"c" is the speed of light in a vacuum. Traditional fiber optic cables are very much not a vacuum, with an index of refraction of ~1.5, so light travels through

"c" is the speed of light in a vacuum. Traditional fiber optic cables are very much not a vacuum, with an index of refraction of ~1.5, so light travels through them at ~2/3c. In contrast, light actually travels at nearly c through hollow core cables.