MHz usually refers to the upper frequency band on a cabling system. In a two-way circuit, crosstalk problems are most noticeable when the signal leaves the transmitter. At this point the detector for the adjacent pair is closest and the energy level is highest. The effects that one energized pair of cable has on one other pair within the same jacket. The resistance to the flow of current is expressed in ohms.
Every material offers some resistance or opposition to the flow of electric current.
The size and type of material in a circuit is chosen to provide the lowest resistance possible. Copper is an excellent conductor for the price. A measurement of reflected energy of a transmitted signal due to impedance variations along the length of the cable. Gigabit Ethernet over twisted pair cable.