Previous versions (4.0-4.1) incurred several errors when estimating the fiber length.

By definition, the speed v at which light travels through a medium with refractive index n is v=c/n
Since for the ITU-T G.652 D recommendation:

	"the refractive index profile of the fibre does not generally need to be known"

we chose a value provided by one of the most popular G.652 fiber manufacturers
for 1550 nm in their product information as seen in:
https://www.corning.com/media/worldwide/coc/documents/Fiber/SMF-28%20Ultra.pdf