12/5    What physically distinguishes serial cables and 10 baseT ethernet
        cables (and any other types of communication cables) that allow
        one to transmit at 10MHz and the other to only transmit at
        115Khz?
        \_ In some cases, it's just a matter of certification -- sometimes
           a given cable can do a lot more than what it's rated for.  But
           there can be a lot of deep magic related to RF interference and
           crosstalk under certain circumstances.  (There are stories of
           people running communications spec X on cables intended for some
           other spec Y, and saving the day and lots of money.  But if you
           want to have confidence that weird RF problems won't pop up, you
           should use cables for purposes for which they are rated.)