A smaller NA reduces dispersion by limiting the number of modes. Fiber bandwidth is given in MHZ-km. If you double the distance, you half the frequency. Attenuation Attenuation is loss power. During transit, light pulses lose some of their energy.
Attenuation varies with the wavelength of light.
The 850nm window is perhaps the most widely used because 850-nm devices are inexpensive. The 1300-nm window offers lower loss, but at a modest increase in cost for LEDs.