| Typical failures: |
| Installation errors, especially with wiremap and pair
twisting; always keep the original twist as much as possible. |
| Bad connectors |
| Inappropriate tester setup |
| Patch Cord(s) or cable is bad |
| Cabling effects not accounted for in the link model |
|
|
| Always verify the following: |
| The appropriate test is selected |
| The correct link model is selected |
| The appropriate adapter is being used |
| The tester has been updated with the latest Firmware |
| The NVP is set correctly for the cable being tested |
| The tester is calibrated & within its operating temperature
range |
|
|
| LINK MODELS: |
Choosing the appropriate autotest and link
model is essential. The majority of all field testing should
use the Permanent Link model. When performing Permanent Link
tests make sure you use adapters appropriate for the cable that
is under test. Channel measurements are typically done when
restoring service or verifying cable for application support. |
|
| FINDING CABLE FAULTS: |
This list of each cabling measurement shows possible causes
for cable failure or why the test does not fail when a failure
is expected. |
| |
WIREMAP |
| |
The Wiremap test checks the outlet configuration
to ensure it matches the appropriate test standard.
| WIRE MAP |
| Test
Result |
Possible
Causes |
| Open
|
Wires broken at connections
Cables routed to wrong connection
Damaged connector
Cuts or break in cable
Wires connected to wrong pins at connector or
panel
|
| Short |
Improper connector termination
Connector or cable is Damaged
Conductive material stuck between pins at connection
|
| Reversed Pair |
Wires connected to wrong pins at connector or punch block |
| Crossed Pair |
Wires connected to wrong pins at connector or punch block
Mix of 568A & 568B wiring standards (pins
(1,2) & (3,6) crossed)
Crossover cables used (pins
(1,2) & (3,6) crossed)
|
| Split
Pair |
Wires connected to wrong pins at connector or punchdown block |
|
| LENGTH |
| Test Result |
Possible Causes |
| Exceeds Limits |
Cable is too long check for coiled service loops
& remove in
NVP is set incorrectly |
| Reported is shorter than known |
Intermediate break in the cable length |
| 1 or more pairs significantly shorter |
Damage to cable
Bad connection
|
|
|
| |
DELAY/SKEW |
| |
Propagation Delay Skew (skew) is the difference
between the propagation delay on the fastest and slowest pairs
in a UTP cable. Some cable construction employs different types
of insulation materials on different pairs. This effect contributes
to unique twist ratios per pair and to skew.
| Test Result - Exceeds Limits |
| Cable uses different insulation materials on different
pairs |
| Cable is too long |
|
| |
INSERTION
LOSS |
| |
Recent changes in the standards
now use the term "insertion loss" and not attenuation.
Given that test equipment manufacturers have used the term attenuation
since 1993, attenuation will continue to be seen on test reports.
Electrical signals transmitted by a link lose some
of their energy as they travel along the link. Insertion loss
measures the amount of energy that is lost as the signal arrives
at the receiving end of the cabling link. The insertion loss measurement
quantifies the effect of the resistance the cabling link offers
to the transmission of the electrical signals.
| Test Result - Exceeds Limits |
| Excessive Length |
| Non-twisted or poor quality patch cables |
| High impedence connections |
| Inappropriate cable |
| Incorrect autotest selected |
|
| |
NEXT
& PSNEXT |
| |
When a current flows
through a wire, an electromagnetic field is created which can
interfere with signals on adjacent wires. As frequency increases,
this effect becomes stronger. Each pair is twisted because this
allows opposing fields in the wire pair to cancel each other.
The tighter the twist, the more effective the cancellation and
the higher the data rate supported by the cable. Maintaining this
twist ratio is the single most important factor for a successful
installation.
Power Sum NEXT (PSNEXT) PSNEXT is derived from the
summation of the individual NEXT effects on each pair by the other
three pairs. PSNEXT is important measurements for qualifying cabling
intended to support 4 pair transmission schemes such as Gigabit
Ethernet, although IEE 8023.ab does not specifically require PSNEXT.
If you run the specific Gigabit Ethernet test within the field
tester, you will see that PSNEXT is not recorded. There are four
PSNEXT results at each end of the link per link tested.
| Test Results Fail, *Fail
or *Pass |
| Poor twisting at connection points |
| Poorly matched plug and jack |
| Incorrect link adapter (Cat
5 adapter for Cat 6 links, or incompatible Cat 6 adapter
on Cat 6 links) |
| Poor quality patch cords |
| Connectors or Cable is Bad |
| Split pairs |
| Inappropriate use of couplers |
| Excessive compression caused by plastic cable ties |
| Excessive noise source adjacent to measurement |
| Knots or kinks do not always cause NEXT failures,
especially on good cable |
|
| Unexpected Pass |
| Incorrect autotest selected |
| “Fails” at low frequency on NEXT graph
but passes overall. |
| When using the ISO/IEC standards, the so-called
4dB rule states all NEXT results measured while insertion
loss <4dB cannot fail |
|
|
| |
RETURN
LOSS |
| |
Return Loss (RL) is a measure of all reflections
that are caused by the impedance mismatches at all locations along
the link and is expressed in decibels (dB). Return Loss is of
particular concern in the implementation of Gigabit Ethernet (1000BASE-T)
| Test Results
Fail, *Fail or *Pass |
| Patch cord impedance not 100ohms |
| Installation practices (untwists
or kinks of cable) |
| Excessive amount of cable jammed in outlet box |
| Bad connector |
| Cable impedance not uniform |
| Cable not 100 ohm |
| Impedance mismatch between patch cable and horizontal
cable |
| Poorly matched plug and jack |
| Service loops in Telco Closet |
| Inappropriate autotest selected |
| Defective link adapter |
|
| Unexpected Pass |
| Knots or kinks do not always cause failures,
especially on good cable |
| Incorrect autotest selected. |
| “Fails” at low frequency on RL graph
but passes overall. |
| Due to the 3 dB rule, whereby all RL results measured
while |
| Insertion loss < 3 dB cannot fail. |
|
|
| |
ELFEXT
& PSELFEXT |
| |
ELFEXT is derived by subtracting the attenuation
of the disturbing pair from the Far End Crosstalk (FEXT) this
pair induces in an adjacent pair. This normalizes the results
for length. Consider the FEXT and attenuation measured on two
links constructed of the same materials with the same workmanship,
but different lengths.
| Test Result - Fail, *Fail
or *Pass |
| As a general rule, troubleshoot NEXT problems first. This
normally corrects ELFEXT problems |
| Service Loops with many tightly coiled windings |
|
| |
RESISTANCE |
| |
DC Loop Resistance is the total resistance through
two conductors looped at one end of the link. This is usually
a function of the conductor diameter and varies only with distance.
This measurement is sometimes done to ensure there are no gross
misconnections which can add significant resistance to the link.
Note that the wire map test automatically isolates breaks but
not high resistance connections.
| Test Result - Fail, *Fail or
*Pass |
| Excessive cable length |
| Poor connection due to oxidized contacts |
| Thinner gauge cable |
| Incorrect patch cord type |
|
