thogh in LAN card only 4 wire is active so 2 set of telephone wire can be carrier of Broad band easily. but i dont heard of single telephone wire broadband, is it possible?
Normally power lines can't carry high frequency signals because the power transformers won't pass them. The lines have some kind of bypass built into them at transformer points to allow the high frequency signal to pass on the line, no idea what kind of protocol voltages or frequencies are involved though.