Why Are DCM Modules Required Only for Data Rates 40G and 10G, and Not For Above 40G?

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That because low rates signals like 10G & 40G uses non_ cohrenet receiver that uses normal receiver to detect the signal .

But with 100G and above they use coherent receiver that extract signal and uses DSP digital signal processor to fix the dispersion and recover the orginal signal

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Dear friend!
In the case of non-coherent transmission, the line rate dispersion tolerance is inversely proportional. (The higher the line rate, the lower the dispersion tolerance.) Without dispersion compensation, the 10G line signal cannot exceed 100 km. Therefore, in a WDM network, dispersion compensation is required for non-coherent systems with a line rate higher than 10 Gbit/s.
Lines above 40G usually use coherent transmission and electrical dispersion compensation technologies.

Thank you！