External Interference | Data Cabling

One hindrance to transmitting data at high speed is the possibility that the signals traveling through the cable will be acted upon by some outside force. Although the designer of any cable, whether it's twisted-pair or coaxial, attempts to compensate for this, external forces are beyond the cable designer's control. All electrical devices, including cables with data flowing through them, generate EMI. Low-power devices and cables supporting low-bandwidth applications do not generate enough of an electromagnetic field to make a difference. In addition, some equipment generates radio-frequency interference; you may notice this if you live near a TV or radio antenna and you use a cordless phone.

Devices and cables that use a lot of electricity can generate EMI that can interfere with data transmission. Consequently, cables should be placed in areas away from these devices.

Some common sources of EMI in a typical office environment include the following:

• Motors
• Heating and air-conditioning equipment
• Fluorescent lights
• Laser printers
• Elevators
• Electrical wiring
• Televisions
• Some medical equipment

Note

Talk about electromagnetic interference! An MRI (magnetic resonance imaging) machine, which is used to look inside the body without surgery or x-rays, can erase the magnetic strip on a credit card from 10′ away.

When running cabling in a building, do so a few feet away from these devices. Never install data cabling in the same conduit as electrical wiring.

In some cases, even certain types of businesses and environments have high levels of interference, including airports, hospitals, military installations, and power plants. If you install cabling in such an environment, consider using cables that are properly shielded, or use fiber-optic cable.

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Cabling and Standards

Maximum acceptable values of attenuation, minimum acceptable values of crosstalk, and even cabling design issues—who is responsible for making sure standards are published? The group varies from country to country; in the United States, the predominant standards organization supervising data cabling standards is the Telecommunications Industry Association (TIA). The standard that covers Category 3, 5e, 6, and 6A cabling, for example, is ANSI/TIA-568-C, which is part of the guideline for building structured cabling systems. These standards are not rigid like an Internet RFC but are refined as needed via addendums. The ANSI/TIA-568-C document dictates the performance specifications for cables and connecting hardware.

Alien Crosstalk (AXT)

Alien crosstalk (AXT) occurs when the signal being carried in one cable interferes with the signal being carried in another cable. This can occur in a cable that runs alongside one or more signal-carrying cables. The term alien arises from the fact that this form of crosstalk occurs between different cables in a bundle, rather than between individual wire pairs within a cable.

Alien crosstalk can be a problem because, unlike the simpler forms of crosstalk that take place within a single cable, it cannot be eliminated by traditional phase cancellation. Because AXT resembles noise rather than signals, alien crosstalk degrades the performance of the cabling system by reducing the signal-to-noise ratio of the link. As the signal rate increases in a cable, this form of crosstalk becomes more important. In fact, this is a major source of interference, and a limiting factor, for running 10GBase-T (10Gbps) over UTP cabling. A lot of work has been performed during the creation of the ANSI/TIA-568-C standard in understanding the causes of AXT and potential solutions.

Alien crosstalk can be minimized by avoiding configurations in which cables are tightly bundled together or run parallel to one another in close proximity for long distances. In a typical installation, however, this is difficult and impractical. Category 6A (augmented Category 6) cable tries to solve this problem by increasing the spacing between wire pairs in a cable using separators within a cable to space the conductors apart from one another. This has the added effect of separating the conductors in one cable from the conductors in another. As you can imagine, this increases the diameter of a Category 6A cable compared to a Category 6 cable.

Another recommendation for reducing AXT is to avoid using tie-wraps to bundle cable together and to try to separate the cables in a rack as much as possible. This in turn requires more space to run these cables.

Recently developed Category 6A cables use a special core wrap that is not electrically continuous, so it does not require grounding, but that isolates and protects the core from alien crosstalk and other forms of external interference (as you'll see in a moment). These cables can be routed and bundled like traditional UTP cables without the concern of AXT and their size is smaller as well.

The industry has created measurement methods to measure alien crosstalk in the field, but they are very time consuming. The best advice is to ensure all the components are verified to be Category 6A compliant and that they have been tested in a channel or permanent link configuration to work together.

