Testing

Permanent Link

• Definition– Horizontal cabling extending from the wall plate to the cross

connect

WORK AREA

TELECOMMUNICATIONS CLOSET

Horizontal

Cross-connect

Wall Outlet

Maximum Length - 90 Meters

Channel

• Definition

– The end-to-end transmission path connecting any two points at

which application specific equipment is connected. Equipment

and work area cables are included in the channel

Channel

WORK AREA

TELECOMMUNICATIONS CLOSET

Horizontal Cross-connect

Wall Outlet

ConcentrationPoint

EquipmentCable

PatchCable

Maximum Length 100 Meters

Permanent Link Test

• Configuration

Horizontal Cross-connect

WORK AREA

TELECOMMUNICATIONS CLOSET

Wall Outlet

Test Set

Test Set

Test Set Connection Cable (2 meters)

Maximum Length 94 meters

Channel Test

• Configuration

TELECOMMUNICATIONS CLOSET

Horizontal

ConcentrationPoint

PatchCable

Maximum Length 100

Meters

WORK AREA

Cross-connect

Wall Outlet

EquipmentCable

Test Set

Test Set

Twisted Pair

• Before we look at each of the basic tests we need to review the

– Physical– Electrical characteristics of a wire pair

Anatomy of a Wire Pair

• What we think of when we envision a wire pair.

Anatomy of a Wire Pair

• However, to a signal it looks like:– Resistors– Capacitors– Inductors

• The cable is not made up of discrete components but has the characteristics of these components

– Resistance– Capacitance– Inductance

• The characteristics are changed when a cable is:– Stretched (pulling / laying)– Kinked (handling / storage / tight corners)– Distorted (cable ties / stood upon / Heavy cable

bundles)

Damaging the cable

Physical Layer TestsTest Parameter TIA-568-B ISO 11801:2002

Wiremap Pass/Fail Pass/Fail

Propagation Delay Pass/Fail Pass/Fail

Delay Skew Pass/Fail Pass/Fail

Cable Length Pass/Fail Information only

Insertion Loss (IL) Pass/Fail Pass/Fail

Return Loss (RL) Pass/Fail (except Cat3)

Pass/Fail

Near-End Crosstalk (NEXT) Pass/Fail Pass/Fail

Power Sum NEXT (PSNEXT) Pass/Fail Pass/Fail

Equal-Level Far-End Crosstalk (ELFEXT)

Pass/Fail Pass/Fail

Power Sum ELFEXT (PSELFEXT) Pass/Fail Pass/Fail

Attenuation-to-Crosstalk Ratio (ACR) Information only Pass/Fail (except Class C)

Power sum ACR (PSACR) Information only Pass/Fail (except Class C)

DC Loop Resistance Pass/Fail

Wire Map

• Verification of the physical connection at each end of the cable

• Checks for– Opens

– Shorts

– Crosses

– Reverses and any other misfiring

– Splits• Difficult to identify with a DC wire map tester• Must use an advanced tester• Will fail on crosstalk

Length

• Physical– Calculated based on the length markings on the cable

• Maximum physical length of a permanent link is 90 meters

• Maximum physical length of a channel is 100 meters

Length

• Electrical– Based on the propagation delay of a signal over the

cable pair• Accomplished using a TDR (Time Domain Reflectometer)

• Calculation base on the Nominal Velocity of Propagation (NVP) of the signal over the pair being tested

• An analogy is friction

• Some of the signal is lost over distance

• Energy is dissipated in the form of heat

• Message may become to weak to be understood

DATA DATA DATA DATA DATA DATA DATA DATA DATA

Attenuation

Decibels dB’s

• Before we go any further we must talk a little about decibels (dB’s)

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• The deciBel is a convenient means of expressing the ratio of two values

– it is commonly used for example to express the Gain or Attenuation of a signal path:

Signal Path

Electrical/ Optical Signal

Input

Electrical/ Optical Signal

Output

Gain or Attenuation = Output Signal ÷ Input Signal

Decibels dB’s

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• deciBel expressions for the power and voltage level of a signal are slightly different:

– Voltage Gain/Attenuation =

– Power Gain/Attenuation =

Note: For Optical Fibre only the Power expression is relevant

IN

OUT

V

VLog1020

IN

OUT

P

PLog1010

Decibels dB’s

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A NEXT reading is -40 dB for Pair 1 to Pair2. What percentage of the Pair 1 input voltage appears at the input of Pair 2 ?

ANSWER

This result says that 1% of whatever is injected into Pair 1 will appear at the input of Pair 2

dBV

VLog

IN

NEXT 40201,

2,10

220

40

1,

2,10

IN

NEXT

V

VLog

%101.010 2

1,

2, orV

V

IN

NEXT

Example 1

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A FEXT reading is -50 dB from Pair 3 to Pair2. What percentage of the Pair 3 input voltage appears at the output of Pair 2 ?

ANSWER

This result says that 0.3% of whatever is injected into Pair 3 will appear at the output of Pair 2

dBV

VLog

IN

FEXT 50203,

2,10

5.220

50

3,

2,10

IN

FEXT

V

VLog

%3.0003.010 5.2

3,

2, orV

V

IN

FEXT

Example 2

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Pair 4 has an Attenuation reading of 20 dB. What percentage of the Pair 4 input voltage appears at the output of Pair 4 ?

