Copper Cable Testing
Twisted pair alternatives have replaced coaxial cabling on
today's LANs. At the Category 5 performance level or above, there are a
bewildering number of options. Let's first identify, compare and contrast these
cabling alternatives.
| Name |
Cross Section |
Construction |
Expected Performance |
|
Cat 5 UTP
|
 |
Cable consists of
4 pairs of 24 AWG (0.50mm) copper with thermoplastic polyolefin or flourinated
ethylene propylene (FEP) jacket. Outside sheath consists of polyvinyl-clorides
(PVC), a fire retardant polyolefin (FRP) or fluoropolymers.
|
Mixed and matched
cables and connecting hardware from various manufacturers that have a
reasonable chance of meeting TIA Cat 5 Channel and ISO Class D requirements. No
manufacturers warranty is normally involved. |
| Cat 5E UTP |
 |
Cable consists of
4 pairs of 24 AWG copper with thermoplastic polyolefin or FEP jacket. Outside
sheath consists of PVC, FRP or fluoropolymers. Higher care taken in design and
manufacturing. |
Category 5
components from one supplier or from multiple suppliers where components have
been deliberately matched for improved impedance and balance. Designed to meet
TIA 568A-A5; often includes a 10 year or greater warranty. |
|
Cat 6 UTP
|
 |
Cable consists of
4 pairs of 0.50 - 0.53mm copper with thermoplastic polyolefin or FEP jacket.
Outside sheath consists of PVC, FRP, or fluoropolymers. Extremely high care
taken in design and manufacturing. |
Category 6
components from one supplier that are extremely well matched. Channel zero
PSACR point (effective bandwidth) is guaranteed to 200 MHz or beyond. |
| FTP |
|
Cable consists of 4 pairs of
24 AWG (0.50mm) copper with thermoplastic polyolefin or FEP jacket plus a drain
wire. Pairs are surrounded by a common metallic foil shield. Outside sheath
consists of PVC, FRP, or fluoropolymers. |
Category 5 or 5E components
from one supplier or from multiple suppliers that have been deliberately
designed to minimize EMI susceptibility and maximize EMI immunity. Various
grades may offer increased ACR performance. |
| S-FTP |
 |
Cable consists of 4 pairs of
24 AWG (0.50mm) copper with thermoplastic polyolefin or FEP jacket. Pairs are
surrounded by a common metallic foil shield, followed by a braided metallic
shield. Outside sheath consists of PVC, FRP or fluoropolymers. |
Category 5 or 5E components
from one supplier or from multiple suppliers that have been deliberately
designed to minimize EMI susceptibility and maximize EMI immunity. Offers EMI
protection that is superior to FTP. |
| SSTP
or
PIMF
|
 |
Also called PiMF (for Pairs in
Metal Foil), SSTP consists of 4 pairs of 22-23 AWG copper with a thermoplastic
polyolefin or FEP jacket. Pairs are individually surrounded by a helical or
longitudinal metallic foil shield, followed by a braided metallic shield.
Outside sheath consists of PVC, FRP or fluoropolymers. |
Category 7. Provides positive
ACR to 600 MHz. Shielding on the individual pairs gives it outstanding NEXT
performance. No standard yet for connector. Many experts doubt Category 7
connectors can achieve compatibility with RJ45 Category 5, 5E, or 6 connectors. |
At the simplest level, installers use
continuity checkers to verify end to end connections. One step up from that are
a series of very helpful tools, such as the
Fluke MicroScanner Pro. This kind of tool is great for testing
voice cabling, making quick checks on data cabling, and for the growing
residential LAN wiring market.
The focus of this site is on Category 5 and higher premise wiring
installation and test issues. Today's cable standards for this kind of cabling
requires several measurements to be made in order to certify the cabling meets
stated performance requirements. Some tests are performed worldwide, while
others are specific to the U.S. or Europe. Each of these standards has unique
pass/fail limits, which vary depending upon the category and link definition.
All standards require that installed links pass three tests. The
first is called wire map. Wire map verifies end-to-end pin-to-pin connectivity,
as well as checking for split pairs. Any miswires, breaks, opens, shorts,
crossovers, or splits should be detected.
Another key measurement used to qualify LAN cabling is attenuation.
Every electromagnetic signal loses strength as it propagates away from its
source, and LAN signals are no exception. Attenuation increases with
temperature and frequency. Higher frequency signals are attenuated much more
than low frequency signals, which is one of the reasons why a cable may have
correct pin to pin continuity, pass low speed traffic like 10BASE-T perfectly,
yet not be able to handle 100BASE-T. With Category 5 copper cabling,
attenuation is remarkably consistent from manufacturer to manufacturer. Cables
that have attenuation performance much better than standard Category 5 numbers
usually have increased copper diameter or slightly higher impedance.
