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Twisted Pair Cable

Twisted pair cable consists of color-coded pairs of insulated copper wires, each with a diameter of 0.4 to 0.8mm (approximately the diameter of a straight pin). Every two wires are twisted around each other to form pairs, and all the pairs are encased in a plastic sheath, as shown in Figure 3-19. The number of pairs in a cable varies, depending on the cable type.

Figure 3-19. Twisted pair cable

The more twists per foot in a pair of wires, the more resistant the pair will be to cross talk. Higher-quality, more expensive twisted pair cable contains more twists per foot. The number of twists per meter or foot is known as the twist ratio. Because twisting the wire pairs more tightly requires more cable, however, a high twist ratio can result in greater attenuation. For optimal performance, cable manufacturers must strike a balance between minimizing cross talk and reducing attenuation.

Because twisted pair is used in such a wide variety of environments and for a variety of purposes, it comes in hundreds of different designs. These designs vary in their twist ratio, the number of wire pairs that they contain, the grade of copper used, the type of shielding (if any), and the materials used for shielding, among other things. A twisted pair cable may contain from 1 to 4,200 wire pairs. Modern networks typically use cables that contain four wire pairs, in which one pair is dedicated to sending data and another pair is dedicated to receiving data.

In 1991, two standards organizations, the TIA/EIA, finalized their specifications for twisted pair wiring in a standard called “TIA/EIA 568.” Since then, this body has continually revised the international standards for new and modified transmission media. Its standards now cover cabling media, design, and installation specifications. The TIA/EIA 568 standard divides twisted pair wiring into several categories. The types of twisted pair wiring you will hear about most often are Cat (category) 3, 4, 5, 5e, 6, 6e, and 7. All of the category cables fall under the TIA/EIA 568 standard. Modern LANs use Cat 5 or higher wiring.

Twisted pair cable is relatively inexpensive, flexible, and easy to install, and it can span a significant distance before requiring a repeater (though not as far as coax). Twisted pair cable easily accommodates several different topologies, although it is most often implemented in star or star-hybrid topologies. Furthermore, twisted pair can handle the faster networking transmission rates currently being employed. Due to its wide acceptance, it will probably continue to be updated to handle the even faster rates that will emerge in the future. All twisted pair cable falls into one of two categories: STP (shielded twisted pair) or UTP (unshielded twisted pair).

STP (Shielded Twisted Pair)

STP (shielded twisted pair) cable consists of twisted wire pairs that are not only individually insulated, but also surrounded by a shielding made of a metallic substance such as foil. Some STP use a braided copper shielding. The shielding acts as a barrier to external electromagnetic forces, thus preventing them from affecting the signals traveling over the wire inside the shielding. It also contains the electrical energy of the signals inside. The shielding may be grounded to enhance its protective effects. The effectiveness of STP’s shield depends on the level and type of environmental noise, the thickness and material used for the shield, the grounding mechanism, and the symmetry and consistency of the shielding. Figure 3-20 depicts an STP cable.

Figure 3-20. STP cable

UTP (Unshielded Twisted Pair)

UTP (unshielded twisted pair) cabling consists of one or more insulated wire pairs encased in a plastic sheath. As its name implies, UTP does not contain additional shielding for the twisted pairs. As a result, UTP is both less expensive and less resistant to noise than STP. Figure 3-21 depicts a typical UTP cable.

Figure 3-21. UTP cable

Earlier, you learned that the TIA/EIA consortium designated standards for twisted pair wiring. To manage network cabling, you need to be familiar with the standards for use on modern networks, particularly Cat 3 and Cat 5 or higher:

  • Cat 3 (Category 3) —A form of UTP that contains four wire pairs and can carry up to 10 Mbps of data with a possible bandwidth of 16MHz. Cat 3 has typically been used for 10Mbps Ethernet or 4Mbps token ring networks. Where it remains, network administrators are replacing their existing Cat 3 cabling with Cat 5 or better cabling to accommodate higher throughput.

  • Cat 4 (Category 4) —A form of UTP that contains four wire pairs and can support up to 16Mbps throughput. Uncommon on new networks, Cat 4 may be found on older 16Mbps token ring or 10Mbps Ethernet networks. It is guaranteed for signals as high as 20MHz and provides more protection against cross talk and attenuation than Cat 3.

  • Cat 5 (Category 5) —A form of UTP that contains four wire pairs and supports up to 1,000Mbps throughput and a 100MHz signal rate. Figure 3-22 depicts a typical Cat 5 UTP cable with its twisted pairs untwisted, allowing you to see their matched color coding. For example, the wire that is colored solid orange is twisted around the wire that is part orange and part white to form the pair responsible for transmitting data.

