INTRODUCTION
Recently, debate has arisen on the advantages and disadvantages of shielded
twisted-pair (STP) cable and unshielded twisted-pair (UTP)
cable. Advocates of STP cable (a category that includes screened twisted-pair
cable and foil twisted-pair cable) have attempted to claim that their product
is superior to UTP
cable without adequately presenting both sides of the story. While it is true
that STP cable and UTP
cable are inherently different in design and manufacture, their purpose should
be the same, to provide reliable connectivity of electronic equipment.
Although, in theory, both types of cable should perform this task
successfully, the true test comes when you look at the performance of each of
these cable types within its respective end-to-end system.
UTP Cable vs STP Cable
Two copper wires, each encased in its own color-coded insulation, are
twisted together to form a twisted pair. Multiple twisted pairs are packaged
in an outer sheath, or jacket, to form twisted-pair cable. By varying the
length of the twists in nearby pairs, the possibility of interference between
pairs in the same cable sheath can be minimized.
Twisted-pair cable has been around for quite a while. In fact, early
telephone signals were sent over a type of twisted-pair cable, and just about
every building today still uses twisted-pair cable to carry telephone and
other signals. However, signals have become more complex over the years,
evolving from 1200 bps to over 100 Mbps. And there are many more sources of
interference that might disrupt those signals today than there were at the
turn of the century. Coaxial cable and fiber optic cable were developed to
handle higher-bandwidth applications, and to support emerging technologies.
But twisted-pair cable, too, has evolved so that it can now carry
high-data-rate signals.
Some twisted-pair cables contain a metal shield to reduce the potential for
electromagnetic interference (EMI). EMI is caused by signals from other
sources such as electric motors, power lines, high-power radio and radar
signals in the vicinity that may cause disruptions or interference, called
noise. Shielded twisted-pair (STP) cable encases the signal-carrying wires in
a conducting shield. At first glance, it may appear that because STP cable is
physically encased in a shield, all outside interference is automatically
blocked; however, this is not true.
Just like a wire, the shield acts as an antenna, converting received noise
into current flowing in the shield when it has been properly grounded. This
current, in turn, induces an equal and opposite current flowing in the twisted
pairs. As long as the two currents are symmetrical, they cancel each other out
and deliver no net noise to the receiver. However, any discontinuity in the
shield or other asymmetry between the current in the shield and the current in
the twisted pairs is interpreted as noise. STP cable is only effective at
preventing radiation or blocking interference as long as the entire end-to-end
link is shielded and properly grounded. To work properly, every component of a
shielded cabling system must be just that fully shielded.
STP cable also has drawbacks; for example, its attenuation may increase at
high frequencies, and its balance (or longitudinal conversion loss) may
decrease if the effects of the shield are not compensated for, which leads to
crosstalk and signal noise. The shielding effectiveness depends on the
material of the shield, its thickness, the type of EMI noise field, its
frequency, the distance from the noise source to the shield, any shield
discontinuity, and the grounding structure used. Nor can it always be
guaranteed that the shield itself will contain no imperfections.
Some STP cables use a thick braided shield. These cables are heavier,
thicker, and harder to install than their
UTP counterparts. Some
STP cables only use a relatively thin overall outer foil shield. These cables,
called screened twisted-pair (ScTP) cables or foil twisted-pair (FTP) cables,
are thinner and less expensive than braided STP cable. However, they are not
any easier to install the minimum bending radius and maximum pulling tension
force must be rigidly observed when these cables are installed; otherwise, the
shield may experience a tear.
Unshielded twisted-pair (UTP)
cable, on the other hand, does not rely on physical shielding to block
interference, but on balancing and filtering techniques through media filters
and/or baluns. Noise is induced equally on two conductors, which cancels out
at the receiver. With properly designed and manufactured
UTP cable, this
technique is easier to maintain than the shielding continuity and grounding of
an STP cable.
