Twisted-Pair Cable
In traditional installations, the most economical and widely installed cabling today is twisted-pair wiring. Not only is twisted-pair wiring less expensive than other media, installation is also simpler, and the tools required to install it are not as costly. UTP and STP are the two primary varieties of twisted-pair on the market today. ScTP is a variant of STP.
- Unshielded Twisted-Pair (UTP)
Though it has been used for many years for telephone systems, UTP for LANs first became common in the late 1980s with the advent of Ethernet over twisted-pair wiring and the 10Base-T standard. UTP is cost effective and simple to install, and its bandwidth capabilities are continually being improved.
NOTE: Alexander Graham Bell invented and patented twisted-pair cabling and an optical telephone in the 1880s. During that time, Bell offered to sell his company to Western Union for $100,000, but it refused to buy.
UTP cabling typically has only an outer covering (jacket) consisting of some type of nonconducting material. This jacket covers one or more pairs of wire that are twisted together. Four-pair cable is the most commonly used horizontal cable in network installations today. The characteristic impedance of UTP cable is 100-ohms plus or minus 15 percent, though 120-ohm UTP cable is sometimes used in Europe and is allowed by the ISO/IEC 11801 Ed. 2 cabling standard.
A typical UTP cable is shown below. This simple cable consists of a jacket that surrounds four twisted pairs. Each wire is covered by an insulation material with good dielectric properties. For data cables, this means that in addition to being electrically nonconductive, it must also have certain properties that allow good signal propagation.
UTP cabling seems to generate the lowest expectations of twisted-pair cable. Its great popularity is mostly due to the low cost and ease of installation. With every new generation of UTP cable, network engineers think they have reached the limits of the UTP cable’s bandwidth and capabilities. However, cable manufacturers continue to extend its capabilities. During the development of 10Base-T and a number of pre-10Base-T proprietary UTP Ethernet systems, critics said that UTP would never support data speeds of 10Mbps. Later, the skeptics said that UTP would never support data rates at 100Mbps. After that, the IEEE approved the 1000Base-T (1 Gb/s) standard in July 1999, which allows Gigabit Ethernet to run over Category 5 cable. Just when we thought this was the end of copper UTP-based applications, in 2006 the IEEE approved the 10GBase-T standard, which allows 10 Gigabit Ethernet over unshielded Category 6 and 6A cable!
- Shielded Twisted-Pair (STP)
STP cabling was first made popular by IBM when it introduced type classification for data cabling. Though more expensive to purchase and install than UTP, STP offers some distinct advantages. The current ANSI/TIA-568-C cabling standard recognizes IBM Type 1A horizontal cable, which supports frequency rates of up to 300MHz, but does not recommend it for new installations. STP cable is less susceptible to outside electromagnetic interference (EMI) than UTP cabling because all cable pairs are well shielded.
Some STP cabling, such as IBM types 1 and 1A cable, uses a woven copper-braided shield, which provides considerable protection against EMI. Inside the woven copper shield, STP consists of twisted pairs of wire (usually two pairs) wrapped in a foil shield. Some STP cables have only the foil shield around the wire pairs.
The figure below shows a typical STP cable. In the IBM design, the wire used in STP cable is 22 AWG (just a little larger than the 24 AWG wire used by typical UTP LAN cables) and has a nominal impedance of 150 ohms, but category versions can have a nominal impedance of 100 ohms.
Constructions of STP in 24 AWG, identical in copper conductor size to UTP cables, are more commonly used today.
Simply installing STP cabling does not guarantee you will improve a cable’s immunity to EMI or reduce the emissions from the cable. Several critical conditions must be met to achieve good shield performance:
- The shield must be electrically continuous along the whole link.
- All components in the link must be shielded. No UTP patch cords can be used.
- The shield must fully enclose the pair, and the overall shield must fully enclose the core. Any gap in the shield covering is a source of EMI leakage.
- The shield must be grounded at both ends of the link, and the building grounding system must conform to grounding standards (such as J-STD-607-A).
If even one of these conditions is not satisfied, shield performance will be badly degraded. For example, tests have shown that if the shield continuity is broken, the emissions from a shielded cabling system increase by 20dB on the average.
- Screened Twisted-Pair (ScTP)
A recognized cable type in the ANSI/TIA-568-C standard is ScTP cabling, a hybrid of STP and UTP cable. ScTP cable contains four pairs of unshielded 24 AWG, 100 ohm wire (see the figure below) surrounded by a foil shield or wrapper and a drain wire for grounding purposes. Therefore, ScTP is also sometimes called Foil Twisted-Pair (FTP) cable because the foil shield surrounds all four conductors. This foil shield is not as large as the woven copper-braided jacket used by some STP cabling systems, such as IBM types 1 and 1A. ScTP cable is essentially STP cabling that does not shield the individual pairs; the shield may also be smaller than some varieties of STP cabling.
The foil shield is the reason ScTP is less susceptible to noise. To implement a completely effective ScTP system, however, the shield continuity must be maintained throughout the entire channel, including patch panels, wall plates, and patch cords. Yes, you read this correctly; the continuity of not only the wires but also the shield must be maintained through connections. Like STP cabling, the entire system must be bonded to ground at both ends of each cable run, or you will have created a massive antenna, the frequencies of which are inversely proportional to the length of the shield. The net effect is that the noise is out of band.
Standard eight-position modular jacks (commonly called RJ-45s) do not have the ability to ensure a proper ground through the cable shield. So special mating hardware, jacks, patch panels, and even tools must be used to install an ScTP cabling system. Many manufacturers of ScTP cable and components exist (just be sure to follow all installation guidelines).
ScTP is recommended for use in environments that have abnormally high ambient electromagnetic interference, such as industrial work spaces, hospitals, airports, and government/military communications centers. For example, ScTP is used in fast-food restaurants that use wireless headsets for their drive-through-window workers; some wireless frequencies can interfere with Ethernet over copper. The value of an ScTP system in relation to its additional cost is sometimes questioned, as some tests indicate that UTP noise immunity and emissions characteristics are comparable with ScTP cabling systems. Often, the decision to use ScTP simply boils down to whether you want the warm and fuzzy feeling of knowing an extra shield is in place.
- Screened Shielded Twisted-Pair (S/STP or S/FTP)
S/STP cabling, also known as screened fully shielded twisted-pair (S/FTP), contains four individually shielded pairs of 24 AWG, 100 ohm wire surrounded by an outer metal shielding covering the entire group of shielded copper pairs. This type of cabling offers the best protection from interference from external sources, and also eliminates alien crosstalk (discussed later), allowing the greatest potential for higher speeds.
Category 7 is an S/STP cable standardized in ISO 11801 Ed. 2, which offers a usable bandwidth to 600MHz. Its intended use is for the 10 Gigabit Ethernet, 10GBase-T application. S/STP cable looks similar to the STP cable, but has four individually shielded conductor pairs.