ANSWER

This result says that 10% of whatever is injected into Pair 4 will appear at the output of Pair 4

dBV

VLog

IN

OUT 20204,

4,10

120

20

4,

4,10

IN

OUT

V

VLog

%101.010 1

4,

4, orV

V

IN

OUT

Example 3

• The loss of signal strength over the cable pair,

measured in dB

– Increases as the carrier frequency increases

– Pass/fail based on the worst case attenuation of all

pairs

Back To Attenuation

Attenuation

Crosstalk

• Defined as the induction of a portion of the signal from one pair into the adjacent pairs

– Measured in dB

– NEXT• NEAR END CROSS TALK

– ELFEXT• EQUAL LEVEL FAR END CROSS TALK

– PSNEXT • POWER SUM NEAR END CROSS TALK

– PSELFEXT • POWER SUM EQUAL LEVEL FAR END CROSS TALK

Cross Talk Types

NEXT and ELFEXT

• A final point is that ELFEXT depends greatly on the length of cable being examined, whereas the NEXT is much less dependent

Crosstalk Components

• Inductive and Capacitive Relationships:

• SPILL OVER

• ELECTRONS ARE LOST TO THE ADJACENT CABLE PAIRS

• THE MESSAGE IS CORRUPTED

Cross Talk

DATA DATA ATA DATA DATA DATA DAA DATA DATA

DATA DATA DDATA DATA DATA DATTA DATA DATA

• NEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS UPON THE OTHER WITH RESPECT TO THE NEAR END OF THE CABLE

NEXT

Near-End Crosstalk (NEXT)

Power Sum

• Use of the TIA Algorithm for Calculating Power Sum– Calculation is derived from the conventional NEXT measurement.– Only one pair is energized at a time; formula “sums” the crosstalk

effect of three energized pairs on the remaining fourth pair.

• POWER SUM NEXT IS THE COMPUTED EFFECT OF THREE PAIRS UPON THE FOURTH WITH RESPECT TO THE NEAR END OF THE CABLE

PSNEXT

• ELFEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS UPON THE OTHER WITH RESPECT TO THE FAR END OF THE CABLE

ELFEXT

• POWER SUM ELFEXT IS THE COMPUTED EFFECT OF THREE PAIRS UPON THE FOURTH WITH RESPECT TO THE FAR END OF THE CABLE

PSELFEXT

• NEXT and ELFEXT (and crosstalk in general) depend very much on the frequency at which they are measured

– they increase as the frequency increases– on average they may be described as follows:

NEXT and ELFEXT

23

frequencyNEXT 2frequencyFEXT

NEXT and ELFEXT

• In practise this means as the Cat6 specification includes frequencies up to 250 MHz, that both the NEXT and ELFEXT measurements will increase dramatically

– For example the NEXT at 250 MHz should be about 4 times higher than the NEXT at 100MHz for any given combination of pairs

– Even worse the ELFEXT at 250 MHz should be over 6 times higher than the value at 100 MHz for any given combination of pairs

– For these reasons (and more) cable that passes Cat 5/5e quite easily may still fail the Cat 6 standard

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CAT 5 to CAT 7 NEXT values at 100 MHz

NEXT dB Calculations

SPECIFICATION NEXT Percentage

CATEGORY 5 32 dB 2.5%

CATEGORY 5e 35 dB 1.8%

CATEGORY 6 43 dB 0.7%

CATEGORY 7 71 dB 0.03%

Attenuation to Crosstalk Ratio

• Definition:– A ratio comparing the received signal with the near-end crosstalk on a

cable

• It is not a direct test– It is a comparison of the attenuation test and NEXT test– Used to give an indication of how problem-free the cable line will be

Attenuation-to-Crosstalk ratio (ACR) is the difference between the signal attenuation produced and NEXT and is measured in decibels (dB). The ACR indicates how much stronger the attenuated signal is than the crosstalk at the destination (receiving) end of a communications circuit. The ACR figure must be at least several decibels for proper performance. If the ACR is not large enough, errors will be frequent. In many cases, even a small improvement in ACR can cause a dramatic reduction in the bit error rate. Sometimes it may be necessary to switch from un-shielded twisted pair (UTP) cable to shielded twisted pair (STP) in order to increase the ACR.

Attenuation to Crosstalk Ratio (ACR)

Propagation Delay

• Propagation Delay– The time needed for the signal to travel from the transmitter to

the receiver over a 100 Ohm 4-pair cable.

• THE TRANSMISSION TIME

T1 T2

Delay

• THE DIFFERENCE IN TIME BETWEEN THE FASTEST AND SLOWEST PAIR

T2

T1 T3Delay Skew = Difference between T2 and T3

Delay Skew

• DELAY – THE TRANSMISSION TIME

• DELAY SKEW (X)– THE DIFFERENCE IN TIME BETWEEN THE FASTEST AND

SLOWEST PAIR

Delay / Delay skew

X

Propagation Delay

DC Loop Resistance

• DC Loop Resistance measures the total resistance through one wire pair looped at one end of the connection. This will increase with the length of the cable. DC resistance usually has less effect on a signal than insertion loss, but plays a major role if you require power over Ethernet. Also measured in ohms is the characteristic impedance of the cable, which is independent of the cable length

Return Loss

• It measures the difference between test signal’s amplitude and the amplitude of signal reflections returned by the cable.

• Information Provided:– Indicates how well the cable’s characteristic impedance matches its

rated impedance

Return Loss is the measurement (in dB) of the amount of signal that is reflected back toward the transmitter. The reflection of the signal is caused by the variations of impedance in the connectors and cable and is usually attributed to a poorly terminated wire. The greater the variation in impedance, the greater the return loss reading. If 3 pairs of wire pass by a substantial amount, but the 4 pair barely passes, it usually is an indication of a bad crimp or bad connection at the RJ45 plug.

Return Loss