What is Crosstalk?

Posted by Patch Cords Online on 3/6/2018 to Cable Technology
What is Crosstalk?

Near and Far End CrosstalkCrosstalk is a complicated subject to grasp, especially where the "Near" and "Far" ends are. In reality both end of the cable are both! They are only reference points to demonstrate where the signal is originating from for the purpose of the test…

The first diagram in this article shows exactly what happens, and how the "concept" of what is the Near and Far ends depending on which end the signal originates from. The diagram show a cable that is tested from either end and where the two ends are and where the NEXT and FEXT signals are in relation to the originating signal.

When a signal legitimately travels down a pair of wires in a cable the signal emerges from the other end as it is supposed to, but the field that the signal creates around that pair "bleeds" over to the other pair in the test. The crosstalk creates a certain voltage in that wire that will emerge out of both ends, the near end and the far end. The following sections describe the different types of crosstalk and what the measurements mean.

Near End Cross Talk (NEXT)

The technical definition is "Interference between two pairs in a cable, this is measured at the same end of the cable as the "interfering transmitter". What this really means is that one pair can interfere with another pair at the end that is doing the transmitting at the time. In reality the transmitting cable will interfere all the way down the cable to the other end, but the interference will be the greatest at the end where the interfering signal is transmitted from, called the "Near" end.

As an example, a cable can be plugged into a network switch and that switch is sending data (sending an electrical signal), that electrical signal will travel down the transmit pair interfering with the other pairs in the cable at the connector. That is where the NEXT measurement is taken, it is one of the most important measurements when testing a cable (except for Power Sum NEXT), it is a fairly conclusive test for the actual manufacturing quality of the cable itself or whether the cable itself is damaged. One of the causes of NEXT is the way that the pairs are inserted into the connector itself, if the individual pairs are not twisted right up to where they enter the connector then there is a potential for crosstalk issues, commonly found with homemade cables.It's hard to make cables correctly and the temptation is to untwist the pairs more than they should be to make the connector fit easily, but this will introduce a potentially unacceptable amount of NEXT interference.

So how does one twisted pair interfere with another? When a current flows through any wire, it creates an electromagnetic field. This electromagnetic field from the transmit pair can "bleed" over to receive pair. This causes a multitude of issues, such as slow data speeds etc. if you have a managed network switch you can look at the interface statistic. NEXT will manifest itself by causing Runts, CRC's, framing errors. This electromagnetic field becomes stronger as the frequency of the signal increases, until eventually the electromagnetic field is so strong it will make the connection fail. You will see a frequency figure on the jacket of network cables, this is the frequency that the manufacturer has tested the cable up to before it reaches failure.

Power Sum Near End Cross Talk (PS-NEXT)

This is the same as NEXT, but is a calculation of the sum of unwanted signals from all the other pairs (in every combination) in the cable that affect the "victim" pair. Standards such as 10BASE-T and 100BASE-T only use two pairs in a cable, but the 1000BASE-t standard uses all four pairs in a network cable, which is where the Power Sum figure may be more relevant.

A cable that passes the PS-NEXT test should deliver superior performance for Gigabit and ten Gigabit networking deployments or any other network protocol that uses simultaneous parallel transmission. Using the PS-NEXT test cables can be "graded" rather than just being given a Pass or a Fail score.Cable grading is measured, in decibels, at a variety of frequencies that fall within the operating bandwidth of the that cable. There are 7 grade levels each one of which is 3db wide, the higher the grade the better the performance of the cable. The 3db measurement is in the NEXT/PS-NEXT margin at 100Mhz and a signal power difference of 2X.

Far End Cross Talk (FEXT)

As the name suggests it’s the same measurement as above (measuring the effect on one pair to another) but a the other end of the cable from the transmitting device. The effect is less than the near end measurement as it is measured at the far end of the cable.

Power Sum Equal-Level Far End Crosstalk (PS-ELFEXT)

This measures the sum of the unwanted signals from pairs on the far end onto a pair on the near end.

Alien Cross Talk (AXT)

Alien Crosstalk Interference from cables around the cable in question. You can look at it as one pair as the "intruder" or the "aggressor" and the effected pair as the "victim". Unlike the other cross talk types mentioned earlier, it is only caused by the balanced "signal coupling" or the signal from one cable intruding on the other. Alien cross talk is not too much of an issue when using Cat5e and Cat6 for 100 Mbps or gigabit applications, but is the most impactful issue for 10 gigabit applications. Even though Cat6 is capable of 10 Gigabit speed (under certain situations, described in the Category Comparison article) Cat6a cable has a spline that separates the pairs at a consistent distance (just like early Cat6), to keep Alien Cross Talk to a minimum. Shielded Cat6a will eliminate Alien Cross Talk all together.

So what are the actual effects of Alien Cross Talk? The main symptoms are re-transmissions, which will cause a significant slow downs, these slow downs will be random in nature causing unpredictable results. In extreme cases the network equipment will negotiate a slower speed.

The following images show a cable that has been cut to expose the spline and the cables so you can see the actual construction without having to cut open one of your own cables! It shows the way that the spline separates the individual pairs. The other image shows a bundle of cables and how Alien Cross Talk acts between the individual cables in that bundle.

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