/24 12:46, Joel Busch via NANOG wrote:
I agree on the struggle with bad fiber, and we've had to use 100G ER4 "Lite" on links around 10km before too. But I think you are a little too pessimistic with putting the limit of what NRZ can do at 10km.
Not at all. NRZ is resilient at those distances. I was referring more to PAM4... should have made that clearer :-).
From my experience 30km is also possible with NRZ. We have a few ER4 and ER4 "Lite", that is Flexoptix Q.161HG.40 and Q.161HG.25, running between cities, with one span being a bit over 30km. That one is muxed parallel to our DWDM, so take an additional 2.5dB for our filters.
Yep, NRZ will be fine at those distances. I have ran a 100G-ZR4 NRZ plug on a 32km link successfully, where the 100G-ER4 struggled, due to the fibre quality. But those are grey plugs, which is why to get a cheap DWDM-based PAM4 option means you will need amplification and DCM to get beyond 10km. Either that, or go coherent.
That's very interesting, even if we don't use PON.
PON is not the primary use-case for OpenXR plugs, it just happens to line up with the technology.
I hadn't heard of this before. On the face of it, it sounds expensive to have to use a 400G capable coherent receiver for a 25G customer.
Not necessarily. The 25G is not fixed. You can configure the rate of each sub-carrier and balance capacity with channel count. But the killer app is being able to terminate multiple high-capacity customers on a single uplink port in a p2mp fashion. This was initially envisaged as a way to save optical operators burning expensive transponder ports. But I could see a use-case for router/switch operators as well, in the Access layer. The second killer app is its BiDi capability, which will deliver a 200G channel on single-core fibre at the kinds of distances only coherent can support without DCM. Prior to this, nearly all coherent BiDi plugs came in a CFP2 form factor, which is bulky and pricey. So having a coherent BiDi plug in a QSFP-DD form factor is great news!
But I'm curious to see how the prices shake out in the end, with potential mass adoption.
Initial price is about US$5,000, which is inline with regular QSFP-DD 400G coherent plugs. I expect these prices to come down toward the end of Q4'25 or early Q1'26, when 800G-ZR+ coherent plugs ship in more batches. Mark.