>To make things worse, a lot of existing medium-haul fiber links are actually twice as long as you'd expect, due to the desire to cancel out dispersion; you first run the fiber e.g. 10km from place to place, and then run it through a large 10km spool (of a slightly different type of fiber) in the datacenter to cancel out the dispersion. This is slowly going away, but only slowly.

Only slowly? I haven't worked with very long haul wdm systems or sonet/sdh but I've never seen this. Maybe you mean much longer distances than 10km as we've had 10G-LR for ages that don't need this.

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Sesse__ 3 days ago | parent [-]

I don't know exactly how long gives you too much dispersion (obviously depends on the fiber), but if it's short enough you just don't need to care, indeed.

I don't work with this myself, but my understanding is that you only start ripping this out when you are positive every wavelength from every customer actually has coherent detection, and that could take a while. :-) Obviously this will differ from site to site, too; I would assume new ones don't care unless there's a lot of legacy involved.

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Hikikomori 3 days ago | parent [-]

Alien wdm isn't common in long haul systems though, if so the provider typically provides the transponder. Usually customers use regular grey (1310nm 10G-LR) to the provider and the provider turns that into something compatible with their dwdm system.

	▲	Sesse__ 2 days ago \| parent [-]
		Huh, interesting, I didn't know this was a thing at all. But you still need to make sure you don't have a stupid customer sending 1000baseLX with direct detection in the other side for this to work. :-)