I need to push 100G over 100 km of dark fiber. Since there are no 100G pluggable optics with this reach (~25 dB), I have been offered coherent transport systems to solve my problem. This is all good and well, except total system costs start from high five figures.
So, my question is, do I have any other options?
I can't help noticing that you can break out a 100G QSFP into four 25G QSFPs. 25G DWDM systems are relatively inexpensive (low five figures), but can you make 25G DWDM go 100 km?
I only need the one 100G, so I don't really need a highly scalable DWDM system. I can't put anything midspan, or if I could it would cost more than just going with a coherent system.
I need to push 100G over 100 km of dark fiber. Since there are no 100G pluggable optics with this reach (~25 dB), I have been offered coherent transport systems to solve my problem. This is all good and well, except total system costs start from high five figures.
100G-ZR4 QSFP28 is on the market and works. Just watch out for power limitations, your typical DC switch might not support it but proper stuff has no problems providing 6.5W of power.
If it’s just a single 100G channel needed you could try 100GBASE-ZR4. Specified for 80km, 30db power budget they could actually reach more the 80km.
Dispersion should also be „no" problem in the 1310nm length. I have to say that I never tried this on 100km distance without coherent solutions.
If you end up marginal with the ZR4 at the receive end, you may be able to use a silicon optical preamp to make up the margin.
If you are at the noise floor already (ZR4 receivers are pretty good), you may just need to dump more power into the line. I recently came across the PDFA (like an EDFA but the fiber is doped with Praseodymium instead of Erbium) which operate on the 1310 band and can push upwards of 20-23dBm of power out. They are not cheap (low 5 figures), but they are cheaper than coherent and will still let you run on 1310 so you don't have chromatic dispersion issues. With good (ER4 or ZR4) receivers good down to -20dBm or so, you've got 30-33dB of link budget which should just about make your 100km if it's decent fiber. They're also usable as a pre-amp (different configuration optimized for gain and noise figure instead of power, similar to EDFA preamps) and have performance potentially better than silicon amps in that situation.
The 1550 PAM4 pizza box EDFA+Mux+DCM that others have mentioned may also work and end up even cheaper, but I normally see those for 40-60km or "up to" 80km with them really pushing the link budget at that point.
Honestly, I'd be tempted to just suck it up and do a coherent solution, though I admit it would probably be at least 2x the cost. You can probably get a 200G carrier, though.
You could break this into 10x 10g coherent lanes, but you’re going to end up back close to coherent 100g prices.
You’re at the threshold distance where you’re past all the short range tech and are seriously pushing it - whereas the 100g coherent tech is just taking off.
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If it’s just a single 100G channel needed you could try 100GBASE-ZR4. Specified for 80km, 30db power budget they could actually reach more the 80km.
Dispersion should also be „no" problem in the 1310nm length. I have to say that I never tried this on 100km distance without coherent solutions.
Just to add to my original suggestion, we just did 100G-ZR4 over 30dB link with pre-FEC BER 3.174E-11.
As OP is asking for a solution for 25dB I don't see any reason why ZR4 would not work and why you would need coherent, amplifiers or any other additional solution except when you are limited by QSFP28 SFF power.
You may also find that 100G PAM4 could work. There are some vendors that
sell the optic, and an outboard EDFA + DCF pizza box.
We are about to deploy these on a couple of dark fibers:
They have amplified and dispersion compensated 8x100G to be used with PAM4 optics, and a pass-through port to connect existing 1G/10G MUXes to (which can have their own amplification if necessary).
They can provide models with different sets of channels of you need that (nice when cascading them with existing 1/10G MUXes). IIRC next year they can also build in a power meter so you can do remote monitoring.
If you're interested I can let you know how much we like them in a few months
4x25Gbit/s, 'directly detected' MAY work but it won't be easy at all. As many already have suggested, coherent detection will give you much much less headaches.
If you want to go with directly detected for financial reasons than first make sure you know the type(s) of fibers and the dispersion you can expect. Fiber-operators tend to splice different types of fiber together. Don't just assume that it will be all G.652 compatible fiber.
If there is G.655 or even G.653 compatible fiber in your path then it is unlikely that you can operate in the O-band (1310nm).
In that case you need to go with the C-band (1550nm), which is easier to amplify than the O-band, because you can use commonly available EDFAs.
You probably need to compensate the chromatic dispersion in the C-band. For that it is again very important to know the fiber type(s) in your path.Because a '100km DCM', assumes 100km G.652 fiber. If your path consists of 50km G.652 and 50km G.655 then you need to compensate for 50km.
Last but not least, your hardware or optics probably need to be able to do FEC to get a bit of a decent BER.
