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UTP vs STP

A Comparison of Cables, Systems, and Performance Carrying High-Data-Rate Signals


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

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