With the use of unshielded twisted-pair copper wire in a network, what causes crosstalk within the cable pairs?

• A. the magnetic field around the adjacent pairs of wire
• B. the use of braided wire to shield the adjacent wire pairs
• C. the reflection of the electrical wave back from the far end of the cable
• D. the collision caused by two nodes trying to use the media simultaneously

Answer: (A)

Crosstalk in unshielded twisted-pair cabling comes from electromagnetic coupling between nearby conductors. When one pair carries a signal, it creates a changing magnetic field that can induce a voltage in adjacent pairs—this inductive (and related capacitive) coupling transfers some energy from one pair to its neighbors. Since the cable is unshielded, there’s nothing to contain these fields, so the interference appears as signal on nearby pairs. Twisting helps reduce this coupling by keeping the loop area small and balancing the fields, but it doesn’t eliminate it entirely. The other scenarios describe different issues: reflections arise from impedance mismatches at cable ends, and collisions relate to multiple devices contending for the medium; shielding would actually reduce crosstalk rather than cause it.