I'm looking at collision detection in communication protocols, in particular Carrier sense multiple access with collision detection (CSMA/CD). According to what I've read on Wikipedia, a collision seems to cause the wave of the outgoing signal and the wave of the incoming signal to overlap. Thus, the signal has values with more amplitude than allowed for a "1-bit". This could be used as a working collision detection algorithm.
However, what's the use of the "Jam signal", as explained in the same Wikipedia article, then? It says that the CRC has to be faulted, but we already have a collision detection algorithm.