displays is a negative number, the larger the
number, the lower the NEXT on the wire pair. As previously
mentioned, the PSNEXT test is actually a calculation based on
combined NEXT effects. The equal-level far-end crosstalk
(ELFEXT) test measures FEXT. Pair-to-pair ELFEXT is expressed
in dB as the difference between the measured FEXT and the
insertion loss of the wire pair whose signal is disturbed by
the FEXT. ELFEXT is an important measurement in Ethernet
networks using 1000BASE-T technologies. Power sum equal-level
far-end crosstalk (PSELFEXT) is the combined effect of ELFEXT
from all wire pairs. Return loss is a measure in decibels of
reflections that are caused by the impedance discontinuities at
all locations along the link. Recall that the main impact of
return loss is not on loss of signal strength. The significant
problem is that signal echoes caused by the reflections from
the impedance discontinuities will strike the receiver at
different intervals causing signal jitter. Web Links
Cable Testing http://www.cabletesting.com/CableTesting/
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Content 4.2 Signals and Noise
4.2.7 Time-based parameters Propagation delay
is a simple measurement of how long it takes for a signal to
travel along the cable being tested. The delay in a wire pair
depends on its length, twist rate, and electrical properties.
Delays are measured in hundredths of nanoseconds. One
nanosecond is one-billionth of a second, or 0.000000001 second.
The TIA/EIA-568-B standard sets a limit for propagation delay
for the various categories of UTP. Propagation delay
measurements are the basis of the cable length measurement.
TIA/EIA-568-B-1 specifies that the physical length of the link
shall be calculated using the wire pair with the shortest
electrical delay. Testers measure the length of the wire based
on the electrical delay as measured by a Time Domain
Reflectometry (TDR) test, not by the physical length of the
cable jacket. Since the wires inside the cable are twisted,
signals actually travel farther than the physical length of the
cable. When a cable tester makes a TDR measurement, it sends a
pulse signal down a wire pair and measures the amount of time
required for the pulse to return on the same wire pair. The TDR
test is used not only to determine length, but also to identify
the distance to wiring faults such as shorts and opens. When
the pulse encounters an open, short, or poor connection, all or
part of the pulse energy is reflected back to the tester. This
can calculate the approximate distance to the wiring fault. The
approximate distance can be helpful in locating a faulty
connection point along a cable run, such as a wall jack. The
propagation delays of different wire pairs in a single cable
can differ slightly because of differences in the number of
twists and electrical properties of each wire pair. The delay
difference between pairs is called delay skew. Delay skew is a
critical parameter for high-speed networks in which data is
simultaneously transmitted over multiple wire pairs, such as
1000BASE-T Ethernet. If the delay skew between the pairs is too
great, the bits arrive at different times and the data cannot
be properly reassembled. Even though a cable link may not be
intended for this type of data transmission, testing for delay
skew helps ensure that the link will support future upgrades to
high-speed networks. All cable links in a LAN must pass all of
the tests previously mentioned as specified in the
TIA/EIA-568-B standard. These tests ensure that the cable links
will function reliably at high speeds and frequencies. Cable
tests should be performed when the cable is installed and
afterward on a regular basis to ensure that LAN cabling meets
industry standards. High quality cable test instruments should
be correctly used to ensure that the tests are accurate. Test
results should also be carefully documented. Web Links
Computer Basics http://www.cabletesting.com/CableTesting/
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Content 4.2 Signals and Noise
4.2.8 Testing optical fiber A fiber link
consists of two separate glass fibers functioning as
independent data pathways. One fiber carries transmitted
signals in one direction, while the second carries signals in
the opposite direction. Each glass fiber is surrounded by a
sheath that light cannot pass through, so there are no
crosstalk problems on fiber optic cable. External
electromagnetic interference or noise has no affect on fiber
cabling. Attenuation does occur on fiber links, but to a lesser
extent than on copper cabling. Fiber links are subject to the
optical equivalent of UTP impedance discontinuities. When light
encounters an optical discontinuity, some of the light signal
is reflected back in the opposite direction with only a
fraction of the original light signal continuing down the fiber
towards the receiver. This results in a reduced amount of light
energy arriving at the receiver, making signal recognition
difficult. Just as with UTP cable, improperly installed
connectors are the main cause of light reflection and signal
strength loss in optical fiber. Because noise is not an issue
when transmitting on optical fiber, the main concern with a
fiber link is the strength of the light signal that arrives at
the receiver. If attenuation weakens the light signal at the
receiver, then data errors will result. Testing fiber optic
cable primarily involves shining a light down the fiber and
measuring whether a sufficient amount of light reaches the
receiver. On a fiber optic link, the acceptable amount of
signal power loss that can occur without dropping below the
requirements of the receiver must be calculated. This
calculation is referred to as the optical link loss budget. A
fiber test instrument checks whether the optical link loss
budget has been exceeded. If the fiber fails the test, the
cable test instrument should indicate where the optical
discontinuities occur along the length of the cable link.
Usually, the problem is one or more improperly attached
connectors. The cable test instrument will indicate the
location of the faulty connections that must be replaced. When
the faults are corrected, the cable must be retested. Web
Links Cable Testing http://www.cabletesting.com/CableTesting/
default.htm
Content 4.2 Signals and Noise
4.2.9 A new standard On June 20, 2002, the
Category 6 (or Cat 6) addition to the TIA-568 standard was
published. The official title of the standard is
ANSI/TIA/EIA-568-B.2-1. This new standard specifies the
original set of performance parameters that need to be tested
for Ethernet cabling as well as the passing scores for each of
these tests. Cables certified as Cat 6 cable must pass all ten
tests. Although the Cat 6 tests are essentially the same as
those specified by the Cat 5 standard, Cat 6 cable must pass
the tests with higher scores to be certified. Cat6 cable must
be capable of carrying frequencies up to 250 MHz and must have
lower levels of crosstalk and return loss. A quality cable
tester similar to the Fluke DSP-4000 series or Fluke
OMNIScanner2 can perform all the test measurements required for
Cat 5, Cat 5e, and Cat 6 cable certifications of both permanent
links and channel links. Figure shows the Fluke DSP-LIA013
Channel/Traffic Adapter for Cat 5e. Lab Activity Lab
Exercise: Fluke 620 Cable Tester – Wire MapIn this lab, the
student will learn the wire mapping feature of the Fluke 620
LAN CableMeter or its equivalent. Lab Activity Lab