    Figure 3-22. A Cat 5 UTP cable with pairs untwisted


    It can be difficult to tell the difference between four-pair Cat 3 cables and four-pair Cat 5 or Cat 5e cables. However, some visual clues can help. On Cat 5 cable, the jacket is usually stamped with the manufacturer’s name and cable type, including the Cat 5 specification. A cable whose jacket has no markings is more likely to be Cat 3. Also, pairs in Cat 5 cables have a significantly higher twist ratio than pairs in Cat 3 cables. Although Cat 3 pairs might be twisted as few as three times per foot, Cat 5 pairs are twisted at least 12 times per foot. Other clues, such as the date of installation (old cable is more likely to be Cat 3), the looseness of the jacket (Cat 3’s jacket is typically looser than Cat 5’s), and the extent to which pairs are untwisted before a termination (Cat 5 can tolerate only a small amount of untwisting) are also helpful, though less definitive.

  • Cat 5e (Enhanced Category 5) —A higher-grade version of Cat 5 wiring that contains high-quality copper, offers a high twist ratio, and uses advanced methods for reducing cross talk. Cat 5e can support a signaling rate as high as 350MHz, more than triple the capability of regular Cat 5.

  • Cat 6 (Category 6) —A twisted pair cable that contains four wire pairs, each wrapped in foil insulation. Additional foil insulation covers the bundle of wire pairs, and a fire-resistant plastic sheath covers the second foil layer. The foil insulation provides excellent resistance to cross talk and enables Cat 6 to support a 250MHz signaling rate and at least six times the throughput supported by regular Cat 5.

  • Cat 6e (Enhanced Category 6) —A higher-grade version of Cat 6 wiring that reduces attenuation and cross talk and allows for potentially exceeding traditional network segment length limits. Cat 6e is capable of a 550MHz signaling rate and can reliably transmit data at multi-Gigabit per second rates.

  • Cat 7 (Category 7) —A twisted pair cable that contains multiple wire pairs, each surrounded by its own shielding, and then packaged in additional shielding beneath the sheath. Although standards have not yet been finalized for Cat 7, cable supply companies are selling it, and some organizations are installing it. One advantage to Cat 7 cabling is that it can support signal rates up to 1GHz. However, it requires different connectors than other versions of UTP because its twisted pairs must be more isolated from each other to ward off cross talk. Because of its added shielding, Cat 7 cabling is also larger and less flexible than other versions of UTP cable. Cat 7 is uncommon on modern networks, but it will likely become popular as the final standard is released and network equipment is upgraded.

Technically, because Cat 6 and Cat 7 contain wires that are individually shielded, they are not unshielded twisted pair. Instead, they are more similar to shielded twisted pair.

UTP cabling may be used with any one of several IEEE physical layer networking standards that specify throughput maximums of 10, 100, 1,000, and even 10,000Mbps. These standards are described in detail in Chapter 5.

Comparing STP and UTP

STP and UTP share several characteristics. The following list highlights their similarities and differences:

  • Throughput —STP and UTP can both transmit data at 10Mbps, 100Mbps, 1Gbps, and 10Gbps, depending on the grade of cabling and the transmission method in use.

  • Cost —STP and UTP vary in cost, depending on the grade of copper used, the category rating, and any enhancements. Typically, STP is more expensive than UTP because it contains more materials and it has a lower demand. It also requires grounding, which can lead to more expensive installation. High-grade UTP can be expensive too, however. For example, Cat 6e costs more per foot than Cat 5 cabling.

  • Connector —STP and UTP use RJ-45 (Registered Jack 45) modular connectors and data jacks, which look similar to analog telephone connectors and jacks. However, telephone connections follow the RJ-11 (Registered Jack 11) standard. Figure 3-23 shows a close-up of an RJ-45 connector for a cable containing four wire pairs. For comparison, this figure also shows a traditional RJ-11 phone line connector. All types of Ethernet that rely on twisted pair cabling use RJ-45 connectors.

    Figure 3-23. RJ-45 and RJ-11 connectors

  • Noise immunity —Because of its shielding, STP is more noise resistant than UTP. On the other hand, signals transmitted over UTP may be subject to filtering and balancing techniques to offset the effects of noise.

  • Size and scalability —The maximum segment length for both STP and UTP is 100m, or 328 feet, on Ethernet networks that support data rates from 1Mbps to 10Gbps. These accommodate a maximum of 1024 nodes. (However, attaching so many nodes to a segment is very impractical, as it would slow traffic and make management nearly impossible.)

