- Testing UTP Cabling
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- Since Cat 5e/6/6A UTP cable is used
to the fullest extent of its performance envelope, comprehensive performance testing is
very important. There are three basic tests that are called for
as part of the EIA/TIA-568 specs for all UTP cables: wiremap,
length and high speed performance. We'll take a look at each
of them and equipment needed to test them.
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- What Is A "Certified"
Cable?
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- Certification has been used by vendors of testers to mean that the
cable was tested and passed by one of the Cat 5e/6/6A "certification"
testers which test all the standard's specified performance parameters.
It means that the cabling meets the minimum specifications of
EIA/TIA standards and should work with any network designed to
operate on a Cat 5e/6 link.
Thus,
a "certification tester" or "certifier" is an instrument that tests the
cabling and compares it to the TIA-568 standards, certifying that the
cable meets the minimum performance specifications required by
the standard.-
- What is "Verification"?
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- Alternatively, cable may be
tested to determine if it will carry the network signals intended
for use on the cabling systems. These testers run network bit error rate
tests (BERT) over the cable as well as checking wiremaps and
length. A "cable verifier" will guarnatee the cabling will support Gigabit Ethernet, for example,
but does not test to the TIA cabling standards, only a problem if
some other usage, such as analog video, may be used.
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- Wiremap
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- Wiremapping
is a simple test that confirms that each wire is hooked up
correctly, with no opens or shorts. UTP intended only for POTS (plain
old telephone service) voice applications actually only needs to be
tested for wiremap. Wiremapping is very straightforward. Structured
cabling standards do not consider simple voice grade cable, only cable
of Categroy 3 or above, so most cable testing will require more than
just wiremapping. Each pair must be connected to the correct pins at
the plugs and jacks, with good contacts in the terminations. A
"wiremapper" is basically a continuity checker that determines if pins
are correctly connected.
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- Most of the failures are simple
enough to understand, like reversed wires in a pair, crossed
pairs, opens or shorts. One possible failure, crossed pairs,
is caused when both wires of a pair are crossed at one termination.
The usual cause of a crossed pair is a 568A termination on one
end and a 568B on the other.
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- The most difficult wiremap problem
is a split pair, when one wire on each pair is reversed on both
ends. It causes the signal to be sent on one wire each of two
pairs. The usual DC wiremap will pass but crosstalk will fail.
It takes a more sophisticated wiremapper or Cat 5e/6/6a tester to
find a split pair, as some wiremappers which use only DC tests
do not check crosstalk. In our experience, a split pair is usually
caused by someone using punchdown color codes on jacks which
splits the pairs.
More information on wiremap testing and troubleshooting. -
- Length
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- Since
568 cables must be less than 90 meters (296 feet) in the permanent link
and 100 meters in the channel (328 feet), cable length must be tested.
This is done with a "time domain reflectometer" (TDR) which is a cable
"radar". The tester sends out a pulse, waits for a reflection from the
far end and measures the time it took for the trip. Knowing the speed
of the pulse in the cable (calibrated for various cable types), it
calculates the length. All cable certification or verification testers
include a TDR to measure length.
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- Besides
measuring length, the TDR looks at the polarity of reflected pulses to
find shorts or opens. If you have a short or open, the TDR will tell
you what and where the problem is by looking at the return pulse,
making it a great tool for
troubleshooting problems. If the return pulse is the same polarity, the
cable is open. If the pulse is of opposite polarity, the cable is
shorted. If no return pulse is seen, the cable is terminated at its
characteristic impedance.
Performance Testing For Certification
Performance
testing for attenuation, crosstalk, etc. requires testing over the full
frequency range of the cable. The frequency range for each cable type
is:
Cat 3: 16 MHz
Cat 5/5e: 100 MHz
Cat 6: 250 Mhz
Cat 6A: 500 MHz-
- Attenuation
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- The proper operation of a LAN
on the cable plant requires the signal strength be high enough
at the receiver end. Thus the attenuation of the cable is very
important. Since LANs send high speed signals through the cable
and the attenuation of the cable is variable with the frequency of the signal,
certification testers test attenuation at many frequencies
specified in the 568 specs.
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- This test requires a tester
at each end of the cable, one to send and one to receive, then
one of them will calculate the loss and record it. There are
pass fail criteria for the cable at Cat 3, 4, 5, 5e, 6 and 6A max
frequencies. Here is how a typical cable attenuation changes with frequency.
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- Crosstalk (NEXT)
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- It's called NEXT for "near
end cross talk" since it measures the crosstalk (signal
coupled from one pair to another) at the end where one pair is
transmitting (so the transmitted signal is largest causing the
most crosstalk.) Crosstalk is minimized by the twists in the
cable, with different twist rates causing each pair to be antennas
sensitive to different frequencies and hopefully not picking
up the signals from it's neighboring pairs. Remember what we've
said repeatedly: you MUST keep the twists as close to the
terminations as possible to minimize crosstalk.
