Issue link: https://www.e-digitaleditions.com/i/1156106
38 I ICT TODAY that, most telephony circuits were operated over a single open wire using the earth as a ground-return circuit. Because this configuration resulted in an unbalanced circuit, electrical interference issues from power lines and electric trams led to a method called transposition, which helped to cancel out the interference, but it was not as effective as twisted pair. The longitudinal balance of the two wires is very important in resisting a given amount of induced vol- tage before service is adversely affected. However, many very knowledgeable people in the industry have the un- fortunate view that twisted pairs eliminate the induced AC voltage. To be sure, the ideal situation would not cause any AC voltage to exist between the two conduc- tors. This condition is usually referred to as the metallic, transverse mode or differential voltage. What is often forgotten is that the longitudinal or common-mode induced voltage is still there! This is the situation that can cause major problems. A balanced circuit has two sides that are electrically alike and symmetrical in respect to ground. In a per- fectly balanced two-wire circuit, longitudinally induced (common-mode) voltages cause an equal flow of current to ground on each conductor. The net metallic (trans- verse mode) voltage from the induction is zero. In practice, of course, perfectly balanced circuits do not exist! Circuit balance, then, is a measurement of the differences in longitudinal impedance of each conductor of a pair or piece of equipment and is expressed in terms of the induced or applied voltage to the resulting metallic voltage: Balance = 20 Log 10 (longitudinal voltage/metallic voltage) or N g - N m . These two expressions, N g or noise-to- ground (sometimes referred to as power influence) and N m or noise metallic (circuit noise) are usually expressed in terms of dBrnC. A table of generally accepted circuit performance values for circuit balance, power influence, and noise metallic are provided in the IEEE standards, but these values only apply to voice-frequency circuits. When data circuits are considered, the C-message weight- ing characteristics are not involved. That is why data circuits are more sensitive to lower frequency induction values and the 3 kHz flat weighting is used to make their measurements. When physical cable pair troubles or faults occur or the equipment it is connected to is inherently unbalanced by design, longitudinal current will not be equal in each conductor of the pair, and a resulting metallic current will flow. When this metallic current flows through a resistance, such as a telephone receiver, metallic voltage results. Depending on the level of its harmonic content, this effect could result in noise that can be heard. Longi- tudinal voltages and currents, on the other hand, cannot be heard, but if they are high enough, saturation of the equipment's components can cause noise to be generated. This effect can also occur with test equipment, which would yield inaccurate readings. By increasing the number of twists, such as found in Cat 6A, Cat 7, and Cat 8 cables, further reductions in near-end crosstalk (NEXT) can allow for higher band- width transmissions. ENTER THE ALIEN CROSSTALK (AXT) ISSUE IN LANS "About 80 percent of the installed base of cabling infra- structures in enterprise LANs consists of Cat 5e that operates at 100 MHz or Cat 6 cabling that operates at 250 MHz—both of which are typically able to support up to 1000 Mb/s [1Gb]." 2 Additionally, as reported in 2017 "…more than 70 billion meters of Cat 5e and Cat 6 cable has been installed over the last 12 years…" 3 FIGURE 5: Long and tight UTP cable bundles are more susceptible to AXT.