AC Noise
AC noise is distortion of the signal on a wire.
Noise on network cable is typically a result of long wiring runs.
Long runs act as antennas and pick up EMI and RFI from other sources.
AC signal noises are called oscillations and can alter digital signals so that they cannot be interpreted correctly by the computer.
Rewiring connections can eliminate noise.
Getting separate breaker boxes for each office reduces wiring run lengths because neutral wires run between wall outlets and breaker boxes where they are grounded.
Since neutral wires are also grounded at the transformer, getting a dedicated transformer for your LAN can also reduce AC noise.