### ACR

The first thing to  understand about testing  data cables is the ACR,  this stands for Attenuation  to Crosstalk Ratio. The  pink area in the graph is  the attenuation, this can be caused by several things  as will be explained below,  and the blue area is the  crosstalk. Attenuation is the reduction in signal strength over the length of the  cable and frequency range, the crosstalk is the external noise that is introduced into the cable. So, if  the two areas meet, the data signal will be lost because the crosstalk noise will be at the same level as  the attenuated signal. ACR is the most important result when testing a link because it represents the overall performance of  the cable.

### So what causes the signal to attenuate?, and where does the crosstalk come from?

Below are of some of the terms used in high performance cable testing, and a description of what they  mean.

### Length

The length of a cable is one of the more obvious causes of attenuation because the longer it is, the  more resistance it has, and therefore less of the signal will get through. To measure the length, a cable  tester uses Time Domain Reflectometry (TDR). A pulse is sent down the cable and when it reaches the  far end it reflects back, by measuring the time it takes to travel down the cable and back again, the  tester can determine how long the cable is. To do this, the tester also needs to know how fast the  pulsed signal is travelling, this is called the Nominal Velocity of Propagation (NVP) and is expressed as  a percentage of the speed of light. The NVP is usually somewhere between 60% and 90% of the speed  of light, with most Cat 5E/6/6a/7/7a cables being around 70%. Due to the twists in the cable, the  measured length will be greater than the physical length, so if a run looks like it might be over 80m it  would be wise to check it before it is tied up and terminated.

### Wire Map

This test is to ensure that the two ends have been  terminated pin for pin, i.e. that pin 1 at the patch panel goes to pin 1 at the outlet, pin 2 goes to pin 2 etc. etc. The wire  map also checks for continuity, shorts, crossed pairs,  reversed pairs and split pairs. A Split pair is probably the  only thing that requires an explanation here, as they are  undetectable with a simple continuity tester, this is because  pin for pin they seem to be correct. As explained on the  Cabling Basics page, balanced line operation requires that  the signal is transmitted over a pair of wires that are twisted  together, with a 'split pair' the signal would be split between  two different pairs.

### Return Loss

When a cable is manufactured there are slight  imperfections in the copper. These imperfections all  contribute to the Structural Return Loss (SRL)  measurement because each one causes an impedance  mismatch which adds to the cables attenuation.

### DC loop resistance

This is simply the resistance between the two conductors of a twisted pair which is looped back at the  far end. The primary purpose of this test is to make sure that there are no high resistance connections  in the link.

### Attenuation

This is the decrease in signal strength (expressed as negative dB) from one end of a cable to the other. The main causes of attenuation are impedance, temperature, skin effect and dielectric loss. Impedance  is the combination of resistance, inductance and capacitance in a cable, it is measured in Ohms and  opposes the flow of current. Skin effect is phenomena which happens at high frequencies where the  signal tries to escape from the confines of the copper and into the air. The signal travels along the outer 'skin' of the copper which effectively reduces the cross sectional area of the cable and therefore  increases its resistance.

### NEXT

This stands for Near End cross  Talk, and it occurs because  alternating current flow  produces an electromagnetic  field around the cable, this field  then induces a current flow in  adjacent cables. The strength of this field increases with the  frequency of the signal, and  because the speed of data  transmissions is ever  increasing, NEXT is a big  problem. The name 'Cross Talk' comes  from the telecommunications  industry, you may have heard a  faint conversation in the  background while on the phone  yourself, this is caused by the  electromagnetic effect between  adjacent telephone wires. In the  transmission of data, cross talk  is at its highest level in the RJ45 connection as it enters the cable, or at the 'Near End'. The term 'Near  End' is slightly confusing because data can travel in both directions, and the NEXT test is carried out in  both directions automatically by the tester, so the NEXT result is relative to the end of the cable that it  was carried out on. The twists in a cable help to cancel out the effects of NEXT and the more twists there are, the better  the cancellation, however, the twists also increase attenuation, so there is a trade off between NEXT  cancellation and attenuation. The twist rates in data cables are optimised for the best overall  performance, the twist rates are also varied for each pair within the cable to help combat crosstalk.

### PSNEXT

This stands for Power Sum Near End Cross Talk and is actually just a calculation. When a tester  carries out the NEXT test it measures the cross talk on each pair as affected by each of the other three  pairs individually, PSNEXT is simply the addition of the three NEXT results for each pair. So this is the  combined effect that a pair would be subject to when used in a network that supports a four pair  transmissions method, e.g.. Gigabit Ethernet.

### PSELFEXT

Basically, Far End Cross  Talk (FEXT) is like NEXT  but it is measured at the  far end (well that seems  logical!). However, on its  own FEXT doesn't mean  much because the length  of the cable determines  how much the signal is  attenuated before it can  affect the pairs at the far  end. To compensate for  this, and to provide a more  meaningful result, the  attenuation is subtracted  from the FEXT test and the  result is then called Equal  Level Far End Cross Talk  (ELFEXT).  And of course, no test  parameter these days would be complete without adding the results together for each pair and calling it  a Power Sum measurement, so now we have Power Sum Equal Level Far End Cross Talk or  PSELFEXT for short. There is now a new term for ELFEXT which is "Attenuation to Crosstalk Ratio,  Far-end" (or ACRF).

### Delay

This is the propagation delay or the time it takes for the signal to travel from one end of the cable to the other, it is not very important on it own because its value is directly proportional to the length of the  cable. What is important is the relationship between the delays on each of the four pairs. This brings us nicely on to .........................

### Delay Skew

Now this is important, Delay Skew is the difference  between the fastest and slowest pairs. Some  networks use a four pair transmission method, this  means that the signal is split into four, sent down the  four pairs in the cable and re-combined at the far  end. It is essential that the signals reach the far end  at near enough the same time, otherwise the signal  will not be re-combined correctly.