In STP-A cables, exterior braids reduce low-frequency noise and provide immunity to other radio-frequency interference.

• A. They reduce high-frequency noise
• B. They increase cable flexibility
• C. They reduce low-frequency noise and provide immunity to RF interference
• D. They are purely cosmetic

Answer: (C)

Exterior braids around STP cables serve as a conductive shield that intercepts external electromagnetic fields before they reach the inner conductors. When this shield is properly grounded, it provides a low-impedance path for interference to go to ground, so the signal-carrying pair experiences less stray voltages and currents. This is especially effective against low-frequency noise, like power-line hum, which can couple magnetically to the pair, and it also helps block other RF sources from injecting noise. In short, the shield around the cable improves immunity to external RF interference and reduces low-frequency noise that can distort the signal. It isn’t primarily about increasing flexibility or appearance—the shielding’s main purpose is interference suppression through a grounded barrier.