Shielded and unshielded twisted-pair cable revisited

Recently, the advantages and disadvantages of shielded and unshielded twisted-pair cable have been under debate. Advocates of STP cable, which includes screened twisted-pair and foil twisted-pair cables, claim that it is superior to UTP cable. However, these advocates have not presented the other side of the story.

John T. Bolcar

AT&T Network Systems

Recently, the advantages and disadvantages of shielded and unshielded twisted-pair cable have been under debate. Advocates of STP cable, which includes screened twisted-pair and foil twisted-pair cables, claim that it is superior to UTP cable. However, these advocates have not presented the other side of the story.

STP and UTP cable differ in design and manufacture. But their purpose should be the same--to provide reliable connectivity of electronic equipment. In theory, both types of cable should do this equally well. The true test comes when you look at how each of these cable types performs that task within its respective end-to-end system.

Shielded twisted-pair cable

Shielded twisted-pair cable encases the signal-carrying wires in a conducting shield as a means of reducing the potential for electromagnetic interference. How effective the shielding is depends on the material used for the shield--its thickness and frequency, the type of electromagnetic noise field, the distance from the noise source to the shield, any shield discontinuity and the grounding practices. Also, crosstalk and signal noise can increase if the effects of the shield are not compensated for.

Some STP cables, for example, use a thick braided shield that makes a cable heavier, thicker and more difficult to install than its UTP counterpart. Other STP cables use only a thin outer foil shield. These cables, known as screened twisted-pair cables or foil twisted-pair cables, are thinner and less expensive than braided STP cable; however, they are not any easier to install. Unless the minimum bend radius and maximum pulling tension are rigidly observed when these cables are installed, the shield can be torn.

Unshielded twisted-pair cable

Unshielded twisted-pair cable does not rely on physical shielding to block interference. It relies instead on balancing and filtering techniques using media filters, baluns or both. Noise is induced equally on two conductors and is canceled out at the receiver. With properly designed, manufactured and installed UTP cable, the network is easier to maintain than one in an STP cable plant, with its shielding continuity and grounding issues.

UTP cable has evolved during the years, and different types are available for different needs. Basic telephone cable, also known as direct-inside wire, is still available. Improvements over the years--variations in the twists or in individual wire sheaths or overall cable jackets--have led to the development of Category 3 cable that is compliant with the Electronic Industries Association/ Telecommunications Industry Association-568 standard for transmission rates up to 16 megahertz. Category 4 UTP cable is specified for signal bandwidths to 20 MHz, and Category 5 UTP for specifications to 100 MHz--and possibly higher.

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 this cabling medium. This is especially true for high-data-rate applications. For best performance, this UTP cable should be used as part of a well-engineered structured cabling system.

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, in turn, can result in degraded emission and immunity performance. Therefore, for a shielded cabling system to totally reduce interference, every component within that system must be fully and seamlessly shielded, as well as properly installed and maintained.

An STP cabling system also requires good grounding and earthing practices because of the presence of the shield. 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 at which a given application is running. For high-frequency signals, an STP cabling system must be grounded, at minimum, at both ends of the cable run, and it must be continuous. A shield grounded at only one end is not effective against magnetic-field interference.

The length of the ground conductor itself can also cause problems. If it is too long, it no longer acts as a ground. Therefore, because specific grounding requirements depend on the application, a general grounding policy that ensures the best results for an STP cabling system is not possible.

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 or installation with adequately shielded and grounded components, a UTP cabling system inherently has fewer potential trouble spots and is easier to install.

Another factor to consider when you are choosing a cabling system, in addition to precision design and manufacture and end-to-end integrity, is electromagnetic compatibility. EMC refers to the ability of an electronic system to function properly in its environment--that is, in an environment where several pieces of equipment are located in the same workspace, each radiating electromagnetic emissions. With more and more electronic equipment in the average workspace, EMC becomes increasingly important. Excess radiation from one piece of equipment can adversely affect performance of other devices. In some countries, including the United States and Germany, EMC regulations have existed for years. However, the implementation of the European EMC Directive in 1989 has refocused attention on this problem. This directive, also known as 89/336/European Economic Community, states that all electronic equipment and apparatus (that is, electronic systems) must comply with the directive, and that systems using this equipment and apparatus must pass the essential requirements of the directive before they can be sold anywhere in the EEC.

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 will have to be tested. Those that do not pass will not be able to be sold in the EEC. This means that every electronic system that includes either an STP or a UTP cabling system must meet this ECC directive.

How well do UTP- and STP-based systems stand up to rigorous EMC testing? Contrary to popular assumption, 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-megabit-per-second Token Ring local area network applications, in accordance with ISO-8802.5 standards, and using personal computers with IBM Token Ring adapter cards.

According to EMC Fribourg, the test results showed that in radiated emissions testing for a frequency range of 30 MHz to 1 gigahertz in an anaechoic chamber (a test facility that eliminates outside electrical sources) and in an open-area test site, the UTP system more than adequately met the International Special Committee on Radio Interference, or 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.

In conducting emissions on signal-port testing at lower frequencies (150 kilohertz to 30 MHz) with a current probe, the UTP system met the proposed CISPR 22/EN55022 Class B requirements. In International Electrotechnical Commission-801.4 electrical fast transient noise-burst testing, the UTP system did not fail even when subjected to the most strenuous test at 4000 volts. None of the STP cabling systems survived to that level. In IEC-801.3 radiated immunity testing, which evaluates the ability of a system to withstand electromagnetic interference at defined severity levels (26 MHz to 1 GHz), the UTP system experienced no errors. The test laboratory concluded that UTP cabling systems can meet the stated EMC requirements.

Additional testing indicates that UTP cabling systems can meet standards specifications for transmitting high-speed data within acceptable levels and can pass all required EMC tests. In this case, EMC tests were conducted on an ISO-8802.3 10 Mbits/sec 10Base-T system and ISO-9314 (American National Standards Institute X3T9.5) 100-Mbit/sec twisted-pair/physical medium-dependent local area network that used Systimax SCS 1061 Category 5, 24-AWG, high-performance, four-pair UTP cable, with Category 5 patch panels, Category 5 M100-type input/output devices and Category 5 patch cords, along with 486-type PCs and electronics from several major vendors.

The tests were conducted at AT&T Bell Laboratories Global Product Compliance Laboratory (Holmdel, NJ) and were sent for certification to the Bundesamt fur Zulassungen in der Telekommunikation, a German organization. The UTP cabling system passed every test, including radiated emissions, conducted emissions, electrostatic discharge immunity, radiated field immunity and Electrical Fast Transient/burst immunity.

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