UTP cable has
evolved over the years, and different varieties are available for different
needs. Basic telephone cable, also known as direct-inside wire (or DIW), is
still available. Improvements over the years, such as variations in the twists
or in individual wire sheaths or overall cable jackets, have led to the
development of EIA/TIA-568 standard-compliant Category 3 (for specifications
on signal bandwidth up to 16 MHz), Category 4 (for specifications on signal
bandwidth up to 20 MHz), and Category 5 (for specifications on signal
bandwidth up to 100 MHz and greater)
UTP cable. Because
UTP cable is
lightweight, thin, and flexible, as well as versatile, reliable, and
inexpensive, millions of nodes have been and continue to be wired with
UTP cable, even for
high-data-rate applications. For the best performance,
UTP cable should be
used as part of a well engineered structured cabling system.
UTP Cabling Systems vs STP
Cabling Systems
If STP cable is combined with improperly shielded connectors, connecting
hardware, or outlets, or if the foil shield itself is damaged, overall signal
quality will be degraded. This can result in degradation of emission and
immunity performance. Therefore, for a shielded cabling system to succeed
totally in interference reduction, every component within that system must be
fully and seamlessly shielded, as well as properly installed and maintained.
Likewise, an STP cabling system requires good grounding and earthing
practices. An improperly grounded system can be a primary source of emissions
and interference. Whether this ground is at one end or both ends of the cable
run depends on the frequency of the application. For high-frequency signals,
an STP cabling system must be grounded, at a minimum, at both ends of the
cable run, and it must be continuous. A shield grounded at one end only has no
effect against magnetic field interference. The length of the ground conductor
itself can also be a source of problems. If it is too long, it no longer acts
as a ground. Therefore, optimum grounding for an STP cabling system is not
possible, since it depends on the application.
UTP cabling systems do
not have this problem.
While an STP cabling system is dependent on such factors as physical
continuity of the cable shield itself or installation with adequately shielded
and grounded components, a
UTP cabling system inherently has fewer points for potential failure and
is easier to install. For
UTP cabling systems such as Lucent Technologies's SYSTIMAX Structured
Cabling Systems (SCS), all of the individual products certified for use are
manufactured by Lucent Technologies and individually tested, as well as tested
in conjunction with other products in the SYSTIMAX SCS offering. All cables,
for example, are tested on the reel at the point of manufacture, and are also
tested as a complete cabling system within the individual applications for
which they are certified. All products certified for use in SYSTIMAX SCS also
carry a 15-year warranty.
SYSTIMAX SCS utilizes Lucent Technologies Bell Laboratories-developed
design rules for all certified end-to-end applications. Such design rules,
which are fully documented in Lucent Technologies application guidelines, cite
which products may be used (for example, only Category 5 products for some
higher-speed applications), how cable must be terminated and administered, and
maximum distances for cable runs. All applications are also tested in Lucent
Technologies Bell Laboratories test labs and are certified for a period of 15
years. Consequently, both products and systems are fully tested and warranted.
UTP vs STP Cabling Systems
and EMC
In addition to precision design and manufacture, as well as end-to-end
integrity, another factor to consider when choosing a cabling system relates
to the recent adoption of electromagnetic compatibility (EMC) directive. EMC
refers to the ability of an electronic system to function properly in its
environment that is, an environment where several pieces of equipment are
located in the same workspace, each radiating electromagnetic emissions. With
the increased amount of electronic equipment in the average workspace, EMC
becomes increasingly more important excess radiation from one piece of
equipment can adversely affect performance of another piece of equipment. This
means that every electronic system, which includes either an STP or
UTP cabling system,
must meet this directive.
In some countries, such as the U.S. and Germany, EMC regulations have
existed for years. However, the implementation of the European EMC Directive
in 1989 has refocused attention on EMC. The European EMC Directive 89/336/EEC
states that all electronic equipment and apparatus must comply with the
directive. These systems must pass the essential requirements of the directive
before they can be sold anywhere in the European Economic Area (EEA). Some
national regulations (such as Amtsblatt Verfugung 243/91 of Germany) currently
exempt STP based systems from immunity testing. However, as of January 1,
1996, these national regulations will no longer apply, and all systems must be
tested. Those that do not pass will not be able to be sold in the EEA.