Pair-to-Pair Crosstalk & Power-Sum Crosstalk

Pair-to-Pair Crosstalk

For both near-end crosstalk and far-end crosstalk, one way of measuring crosstalk is the pair-to-pair method. In pair-to-pair measurement, one pair, the disturber, is energized with a signal, and another pair, the disturbed, is measured to see how much signal transfer occurs. The following six combinations are tested in a four-pair cable:

• Pair 1 to pair 2
• Pair 1 to pair 3
• Pair 1 to pair 4
• Pair 2 to pair 3
• Pair 2 to pair 4
• Pair 3 to pair 4

The test is repeated from the opposite end of the cable, resulting in 12 pair-to-pair combinations tested. The worst combination is what is recorded as the cable's crosstalk value. See Figure 1.

 

Figure 1: Cutaway of a UTP cable, showing pair-to-pair crosstalk

Power-Sum Crosstalk

Power-sum crosstalk also applies to both NEXT and FEXT and must be taken into consideration for cables that will support technologies using more than one wire pair at the same time. When testing power-sum crosstalk, all pairs except one are energized as disturbing pairs, and the remaining pair, the disturbed pair, is measured for transferred signal energy. Figure 2 shows a cutaway of a four-pair cable. Notice that the energy from pairs 2, 3, and 4 can all affect pair 1. The sum of this crosstalk must be within specified limits. Because each pair affects all the other pairs, this measurement will have to be made four separate times, once for each wire pair against the others. Again, testing is done from both ends, raising the number of tested combinations to eight. The worst combination is recorded as the cable's power-sum crosstalk.

 

Figure 2: Power-sum crosstalk

NEXT, FEXT, ELFEXT | Types of Crosstalk

Near-End Crosstalk (NEXT)

When the crosstalk is detected on the same end of the cable that generated the signal, then near-end crosstalk has occurred. NEXT is most common within 20 to 30 meters (60 to 90 feet) of the transmitter. Figure 1 illustrates near-end crosstalk.

 

Figure 1: Near-end crosstalk (NEXT)

Crosstalk on poorly designed or poorly installed cables is a major problem with technologies such as 10Base-T and 100Base-TX. However, as long as the cable is installed correctly, NEXT is less of an issue when using 1000Base-T because the designers implemented technologies to facilitate NEXT cancellation. NEXT-cancellation techniques with 1000Base-T are necessary because all four pairs are employed for both transmitting and receiving data.

Note

Cables that have had their twists undone (untwisted) can be problematic because the twists help cancel crosstalk. Twists are normally untwisted at the ends near the patch panels or connectors when the cable is connected. On the receiving pair of wires in a cable, the signal received at the end of the cable will be the weakest, so the signal there can be more easily interfered with. If the wires on adjacent transmit pairs are untwisted, this will cause a greater amount of crosstalk than normal. A cable should never have the wire pairs untwisted more than 0.5″ for Category 5e, and 0.375″ maximum for Category 6 cables.

Far-End Crosstalk (FEXT)

Far-end crosstalk (FEXT) is similar to NEXT except that it is detected at the opposite end of the cable from where the signal was sent. Due to attenuation, the signals at the far end of the transmitting wire pair are much weaker than the signals at the near end.

The measure of FEXT is used to calculate equal-level far-end crosstalk (ELFEXT). More FEXT will be seen on a shorter cable than a longer one because the signal at the receiving side will have less distance over which to attenuate.

Equal-Level Far-End Crosstalk (ELFEXT)

Equal-level far-end crosstalk (ELFEXT) is the crosstalk coupling between cabling pairs measured at the end of the cable opposite to the end of the signal source, taking into account signal loss. ELFEXT is calculated, not measured, by subtracting the attenuation of the disturber pair from the far-end crosstalk (FEXT) on the disturbed pair. The calculation describes the ratio of disturbance to the level of the desired signal; it is another indication of signal-to-noise ratio. Another way of looking at it is that the value represents the ratio between the strength of the noise due to crosstalk from end signals compared to the strength of the received data signal. You could also think of ELFEXT as far-end ACR (attenuation-to-crosstalk ratio, described later in this chapter).

Each pair-to-pair combination is measured, as the attenuation on each pair will be slightly different. If the ELFEXT value is very high, it may indicate that either excessive attenuation has occurred or that the far-end crosstalk is higher than expected.