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In optical DWDM networks, the spectrum is classically divided into several 50 or 100 GHz channels.

So, let's assume some channel ranges from x to x+50 GHz. On which frequency/frequencies is the signal sent?

It would be a reasonable choice to use x+25 GHz as a frequency to transmit the signal on, however, I was wondering whether one could also use x+12.5, x+25 and x+37.5 for instance, or even more frequencies simultaneously.

How is this usually handled in DWDM networks?

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In WDM (Wavelength Division Multiplexing) networks, the channel frequency given is the nominal optical wavelength. DWDM (Dense WDM) the channels are more closely spaced, to allow more channels. DWDM requires more stable and precise laser wavelength control and narrower accurate receive filters.

To fit more optical channels between the existing DWDM channels becomes more expensive and technically difficult. Also, as the channels become narrower, at some point you have to reduce the modulated data rate or change the modulation method.

This is all a cost / difficulty / need / standardization and interoperability tradeoff.

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