How well do
UTP and
STP based systems stand up to rigorous EMC testing? Contrary to some popular
assumptions, not all STP based systems can automatically pass EMC tests, while
a well designed UTP
cabling system can.
EMC Fribourg, a Swiss testing facility, conducted comparative EMC tests on
four STP cabling systems and one
UTP cabling system. All
were configured to support the IBM 16-Mbps Token Ring local area network (LAN)
application according to ISO 8802.5 standards, using personal computers (PCs)
with IBM Token Ring Adapter Cards.
For the
UTP cabling
system, SYSTIMAX SCS 1061 Category 5 24-AWG High-Performance 4-Pair
UTP cable was chosen,
with an M1000 MULTIMAX Panel and M100-type information outlets (IOs) used as
the connecting hardware, and a 370C1 Adapter (media filter) used to link the
IBM card to the SYSTIMAX SCS
UTP system.
Test results were as follows:
-
In radiated emissions testing for a frequency range of 30 MHz 1 GHz in
an anaechoic chamber and in an open area test site (OATS), the SYSTIMAX SCS
UTP system met CISPR
22/EN5022 Class B requirements (Class B requirements are for residential
use, and are more stringent than the Class A requirements for commercial
use) with a more-than-adequate margin.
-
In conducting emissions on signal port testing at lower frequencies (150
kHz 30 MHz) with a current probe, the SYSTIMAX SCS
UTP system met the
proposed CISPR 22/EN55022 class B requirements.
-
In IEC 801.4 electrical fast transient (EFT) noise-burst testing, the
SYSTIMAX SCS UTP
system did not fail even when subjected to the most strenuous test at 4,000
V. None of the STP cabling systems survived to that level.
-
In IEC 801.3 radiated immunity testing, which tests the ability of a
system to withstand electromagnetic interference at defined severity levels
(26 MHz 1 GHz), the SYSTIMAX SCS
UTP system
experienced no errors. The one STP cabling system tested experienced errors
when the media filter was used instead of the shielded work area cable at
the PC.
EMC Fribourg concluded that
UTP cabling systems,
and, more specifically, SYSTIMAX SCS
UTP systems, can meet
the above EMC requirements.
UTP Cabling Systems and
High-Speed Data Transmittion
Tests conducted in well known testing facilities show that
UTP cabling systems
and, specifically, SYSTIMAX SCS using
UTP cable, can meet
standards specifications for transmitting high-speed data within acceptable
levels and can pass all required tests.
EMC tests were conducted on an ISO 8802.3 10-Mbps 10BASE-T system that used
SYSTIMAX SCS 1061 Category 5 24-AWG High-Performance 4-Pair
UTP cable, with
Category 5 patch panels, Category 5 M100-type IOs, and Category 5 patch cords,
along with 486-type PCs and electronics from several major vendors. The tests
were done at the Lucent Technologies Bell Laboratories Global Product
Compliance Laboratory in Holmdel, NJ, and were sent to a German notified and
competent body, Bundesamt fur Zulassungen in der Telekommunikation (BZT) for
certification. The SYSTIMAX SCS
UTP system passed every
test, in some cases even exceeding the current requirements under the EMC
Directive.