Terminating Twisted Pair Cable

Imagine you have been sent to one of your employer’s remote offices and charged with upgrading all the old Cat 3 patch cables in a data closet with new, Cat 6 patch cables. A patch cable is a relatively short (usually between 3 and 25 feet) length of cabling with connectors at both ends. Based on the company’s network documentation, you brought 50 pre-made cables with RJ-45 plugs on both ends, which you purchased from an online cable vendor. At the remote location, however, you discover that its data closet actually contains 60 patch cables that need replacing. No additional premade cables are available at that office, and you don’t have time to order more. Luckily, you have brought your networking tool kit with spare RJ-45 plugs and a spool of Cat 6 cable. Knowing how to properly terminate Cat 6 cables allows you to make all the new patch cables you need and complete your work. Even if you are never faced with this situation, it’s likely that at some point you will have to replace an RJ-45 connector on an existing cable. This section describes how to terminate twisted pair cable.

Proper cable termination is a basic requirement for two nodes on a network to communicate. Beyond that, however, poor terminations can lead to loss or noise—and consequently, errors—in a signal. Closely following termination standards, then, is critical. TIA/EIA has specified two different methods of inserting twisted pair wires into RJ-45 plugs: TIA/EIA 568A and TIA/EIA 568B. Functionally, there is no difference between the standards. You only have to be certain that you use the same standard on every RJ-45 plug and jack on your network, so that data is transmitted and received correctly. Figure 3-24 depicts pin numbers and assignments (or pinouts) for the TIA/EIA 568A standard when used on an Ethernet network. Figure 3-25 depicts pin numbers and assignments for the TIA/EIA 568B standard. (Although networking professionals commonly refer to wires in Figures 3-24 and 3-25 as transmit and receive, their original T and R designations stand for Tip and Ring, terms that come from early telephone technology but are irrelevant today.)

Figure 3-24. TIA/EIA 568A standard terminations

Figure 3-25. TIA/EIA 568B standard terminations

If you terminate the RJ-45 plugs at both ends of a patch cable identically, following one of the TIA/EIA 568 standards, you will create a straight-through cable. A straight-through cable is so named because it allows signals to pass “straight through” from one end to the other. This is the type used to connect a workstation to a hub or router, for example. However, in some cases you may want to reverse the pin locations of some wires—for example, when you want to connect two workstations without using a connectivity device or when you want to connect two hubs through their data ports. This can be accomplished through the use of a crossover cable, a patch cable in which the termination locations of the transmit and receive wires on one end of the cable are reversed, as shown in Figure 3-26. In this example, the TIA/EIA 568B standard is used on the left side, whereas the TIA/EIA 568A standard is used on the right side. Notice that only pairs 2 and 3 are switched, because those are the pairs sending and receiving data.

Figure 3-26. RJ-45 terminations on a crossover cable

The tools you’ll need to terminate a twisted-pair cable with an RJ-45 plug are a wire cutter, wire stripper, and crimping tool, which are pictured in Figures 3-27, 3-28, and 3-29, respectively. (In fact, you can find a single device that contains all three of these tools.)

Figure 3-27. Wire cutter

Figure 3-28. Wire stripper

Figure 3-29. Crimping tool

Following are the steps to create a straight-through patch cable. To create a crossover cable, you would simply reorder the wires in step 4 to match Figure 3-26. The process of fixing wires inside the connector is called crimping, and it is a skill that requires practice—so don’t be discouraged if the first cable you create doesn’t reliably transmit and receive data.

Using the wire cutter, make a clean cut at both ends of the twisted-pair cable.

Using the wire stripper, remove the sheath off of one end of the twisted-pair cable, beginning at approximately one inch from the end. Be careful to neither damage nor remove the insulation that’s on the twisted pairs inside.

Separate the four wire pairs slightly. Carefully unwind each pair no more than ½ inch.

To make a straight-through cable, align all eight wires on a flat surface, one next to the other, ordered according to their colors and positions listed in Figure 3-25. (It might be helpful first to “groom”—or pull steadily across the length of—the unwound section of each wire to straighten it out and help it stay in place.)

Keeping the wires in order and in line, gently slide them all the way into their positions in the RJ-45 plug.

After the wires are fully inserted, place the RJ-45 plug in the crimping tool and press firmly to crimp the wires into place. (Be careful not to rotate your hand or the wire as you do this; otherwise, only some of the wires will be properly terminated.) Crimping causes the internal RJ-45 pins to pierce the insulation of the wire, thus creating contact between the two conductors.

Now remove the RJ-45 connector from the crimping tool. Examine the end and see whether each wire appears to be in contact with the pin. It may be difficult to tell simply by looking at the connector. The real test is whether your cable will successfully transmit and receive signals.

Repeat steps 2 through 7 for the other end of the cable. After completing step 7 for the other end, you will have created a straight-through patch cable.

Even after you feel confident making your own cables, it’s a good idea to verify that they can transmit and receive data at the necessary rates using a cable tester. Cable testing is discussed in Chapter 13.

In this section you’ve learned about twisted pair wiring, the most common network transmission medium in use today. The next section describes a transmission medium that, due to its many advantages, is enjoying ever-growing popularity.

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