• 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.
• Also checks for continuity, shorts, crossed pairs, reversed pairs and split pairs.
Balanced Cabling Testing
Wire Map
The Wire Map 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.
The test also checks for continuity, short circuits, 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. Balanced Twisted-pair 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.
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Balanced Cabling Testing
Length
This is actual cable length (jacketed length) and is measured electrically by converting the time it takes for a signal to travel down the conductors and converting it to length.
PASS/FAIL is based on the length of the shortest conductor pair.
Length
Length
When a tester carries out the Length test it makes the measurement by sending an electrical signal down each pair and measuring the time it takes for the signals to be reflected back from the far end. Each cable pair has a different length from its
neighbours because of the different twist lengths so PASS/FAIL is based on the length of the shortest pair.
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Balanced Cabling Testing
NVP
For an accurate measurement, the correct NVP (Nominal Velocity of Propagation) must be entered during the test set-up.
An incorrect NVP setting will result in length readings that are too long or too short.
Length
NVP
The speed at which an electrical signal travels down a cable is expressed as the Nominal Velocity of Propagation (NVP). The value is shown as a percentage of the speed of light in a vacuum (100%). Balanced twisted-pair cables normally have an NVP of ~ 70%. The actual value is mainly dependent on the length of copper in the cable pair and the material used for the insulation.
The NVP value for a specific cable type can be found in the manufacturer’s specification sheet. Test instruments also hold the NVP for major manufacturers’
cables in a table from which the appropriate part number can be selected.
Instruments also have a facility that allows NVP to be measured from a known cable length.
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Insertion loss/Attenuation
This is the decrease in signal strength (expressed as negative dB) from one end of a cable to the other.
Balanced Cabling Testing
Insertion Loss
The conductor and insulation are the major contributions to Insertion Loss. In
general, the higher the signal frequency and/or the greater the distance, or length, of cable, the greater the loss.
The main causes of Insertion Loss 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 a phenomenon 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.
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NEXT (Near end Crosstalk loss)
NEXT is a measure of how much signal energy is radiated from one transmitting pair and capacitively coupled to an adjacent pair (of conductors)
Balanced Cabling Testing
NEXT
NEXT is a measure of how much signal energy is radiated from one transmitting pair and capacitively coupled to an adjacent pair of conductors.
Thus, NEXT is defined as the ratio of the strength of transmitted signal to the
coupled signal measured on the adjacent receiver pair quoted in decibels (dB). The higher the value, the better the cable performance.
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.
NEXT problems are most noticeable at the two ends of the link and are often caused by poor termination practices or faulty connectors.
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PSNEXT (Power-sum Near End Crosstalk)
This is a computation of the unwanted signal coupling from multiple transmitters at the near-end into an adjacent pair measured at the near-end.
Balanced Cabling Testing
PSNEXT
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.
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Attenuation to Crosstalk Ratio (ACR)
• ACR is the difference, expressed as a figure in decibels (dB), between the signal Insertion Loss/Attenuation produced by a wire or cable transmission medium and the near-end crosstalk (NEXT)..
ACR = NEXT – Insertion Loss (at corresponding frequencies)
• The ACR is a quantitative indicator of how much stronger the attenuated signal is than the crosstalk at the destination (receiving) end of a communications circuit.
Balanced Cabling Testing
ACR
Attenuation to Crosstalk Ratio (ACR) a factor in determining how far a signal can be transmitted in any given medium. In order for a signal to be received with an
acceptable bit error rate, the attenuation and the crosstalk must both be minimized.
In practice, the attenuation depends on the length and gauge of the wire or cable transmission medium, and is a fixed quantity. However, crosstalk can be reduced by ensuring that twisted-pair wiring is tightly twisted and is not crushed, and by ensuring that connectors between wire and cable media are properly rated and installed.
Crosstalk can sometimes be reduced by replacing unshielded twisted pair (UTP) wiring with shielded twisted pair (STP) wiring.
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Power Sum Attenuation to Crosstalk Ratio (PSACR)
PSACR = PSNEXT – Insertion Loss (at corresponding frequencies)
Balanced Cabling Testing
PSACR
Instead of the ACR values being measured for all six pair combinations they are calculated for the four pairs in the cable. Modern protocols utilise more than one pair to achieve their high bit rates. In situations such as these more than one signal in each direction could be transmitted at any one time.
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Return Loss
Return Loss (RL) is the difference between the power of a transmitted signal and the power of the signal reflections caused by variations in link and channel impedance.
It is especially important for applications using simultaneous bi-directional transmission.
Balanced Cabling Testing
Return Loss
A return loss plot indicates how well the link and channel's impedance matches its rated impedance over a range of frequencies. High return loss values mean a close impedance match, which results in greater differentiation between the powers of transmitted and reflected signals. Links and channels with high return loss values are more efficient at transmitting LAN signals as less of the signal is lost in reflections.
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Far End Crosstalk (FEXT)
FEXT is the measure of the unwanted signal coupling from a transmitter at the near-end into an adjacent pair measured at the far-end.
Balanced Cabling Testing
FEXT
New protocols use multiple pairs and the signals can travel in opposite directions at the same time. It is no longer sufficient to simply test for Cross Talk at the Near End but a Far End Cross Talk test must also be completed.
The test signal is transmitted from one end of the cabling sample and measured at the other on a different pair. By repeating the tests on all combinations of pairs in both directions a full evaluation of FEXT can be derived.
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Attenuation Crosstalk Ratio Far End (ACR-F)
(ACR-F) is the result of subtracting the Insertion Loss from the FEXT measurement at corresponding frequencies.
ACR-F = FEXT – Insertion Loss(at corresponding frequencies)
Balanced Cabling Testing
ACR-F
Since the signal has been attenuated along the length of the cable, that attenuation is added back to the final measurement to give Equal Level Far End Crosstalk (ELFEXT). This adding back of the attenuation provides a relative measurement of FEXT and allows a true comparison of the level of received signals at the Far End.
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Power Sum Attenuation Crosstalk Ratio Far End (PSACR-F)
Computation of the unwanted signal coupling from multiple transmitters at the near-end into a pair measured at the far-end relative to the received signal level on that pair.