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• Article Continues: Twisted Pair Cable Technology
USB User Guide
The term “twisted pair” is now well ingrained into our vocabulary due to today’s widespread proliferation of high speed data networks. So what exactly does twisted pair cabling do and why is it so popular?
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Twisted pair cabling is a form of wiring in which two conductors are wound together for the purposes of canceling out electromagnetic interference (EMI) from external sources and crosstalk from neighboring wires. Twisted pair cabling is usually made of copper alloy, but silver and other exotic versions have been known to exist. Twisted pair cabling is perhaps one of the oldest types of cable ever invented. Alexander Graham Bell patented twisted pair cabling along with his first telephone circuit on October 21, 1879! |
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Image of exposed twisted copper pairs |
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Differential Signal Block Diagram (Simplified) |
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Twisted pair cabling is sometimes referred to as a differential system. With the differential system each signal is transmitted on two lines at the same time. On one, the signal is transmitted as a POSITIVE (+) signal, on the other as a NEGATIVE (-) signal. At the receiving end of the cable the receiver device gets two signals. Both of them however, have been changed by the EMI noise that penetrated the cable. The changes came in the form of unwanted voltage added to the wanted signal. At this point it is important to note that the unwanted voltage got added to both lines at the same time and by the same amount because the cable is uniformly twisted. The essence of the differential system is that the receiver is designed to take the difference between the two signals on the two lines. In doing that, the noise part of the signal, equal on both lines, gets eliminated, and what remains is clear signal. This same concept helps prevent the signal of one line from interfering with the signal on an adjacent line. This adjacent interference is known as crosstalk.
The interference canceling aspects of twisted pair cabling is generally adequate. That is why Unshielded Twisted Pair (UTP) cabling is the most common. Sometimes, however, electromagnetic interference can be so pervasive in certain environments, that an additional level of shielding is required. This is why Shielded Twisted Pair (STP) cabling is used as an alternative. STP cabling comes in two flavors. Foiled Twisted Pair (FTP) adds a single mylar/foil shield over the entire cable body.
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This protects mainly against EMI. Extremely sensitive installations often use Screened Shielded Twisted Pair (S/STP) cabling. This includes the mylar/foil shield in addition to separate shields over each individual pair. This provides added protection against cross-talk. This type of cable is extremely bulky and hard to terminate, so its use is somewhat limited.
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We’ll explore the merits of shielded cables more in the next section. For now, we’ve established that twisted pair cabling is a proven technology which naturally suppresses interference due to its inherent design characteristics. Twisted pair cabling, especially the unshielded variety, is perhaps most popular due to its favorable cost. Competing technologies such as coaxial and fiber optic cabling must adhere to extremely strict tolerances to maintain signal integrity; a costly requirement. Twisted pair cabling tends to have more “wiggle room” in its design, thereby bringing costs down considerably. This partially explains why twisted pair cabling is increasingly being used for consumer applications such as the HDMI interface for HDTV.
Technical Primer: Terminating Twisted Pair Cable
An often over-looked but all-to-critical aspect of twisted pair cabling is how the cable is terminated to a connector. Twisted pair cabling is designed to maintain an impedance of 100 ohms. The precise distance or space between wire pairs determines a cable’s impedance. The industry standard RJ-45 type networking connector also maintains this spacing and 100 ohm impedance. This versatile connector works wit h all types of Cat5 and Cat6 twisted pair cabling.
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| Standard RJ-45 Connector Network Cable Crimp Tool |
The challenge comes in terminating this connector to the twisted pair cabling, because unwinding the cable will automatically alter this critical spacing. Here at Amphenol Cables on Demand, our networking patch cables are assembled by factory trained professionals with over a million feet worth of combined network cable assembly experience. All cables are electrically screened at the factory before shipment, thereby ensuring the integrity of your network cable infrastructure.
If you elect to wire your own networking cables using bare UTP cabling, RJ-45 connectors, and a crimp tool, you may have questions about properly mapping the different colored wire pairs. The answer to this question has a lot to do with your particular network installation. Generally, networks are made up of two types of cables: patch cables and crossover cables.
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T568A and T568B Patch Cable Wiring Diagram |
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A networking patch cable is the most common cable type in any network. Patch cables are used to connect any two different network devices. Examples include connecting a computer to a router or connecting a cable modem to an X-Box. A patch cable is often referred to as a straight through cable, because a pin on one end of the cable will always connect to its corresponding pin on the other end of the cable. So how do you know what color wire goes to what pin? Technically it doesn’t matter. As long as each wire goes straight through from one end to the other, the signal will get through just the same. Regardless, there are some standards out there that tend to be followed.
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The common network wiring standards used today include both T568A and T568B. As shown in the diagram above, the T568A and T568B wiring standards differ in one simple way: the orange and green wire pairs are interchanged. Generally T568A is used in residential and government installations, while T568B is used in some commercial installations. If you wire a cable where one end follows the T568A standard and the other end follows the T568B standard, a crossover cable will result. A crossover cable is used to connect any two identical type network devices. Examples include connecting a computer to a computer for data transfer purposes. |
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Crossover Cable for 100BASE-T Applications |
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Gigabit Ethernet Crossover Cable Wiring Diagram |
A crossover cable for Gigabit Ethernet is slightly different than a crossover cable for 10BASE-T or 100BASE-T. This is because 100 Megabit Ethernet and below only needs to utilize the orange and green wire pairs for data transfer. Gigabit Ethernet (1000BASE-T) takes advantage of the unused brown and blue wire pairs for its additional throughput. For crossover purposes, the blue wire pair must therefore also be swapped with the brown pair. The striped/solid wires within each of those pairs must also be swapped. Please refer to the chart above for guidance.
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