29/04/2013

1

Testing Copper Cable

HB29:2007

Section 5

Testing of cabling systems falls into two distinct categories, which are thefollowing:

(a) Permanent link transmission performance tests as in Figure 5.1.(These tests are intended to be used to test the performance of thepermanently installed cabling. The tests performed on this portion of acabling as in Clause 5.2.1.)

(b) Channel transmission performance tests as in Figure 5.2.(These tests are used to verify the overall performance of the cablingsystem end to end including all cross‐connects, cabling, jumpering, flyleadsand patch cords.)

Hand‐held testers used for these performance tests should be set up foreither permanent link or channel testing.

Permanent link transmission performance tests

29/04/2013

2

Channel transmission performance tests

Tests to be performed

(a) Wire map.(b) Length.(c) Insertion loss (Attenuation).(d) Near‐end crosstalk (NEXT).(e) Attenuation to crosstalk ratio (ACR).(f) Propagation delay.

There are twelve main tests to be performed on a cabling system, which are the following:

(g) Delay skew.(h) Power sum NEXT.(i) Power sum ACR.(j) ELFEXT.(k) Power sum ELFEXT.(l) Return loss.

Wire Map

(a) Continuity—end to end—of the cable.(Ensures that the conductors are not cut along the cable length or there areno misconnections in either connector.)

(b) Short circuits between conductors.(Shows where two conductors may be joined at the connector via a strandof copper or such like.)

(c) Reversed pairs.(Where the 'A' leg and 'B' leg of a pair are crossed between connectors.)

(d) Transposed pairs.(Where pairs are transposed between connectors.)

(e) Split pairs.(Shows where the 'A' leg and 'B' leg of two pairs are crossed betweenconnectors. Pin to pin continuity is maintained but NEXT could besignificantly affected.)

29/04/2013

3

Wire Map

Length

(a) A time domain reflectometer (TDR) is used. A TDR function isincorporated into a field test instrument, and it is primarily used tomeasure length or identify location of fault condition.

(b) The instrument transmits a signal down the length of the cable.

(c) Where a change in impedance occurs (e.g. short circuit, open circuit orhigh impedance), some of the signal is reflected back to the testinstrument.

(d) The tester measures the time delay of the reflected signal andcalculates the length of the cable or distance to the fault.

(e) It also measures the size of the reflected signal and compares it to thetransmitted signal and can ascertain the likely type of fault (open circuit,short circuit or poor connection).

(f) To be accurate in measuring with a TDR, there is a need to use thecorrect nominal velocity of propagation (NVP) of the cable under test.(This is a measure of the speed of the electrical signal travelling down thecable. Every cable has its own NVP and if incorrectly set reported lengthresults will be inaccurate.)

(g) NVP changes from cable to cable and is specified for each cable typeand typically, NVP is between 0.6c – 0.9c (c = speed of light).(Check with manufacturer for the correct NVP for the type of cable you aretesting.)

(h) There are two different lengths that need to be considered:(i) Physical length, which is measured between the two end points.(ii) Electrical length, which is measured by a TDR.

(There will be a difference between measurements because of the twistrate of the conductor pairs.)

Length

29/04/2013

4

Insertion loss (Attenuation)

This test, shown in Figure 5.5, measures the amount of signal loss (orpower loss) from the source to the end of a cable or link. It is measured indecibels (dB), and the smaller the number of dB, the better theperformance of the cable.

Insertion loss (Attenuation)

It is important to note that Insertion Loss increases as frequency and length increase

Near‐end crosstalk (NEXT)

(a) NEXT is the signal coupling between adjacent conductor pairs, shown inFigure 5.7, and is the measure of the amount of the transmitted signalcoupled back into the received signal (at the near end to the transmitter),expressed in dB as in Figure 5.8. The greater the number, the better NEXTperformance.

(b) When current flows in a conductor, electromagnetic fields are createdwhich may interfere with signals in nearby conductors. This effect increasesas the frequency of the signals on the cable increases.

(c) As a measure to minimize the effect of 'crosstalk', each pair of a multi‐pair cable is twisted. This allows the field in each conductor of the pair tocancel the effect of the field in the other conductor within that pair.

(d) One of the most important practices when installing UTP cables is tomaintain the twist ratio at the point of termination.

29/04/2013

5

Near‐end crosstalk (NEXT)

The greater the number, the better NEXT performance.

Attenuation to crosstalk ratio (ACR)

Signals are at their weakest at the receiver end of a cable pair due to theattenuation of the signal along that cable. It is also at this point that theNEXT is the greatest from the adjacent transmit pair.

In balanced pair cabling networks, insertion loss determines the strength ofthe signal at the end of the cable, while the noise is primarily the NEXT fromthe station's own transmitter.

The ACR is the difference between the crosstalk loss and the insertion lossin the tested pair, as in Figure 5.9.

This is expressed in dB, as in Figure 5.10, and the larger the dB figure thebetter the link is performing in terms of ACR.

ACR(dB) = NEXT(dB) − Insertion Loss(dB)

Attenuation to crosstalk ratio (ACR)

The larger the dB figure the better the link is performing in terms of ACR

29/04/2013

6

Propagation delay

Propagation delay is the time it takes for a pulse to travel down a pair ofconductors and is measured in nanoseconds (ns). There is a different timefor each pair due to different twist rates, as in Figure 5.11.

Delay skew

Delay skew is the difference in propagation delay between the fastest andthe slowest pairs. To determine delay skew we use the propagation delayvalues, as in Figure 5.12.

Power sum NEXT (PSNEXT)

Power Sum NEXT (PSNEXT) is the sum of NEXT in one pair from a signal inthe other three pairs, as in Figure 5.13.

29/04/2013

7

Power sum NEXT (PSNEXT)

Power sum NEXT (PSNEXT)

http://m

yaccount.flukenetw

orks.com/fnet/en‐us/supp

ortAndD

ownloads/KB/Copper‐Testing/DTX

+CableAnalyzer/Power_Sum_N

EXT_%28PSN

EXT%

29

Power sum ACR (PSACR)

PSACR is calculated exactly the same as ACR except PSNEXT is used insteadof NEXT.

PSACR(dB) = PSNEXT(dB) − Insertion Loss(dB)

The larger the number the better the result

29/04/2013

8

Equal level far‐end crosstalk (ELFEXT)

ELFEXT is the measured far end crosstalk (FEXT) calculated for the nearend, as in Figure 5.15. It is calculated by reducing the FEXT by themeasured insertion loss (attenuation). It is combined with the near endcrosstalk (NEXT) to provide a noise figure. The greater the number, thebetter ELFEXT performance.

Power sum ELFEXT (PSELFEXT)

PSELFEXT is the measured combined far end crosstalk (PSFEXT) of all pairscalculated for the near end, as in Figure 5.16. It is calculated by reducing thePSFEXT by the measured insertion loss (attenuation). It is combined withthe power sum near end crosstalk (PSNEXT) to provide a noise figure. Thegreater the number, the better PSELFEXT performance.

Return loss (RL)

Signals are reflected by impedance mismatching along the entire length of achannel, as in Figure 5.17. The worst cast reflections are due mainly to poorcomponent selection (e.g. poor cable to connector matching) and poorinstallation practices (e.g. poor punch‐down techniques, tight bend radiusand tight cable ties).

Return loss is an indication of the severity of the reflections. The greaterthe number, the better RL performance.