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of Structured Cabling
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- Testing Wiring
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- Since Cat 5e/6 cable is used
to the fullest extent of its performance envelope, 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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- Wiremapping
- Wiremapping tests to make sure
the cable is connected properly, according to the standard for
connecting pin-to-pin. Basically, wiremapping is a continuity
test using an inexpensive tester. Just make sure the wiremapper
you buy tests "split pairs" as noted below.
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- What Is A "Certified"
Cable?
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- Certification is one of the
most overused and least understood words in our industry! Did
the cable pass an exam and get a certificate? Well, sort of.
This term has been used by vendors of testers to mean that the
cable was tested and passed by one of the Cat 5e/6 "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.
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- 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 bit error rate
tests (BERT) over the cable as well as checking wiremaps and
length. A Verifier will guarnatee the cabling will support Ethernet,
but does not test to the 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 simple means that
each wire is hooked up correctly, with no opens or shorts. That's
mostly very straightforward. Each pair must be connected to the
correct pins at the plugs and jacks, with good contacts in the
terminations.
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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 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.
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- Length
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- Since 568 cables must be less
than 90 meters (296 feet) in the link and 100 meters in the channel
(328 feet), length must be tested. This is done with a "time
domain reflectometer" which is a fancy term for cable "radar".
The tester sends out a pulse, waits for an "echo" from
the far end and measures the time it took for the trip. Knowing
the speed in the cable, it calculates the length. All cable certification
testers include a TDR to measure length.
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- If you have a short or open,
the TDR will also tell you where the problem is, making it a
great tool for troubleshooting problems.
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- 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 is variable with the frequency of the signal,
certification testers test attenuation at several 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, and 6 max
frequencies.
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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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- More Tests for Gigabit Ethernet
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- The latest generation of test
specs for Category 5e and 6 includes a number of new tests to
insure higher performance from the cable. These tests relate
to higher bandwidth usage of the cable and simultaneous use of
all four pairs, even in both directions at once.
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- Powersum 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, sort
of like 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.
- The "augmented" Cat
6 spec will have reference to "alien crosstalk" or
the signal coupling from one pair in a cable to the same pair
in another cable, a consequence of higher frequencies and the
consistency of twists.
- Testers
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- Wiremappers test the connections
and Cat 5e/6 certification testers test the performance at high
frequencies. Cable Certification testers are mostly automated,
"push a button get a pass/fail" simple. In fact, 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.
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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.
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- Until now, each tester used
personality modules specific to each manufacturers Cat 6 cabling
for testing. The personallity modules insured that the connection
between the adapter and the link under test yield optimum performance
and more passes. The need for separate adapters for each manufacturer's
cabling system was defacto acknowledgement of the incompatibility
of various Cat 6 products.
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- A change in the definition of
the "link" has been 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.
Compliance with this standard could speed compatibility among
Cat 6 cabling systems.
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- Upgrading Testers
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- Category 5 testers generally
cannot be upgraded to 5e or 6 but Category 5e testers can often
be upgraded to Cat 6.
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