When installing and maintaining a system of structured cabling, certain issues are bound to arise. Consider for a moment how much data is being transmitted simultaneously as electrical impulses all in one place. These signals that travel down the wires inside a cable create an electrical field that can sometimes interfere with the other wires inside (or even outside) the cable. Whenever there is electromagnetic interference between wires, it is referred to as crosstalk.
The higher the electromagnetic frequencies– and the more parallel the wires– the larger the incidences of crosstalk. In structured cabling, crosstalk can degrade the signals that travel along each wire and cause them to become confused and cross over each other. This slows down the transmission of data.
There are actually three different kinds of crosstalk that can show up in structured cabling.
- Near end crosstalk, or NEXT, occurs at the end of a cable where the signal originates
- Far end crosstalk, or FEXT, occurs at the far end of the cable from where the signal originates
- Alien crosstalk, or AXT, is somewhat different. Rather than interference occurring between wires within the same cable, the signal being carried in one cable interferes with the signal being carried in a totally different cable
To combat crosstalk, structured cabling systems are constructed using twisted pair wiring. Twisted pair wiring uses a different twist ratio, or pitch, for each of the four pairs of wires inside a cable. This, in a sense, turns them into antennas that are receptive to certain frequencies. Twisted pair wiring is then used in tandem with differential signaling– a method for transmitting information that uses both wires to send a signal. With these two elements working together, the receiver at the end of the cable has an easier time blocking interference added to an original signal sent by the transmitter. The structured cabling system can then reduce interfering crosstalk and transmit information at the expected rate.