The most important test in qualifying the performance of network
cabling is near end crosstalk (NEXT). Crosstalk occurs when signals from one
pair of wires radiate and are picked up by an adjacent pair of wires. Crosstalk
increases with frequency, so that just as for attenuation, a category 3 cable
may be fine for 10BASE-T, but can't handle 100BASE-T.
Keeping the pairs tightly twisted and well-balanced best minimizes
crosstalk. This tight twisting causes opposing electromagnetic fields to more
effectively cancel each other, thus reducing emissions from the pair. Category
5 cable is much more tightly and consistently twisted than category 3 cable,
and uses better insulation materials which further reduce crosstalk and
attenuation. EIA/TIA 568A requires that all UTP terminations be properly
twisted to within 0.5 inches of all connections.
TIA TSB-67 further requires that length
be measured. Length measurement may seem straightforward but actually can be
tricky. In essence, a basic link or permanent link should not exceed 90 meters
and a channel should not exceed 100 meters. The accuracy of a length
measurement is affected by several factors, including nominal velocity of
propagation (NVP) of the cable, twist length vs. physical sheath length, and
impulse dispersion over long lengths.
When you are measuring length with a field test tool, you are
usually measuring time delay, and converting it to a length estimate based upon
an assumption of signal speed.
Nominal Velocity of Propagation (NVP) refers to how quickly signals
travel in a cable. It is expressed as a percentage of the speed of light.
Incorrectly set NVP is a very common error. If your NVP is set for 75% and the
actual cable's NVP is 65%, that's a 10% error off the bat. Furthermore, NVP is
unique to each pair and also varies with frequency. For category 3 cables and
hybrid category 5 cables, NVP can vary by up to 12% between pairs!
In addition, the copper conductors in UTP are twisted, so the
actual length of the wire is longer than the length of the cable jacket. On a
1000 foot spool, you could easily have 1020 feet of copper.
For these reasons, consider length results from hand held testers
to be good approximations, not precise values.
Attenuation to Crosstalk Ratio (ACR) is a measurement that
determines the effective signal-to-noise ratio of a cabling link. ACR is simply
the difference between the NEXT and the attenuation. It is a measure of the
strength of the signal that survives attenuation from the far end relative to
crosstalk noise. For example, imagine an instructor standing in front of a room
giving a lecture. The goal of the instructor is to be heard by the students.
The volume of the instructor's voice is a key factor in determining this, but
it isn't as important as the difference between the instructor's voice and the
background noise. The instructor could be speaking in a very quiet library, so
that even a whisper could be heard. But imagine that same instructor, speaking
at the same volume, at a noisy football game. The instructor would have to
raise his voice, so that the difference between his voice (the desired signal)
and the cheering crowd (the background noise) is enough for him to be heard.
That's ACR.
Emerging standards, such as TSB-95, require the new measurements.
Return loss is the ratio, expressed in decibels, of the fractional amount of
signal reflection caused by an impedance mismatch. Return loss is increasingly
important when trying to get premium performance from UTP. Manufacturers of
very high quality UTP have taken special care to ensure impedance is very
uniform throughout the link, and also that all components are very well
matched. So while return loss wasn't a big issue when Category 5 cabling first
appeared, it is an important differentiator for Cat 5E and Cat 6 cabling.
Power-sum NEXT (PSNEXT) is actually a calculation, not a
measurement. PSNEXT is derived from an algebraic summation of the individual
NEXT effects on each pair by the other three pairs. PSNEXT and FEXT (discussed
below) are important measurements for qualifying cabling intended to support 4
pair transmission schemes such as Gigabit Ethernet.
Far End Crosstalk (FEXT) is similar to NEXT, except the signal is
sent from the local end and crosstalk measured at the far end.
FEXT by itself is not a useful measurement. This is because FEXT is highly
influenced by the length of the cable, since the signal strength inducing the
crosstalk is affected by how much it has been attenuated from its source. For
this reason, Equal Level FEXT or ELFEXT is measured instead. ELFEXT simply
subtracts attenuation from the result, so that the result is normalized for
attenuation (length) effects. Then, just to make things interesting, we also
have power-sum ELFEXT, or PSELFEXT.
Category 6 testing requires three things you don't have in your
current Level II tester:
-
Support to accurately make all the new measurements
-
Dynamic range to measure FEXT and return loss at Category 6 levels
correctly
-
250 MHz of bandwidth
For this reason many installers are in the process of replacing
their field testers with new products designed to fully support Category 5/5E/6
requirements, such as the Microtest OMNIScanner.
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