Providing flow control includes receiving at a router an indication of the ability of each one of multiple ports not directly connected to the router to receive data from the router and controlling transmission of data from the router to the multiple ports based at least on the indication. Also, the filter approximates the frequency characteristic of the acting amount caused by a skin effect in the electric waveguide. It is a further object of this invention to prevent and treat cataract formation, glaucoma, diabetic retinopathy and macular degeneration.