3.1 Signals and Noise
In copper wires the signal is sent electronically as an analogue signal, represented as a series of sine waves, and as such is susceptible to interference from external electronic devices or from poor shielding.
Interference caused by the wiring within the cable is commonly called crosstalk. A signal in one wire is disrupted by a signal in an adjacent wire, causing the data to be corrupted. To test the quality of a cable we use the attenuation crosstalk ratio (ACR). This is the difference between attenuation (reduction in signal strength) and crosstalk at a given frequency along a cable. ACR is a calculation (measured in decibels) used in networking transmission to assure that a signal transmitted across a twisted-pair cable is stronger at the receiving end than any interference signals imposed on that same pair by crosstalk from adjacent pairs.
In twisted pairs, common sources of interference are electromagnetic interference (EMI) and radio frequency interference (RFI). Problems may occur such as the untwisting of pairs. If the pairs open up or even loosen, they will be more susceptible to EMI and RFI. Care should be taken not to untwist more of the wires than required to make the connection to a switch/patch panel or the plug.