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Comment by walrus01 | Hacker News Reader

for long reach, yes, but not for ISP-to-ISP interconnection... the cost of coherent (QPSK, 8PSK, 16QAM) 100G SerDes and modulation/demodulation is considerably higher and only justified if going a great many km. For intra-peering-facility connections I don't see coherent 100G (or multiples of coherent 100G bundled by CWDM into 200G, 400G interfaces) becoming the norm anytime soon.

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I was looking at this the other day; it sounds like 100G-DR1 and 400G-DR4 are not coherent and vendors are aiming to make it commodity. Coherent is more like 200-600G per lambda.

At present 100G-LR4 is the defacto standard for ISP peering interconnections (such as to an IX switch in the same building, or for PNIs between big players within the same carrier hotel). Because it is very low cost. The cheap 25 Gbps x 4 approach and low cost cwdm prism are a big part of that. Since 100G will soon move from qsfp format down to the same size as SFP and SFP+ (fitting 48 in a 1RU height, 17.5" wide line card in a chassis based Arista), I don't foresee them becoming any less popular... Not when the optics are so affordable.

We aren’t talking about long distance and coherent here but direct detect. Coherent is not cost or power competitive with direct detect for short reaches yet.

When I talk about SERDES I’m talking about the electrical signal interface to the optic. Also it’s worth noting that the majority of coherent DWDM interfaces are some kind of embedded optic already.

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