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- Cat 5e /6 testers measure crosstalk
from one pair to all three other pairs for each pair and compare
it to the 568 specs, giving a pass/fail result. Some also calculate
"ACR" or attenuation/crosstalk ratio, as it is a measure
of how big the crosstalk signal is to the attenuated signal at
the receiver. You want this number as big as possible, as it
is an indication of the signal to noise ratio.
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- Tests on Cat 5e/6 for Gigabit Ethernet
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- The additional test
specs for Category 5e and 6 includes a number of new tests to
insure higher performance from the cable to make it compatible with Gigabit Ethernet. These tests relate
to higher bandwidth usage of the cable and simultaneous use of
all four pairs in both directions at once.
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- Powersum Crosstalk (NEXT) is the NEXT on one pair when all three
others are carrying signals. This is realistic with 1000Base-T
Gigabit Ethernet where all pairs carry signals simultaneously.
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- Far end crosstalk, looking at
the effect of the coupling from one pair to another over the
entire length, measured at the far end. As tested, it's ELFEXT
or equal level FEXT, or the ratio of FEXT to attenuation, similar to ACR.
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- Delay Skew measures how much simultaneous pulses
on all 4 pairs spread out at the far end. This measures the speed
on each pair, which may be different due to the variations in
number of twists (more twists means longer wires) or insulation.
Since 1000Base-T Gigabit Ethernet uses all 4 pairs with the signals
split into 4 separate signals, it's necesary to have all arrive
simultaneously. Testers measure Propogation Delay, the
actual transit time on the pairs to calculate Delay Skew.
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- Return Loss is a measure of the reflections from
the cable due to variations in the impedance. These reflections
can cause signal degradation, especially if the pairs are used
in a full-duplex (bidirectional) mode. With 1000Base-T Gigabit
Ethernet transmitting in both directions on each pair, return
loss can cause big problems.
Requirements For Testing Cat 6/6A For 10Gigabit Ethernet
The
development of augmented Cat 6 (Cat 6a) cable for use on 10 Gigabit
Ethernet links added a new test. The cable is so precisely made,
especially the rate of twist in the pairs, that cable pairs can
interfere with the same pair in other cables nearby. This added a
new test for Cat 6A which is called "Alien Crosstalk."
Performing
this test is time consuming and is highly dependent on the physical
location of cables. Some controversy regarding the relevance of this
test exists in the industry, with some cabling vendors not requiring
it.
- Cable Testers
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- Wiremappers
test the connections and Cat 5e/6 certification testers test the
performance at high frequencies. Cable Certifiers test the cable
according to TIA-568 standards. Cable Verifiers test the cable to see
if it will transmit Ethernet signals without errors.
- Cable Certification testers are mostly automated,
"push a button get a pass/fail" simple. Certification
testers test everything, wiremap, length, attenuation and crosstalk
in one connection, give you a pass/fail result, help on troubleshooting
and store the result for printing reports for the customer.
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- Some installers use the certification
tester for all testing, after the cable is installed. But it's
a very expensive unit that needs a trained operator and many
failures are simply wire map problems. Others have each crew
use an inexpensive wiremapper to make sure all connections are
correct before the certification tester is brought in. By having
each crew find and fix their own wiremap problems, testing and
corrections are done as the cable is installed and the cost of
the certification tester is not wasted on simple problems. It's
just provides the high frequency tests and documentation required
by most users.
Cable
Verifiers are a new class of testers that use the Ethernet
communications protocols to ensure the cable supports the system
intended for use on it, generally a LAN or connection to a wireless
access point.
Since some UTP cables are used for non-Ethernet
applications like CCTV, security or building management systems that
are designed to operate on TIA-568 standardized cabling, a
certification tester may be a better choice for them.-
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- Permanent Link Adapters
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- The tester's adapter interface
cable may be the weakest link when testing. Conventional adapter
cords may be the cause for many false failures in the field.
Susceptable to the daily wear and tear associated with rough
field conditions, they degrade with time and contribute to return
loss, crosstalk and attenuation.
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- Until now, each tester used
personality modules specific to each manufacturer's Cat 6/6a cabling
for testing. The personallity modules insured that the connection
between the adapter and the link under test yield optimum performance
and more valid tests.
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- A change in the definition of
the "link" was implemented in EIA/TIA568 B and
ISO 11801 AM2 and it is now called the "permanent link."
The permanent link moves the test reference point to the end
of the test cable at the wall outlet or patch panel jack, including
only the connector on the end of the tester interface cable.
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- Test your comprehension with the section quiz.
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