If the runs are too long, clock recovery is difficult -- if they are too short, the high frequencies might be attenuated by the communications channel.
Because these waves are not guided but diverge, in free space these systems are attenuated following an inverse-square law, inversely proportional to distance squared.
As it passes through the atmosphere, light is attenuated by scattering and absorption; the more atmosphere through which it passes, the greater the attenuation.
