Copyright Goodheart-Willcox Co., Inc. 56 Networking Fundamentals far-end crosstalk (ELFEXT), and alien crosstalk (AXT). Th e fi rst three measurements are taken by a manufacturer to determine cable specifi cations so that the cable can be correctly labeled. Th ey are only taken in the fi eld to see if an existing Category 5 cable could be used as a Category 5e cable. AXT was introduced as part of the Cate- gory 6a standard. Near-End Crosstalk (NEXT) Near-end crosstalk (NEXT) is a measurement of the refl ected loss at the near end, or input end, of a cable, as illustrated in Figure 2-9. Losses are typically expressed in decibels. A decibel (dB) is a unit of measurement that expresses the relationship of power between two electrical forces. Th is measurement is often used to show the output-to-input ratio of a signal. For twisted-pair cable, it is used to compare the diff erence between the amount of power in the input signal and the amount of power generated by crosstalk. For networking fundamentals, it is the relationship of the input signal compared to the crosstalk signal. Far-End Crosstalk (FEXT) As the name implies, far-end crosstalk (FEXT) is a measurement of refl ective loss at the far end, or output end, of the cable. Figure 2-10 provides an example of this loss. Far-end losses are also expressed in decibels. Equal Level Far-End Crosstalk (ELFEXT) Equal level far-end crosstalk (ELFEXT) is calculated by subtracting the eff ects of attenuation from the FEXT. Note that ELFEXT is a calculation, not a measurement. It is found by subtracting attenuation from the FEXT measurement. For example, if Proper Connection Improper Connection Excessive exposed and untwisted wires will produce crosstalk Cable jacket is not secured under crimp Figure 2-8 Example of a twisted-pair cable properly connected to a connector and a twisted-pair cable improperly connected to a connector. Note the exposed, untwisted pairs of wire in the improper connection. Goodheart-Willcox Publisher