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.

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.

FEXT, ELFEXT and

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 .........................