Data Rate and Cable Length
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For imperfectly terminated lines the data rate is limited by the round trip delay i.e. when the reflected wave arrives back at the driver.

Delay = 2*l/(v*c)
where:
 l = length
 v = velocity factor of cable
 c = speed of light

so maximum data rate = 0.5*v*c/l

As v is typically about 0.66, max data rate rule of thumb:
 bits/sec * length(m) < 10^8