Content

| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.6 |  | External interference |

One of the most notable causes of Electromagnetic Interference (EMI) is lightning. When electrical disturbances occur in the environment they impact radio and television broadcast signals. This will often result in crackling and popping sounds in speakers or bright flashes on the screen. Common occurrences in a building utility system, such as main power lines near data cables, cabling runs passing elevator installations, or other features that use electrical motors, can cause interference as well. Loose connections, damaged or aging equipment and dusty insulators are a few of the conditions causing electrical noise from the outside world.

Noise
On any line, even in the absence of a data signal, random fluctuations of the line voltage and current will occur. This effect is known as Line Noise Level, or simply Background Noise. There are three main causes of this noise:

- Cross-talk
- Impulse Noise
- Thermal Noise, which will not be discussed

Cross-talk – Occurs when a signal on one line is picked up by adjacent lines as a small noise signal. Particularly troublesome is Near- End Cross talk (NEXT) caused when a strong transmitter output signal interferes with a much weaker incoming receiver signal.

Impulse Noise – This noise is caused by external activity or equipment and generally takes the form of electrical impulses on the line. These impulses can cause large signal distortion for their duration and can bring the entire network down whenever they occur.

Proper common-mode line terminations must be used for the unused Category 5, UTP cable pairs 4/5 and 7/8. Common-mode termination reduces the contributions to EMI and susceptibility to common-mode sources. Wire pairs 4/5 and 7/8 are actively terminated in the RJ-45, 100BASE-TX port circuitry in the FE-TX port adapter.