testing. permanent link definition –horizontal cabling extending from the wall plate to the cross...
TRANSCRIPT
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)
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 17
• 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
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 18
• 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
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 19
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
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 20
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
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 21
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
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 37
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