The tests were as follows:
-
Radiated Emissions; specifications EN 55022, 1987, Class B Limit
-
Conducted Emissions (AC Mains); specifications EN 55022, 1987, Class B
Limit
-
Conducted Emissions (Signal Ports); specifications EN 50081-1, 1992,
Informative Annex A, CISPR 22 Amendment, CISPR/G (Sec 65), 1993, Class B
Limit
-
Electrostatic Discharge (ESD) Immunity; specifications IEC 801.2, 1991,
IEC CISPR 24, Part 2, prEN 55024, Part 2, Contact Discharge at 4,000 V
(Level 2), Air Discharge at 8,000 V (Level 3)
-
Radiated Field Immunity; specifications IEC 801.3, 1992, IEC CISPR 24,
Part 3, prEN 55024, Part 3, 3 V/m (Level 2), 10 V/m (Level 3)
-
EFT/Burst Immunity; IEC 801.4, 1988, IEC CISPR 24, Part 4, prEN 55024,
Part 4, AC Mains at 1.0 kV (Level 2), Signal/Control Lines at 0.5 kV (Level
2) and 1.75 kV (Level 3)
EMC tests were also conducted on an ISO 9314 (ANSI X3T9.5) 100-Mbps TP-PMD
LAN that used SYSTIMAX SCS 1061 Category 5 24-AWG High-Performance 4-Pair
UTP cable, with
Category 5 patch panels, Category 5 M100-type IOs, and Category 5 patch cords,
along with 486-type PCs and electronics from several major vendors. The tests
were done at the Lucent Technologies Bell Laboratories Global Product
Compliance Laboratory in Holmdel, NJ, and were sent to BZT in Germany for
certification. The SYSTIMAX SCS
UTP system again passed
every test, in some cases even exceeding the current requirements under the
EMC Directive.
The tests were as follows:
-
Radiated Emissions; specifications EN 55022, 1987, Class B Limit
-
Conducted Emissions (AC Mains); specifications EN 55022, 1987, Class B
Limit
-
Conducted Emissions (Signal Ports); specifications EN 50081-1, 1992,
Informative Annex A, CISPR 22 Amendment, CISPR/G (Sec 65), 1993, Class B
Limit
-
ESD Immunity; specifications IEC 801.2, 1991, IEC CISPR 24, Part 2, prEN
55024, Part 2, Contact Discharge at 4,000 V (Level 2), Air Discharge at
8,000 V (Level 3)
-
Radiated Field Immunity; specifications IEC 801.3, 1992, IEC CISPR 24,
Part 3, prEN 55024, Part 3, 3 V/m (Level 2)
-
EFT/Burst Immunity; IEC 801.4, 1988, IEC CISPR 24, Part 4, prEN 55024,
Part 4, AC Mains at 1.0 kV (Level 2), Signal/Control Lines at 0.5 kV (Level
2)
Furthermore, research conducted by the SYSTIMAX SCS Department of Lucent
Technologies Bell Laboratories together with the Advanced Multimedia
Communications Department of Lucent Technologies Bell Laboratories has
demonstrated that SYSTIMAX SCS
UTP systems, using 328
ft (100 m) of 1061 Category 5 24-AWG High-Performance 4-Pair
UTP cable, with M1000
MULTIMAX Panels and M100-type IOs for connecting hardware along with Category
5 D8AU patch cords, can successfully transmit up to 622 Mbps the equivalent of
23,000 pages of text per second.
The test used off-the-shelf high-quality red/green/blue (RGB) video
equipment to provide the data stream. A studio-quality RGB video camera was
used to capture a full-motion high-resolution image. Using a codec, the analog
video signal from the camera was converted to an industry-standard D1 protocol
digital video data stream and, at the transmitter, encoded into a 64-point
Carrierless Amplitude and Phase (64 CAP) signal. The 64 CAP encoding method
was used to partition the data stream into four 155-Mbps "channels" at the
transmitter, which were each then sent over one pair of the 4-pair cable, and
then decoded and recombined into a single 622 Mbps data stream at the receiver
end of the link. A codec at this end converted the signal back into an analog
RGB video signal that was displayed on the monitor.
The Advantages of using
UTP cabling systems
STP cabling systems are more expensive and harder to install and maintain
than UTP cabling
systems, but are not necessarily better. As demonstrated in EMC and other test
results, UTP cabling
systems succeeded even excelled in rigorous testing. Furthermore, because it
was chosen as the representative
UTP cabling system,
SYSTIMAX SCS demonstrated even more fully the benefits of Lucent
Technologies's extensive testing and precision manufacturing under rigid ISO
9000 quality control conditions. This underscores the importance of using a
structured cabling system made up of products designed and manufactured to
work together that meet or exceed international standards.
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November 1994
