Collision detection on ethernet coaxial cable
Back in the 1980s Ethernet used coaxial cable.. and collision detection was implemented with CSMA/CD (Carrier Sense Multiple Access With Collision Detection) 



Collision detection is possible because the receiver is monitoring the line at the same time as the transmitter is transmitting. If there is a difference between transmitted and received, then a collision is presumed to have occurred. 

The transmitter put a current of about 16mA into the 50\$\Omega\$ coax (32mA since it was a T-joint). Gory details from ~35 years ago here.