Testing
Permanent Link
• Definition
– Horizontal cabling extending from the wall plate to the cross
connect
WORK AREA
Wall Outlet
Maximum Length 90 Meters
Horizontal
Cross-connect
TELECOMMUNICATIONS CLOSET
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
Wall Outlet
Equipment
Cable
Concentration
Point
Maximum Length 100 Meters
Horizontal
Cross-connect
Patch
Cable
TELECOMMUNICATIONS CLOSET
Permanent Link Test
• Configuration
WORK AREA
Wall Outlet
Test Set
Test Set Connection Cable (2 meters)
Horizontal
Maximum Length 94 meters
Cross-connect
Test Set
TELECOMMUNICATIONS CLOSET
Channel Test
• Configuration
WORK AREA
Test Set
Equipment
Cable
Maximum Length
100 Meters
Wall Outlet
Cross-connect
Horizontal
Concentration
Point
Patch
Cable
Test Set
TELECOMMUNICATIONS
CLOSET
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
Damaging the cable
• The characteristics
– Stretched
– Kinked
– Distorted
are changed when a cable is:
(pulling / laying)
(handling / storage / tight corners)
(cable ties / stood upon / Heavy cable
bundles)
Physical Layer Tests
Test 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
Attenuation
• 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
Decibels dB’s
• Before we go any further we must talk a
little about decibels (dB’s)
Decibels dB’s
•
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:
Electrical/
Optical Signal
Input
Signal Path
Electrical/
Optical Signal
Output
Gain or Attenuation = Output Signal ÷ Input Signal
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17
Decibels dB’s
•
deciBel expressions for the power and voltage level of a
signal are slightly different:
– Voltage Gain/Attenuation =
 VOUT
20Log10 
 VIN



– Power Gain/Attenuation =
 POUT
10 Log10 
 PIN



Note: For Optical Fibre only the Power expression is relevant
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18
Example 1
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
 VNEXT , 2 
  40dB
20 Log10 

 VIN ,1 
 VNEXT , 2 
40

Log10 
 2

V
20
 IN ,1 
 VNEXT , 2

 V
 IN ,1

  102  0.01 or 1%


This result says that 1% of whatever is injected into Pair 1 will
appear at the input of Pair 2
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19
Example 2
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
 VFEXT , 2 
  50dB
20 Log10 

 VIN ,3 
 VFEXT , 2 
50
    2.5
Log10 

20
 VIN ,3 
 VFEXT , 2 

  102.5  0.003or 0.3%
 V

IN
,
3


This result says that 0.3% of whatever is injected into Pair 3 will
appear at the output of Pair 2
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20
Example 3
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
 VOUT ,4 
  20dB
20 Log10 

V
IN
,
4


 VOUT ,4 
20
    1
Log10 

20
 VIN , 4 
 VOUT ,4 

  101  0.1 or 10%
V

 IN ,4 
This result says that 10% of whatever is injected into Pair 4 will
appear at the output of Pair 4
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21
Back To Attenuation
• 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
Attenuation
Crosstalk
• Defined as the induction of a portion of the signal
from one pair into the adjacent pairs
– Measured in dB
Cross Talk Types
– 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
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:
Cross Talk
•
•
•
SPILL OVER
ELECTRONS ARE LOST TO THE ADJACENT CABLE PAIRS
THE MESSAGE IS CORRUPTED
DATA DATA DDATA DATA DATA DATTA DATA DATA
DATA DATA ATA DATA DATA DATA DAA DATA DATA
NEXT
• NEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS
UPON THE OTHER WITH RESPECT TO THE NEAR END OF THE
CABLE
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.
PSNEXT
• POWER SUM NEXT IS THE COMPUTED EFFECT OF
THREE PAIRS UPON THE FOURTH WITH RESPECT TO
THE NEAR END OF THE CABLE
ELFEXT
• ELFEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS
UPON THE OTHER WITH RESPECT TO THE FAR END OF THE
CABLE
PSELFEXT
• POWER SUM ELFEXT IS THE COMPUTED EFFECT OF
THREE PAIRS UPON THE FOURTH WITH RESPECT TO THE
FAR END OF THE CABLE
NEXT and ELFEXT
•
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  frequency
3
FEXT  frequency
2
2
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
NEXT dB Calculations
CAT 5 to CAT 7 NEXT values at 100 MHz
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%
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37
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.
Delay
•
T1
THE TRANSMISSION TIME
T2
Delay Skew
• THE DIFFERENCE IN TIME BETWEEN THE
FASTEST AND SLOWEST PAIR
T2
T1
Delay Skew = Difference between
T2 and T3
T3
Delay / Delay skew
•
•
DELAY
– THE TRANSMISSION TIME
DELAY SKEW (X)
– THE DIFFERENCE IN TIME BETWEEN THE FASTEST AND
SLOWEST PAIR
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