EMI occurs when magnetic fields intersect network or computer cables, causing interference in the cables. Motors and transformers, which are ubiquitous in an office (in air conditioners, hearters, and so on), are typical source of EMI. A common mistake is to run network cable through an elevator shaft or through a ceiling that hides a bank of transformers in fluorescent lights.
Finding the source of EMI can be challenge. The best approach is to follow a cable with an inexpensive compass, noting strong, odd needle movement. When you find the source of the EMI, You can protect the cable against it by either replacing the cable with a shielded cable (or fiber-optic cable, which is immune to both EMI RFI) or by moving the cable far away from the source of the EMI.
Radio Frequency Interference (RFI) Problems
RFI occurs when radio signals interfere with the normal operation of electronic circuits (com-puters in particular). Everyday sources of RFI include television and radio transmitters, which by nature create a specific radio frequency as part of the transmission process. Other sources are two-way radios and celluar phones.
The only way to protect against both EMI and RFI is to use shielded network cables. Shielded cable, as used in shielded twisted-pair (STP) and coaxial cable, can reduce the effects of RFL. You could also use fiber-optic cable, which is immune to EMI and RFI, throughout your entire network, although this option can get a little pricey.