Twisted-Pair Cable
Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. Hence, the name shielded
twisted-pair (STP). (You might see this type of cabling in Token Ring installations.) More commonly, you see cable without outer shielding; it’s called unshielded twisted-pair (UTP). UTP is
commonly used in twisted-pair Ethernet (10Base-T, 100Base-TX, etc.), star-wired networks. Let’s take a look at why the wires in this cable type are twisted. When electromagnetic signals are conducted on copper wires that are in close proximity (such as inside a cable), some electromagnetic interference occurs. In this scenario, this interference is called crosstalk. Twisting two wires together as a pair minimizes such interference and also provides some protection
against interference from outside sources. This cable type is the most common today. It is popular for several reasons:
- It’s cheaper than other types of cabling.
- It’s easy to work with.
- It permits transmission rates considered impossible 10 years ago.
Category 1 Two twisted wire pairs (four wires). Voice grade (not rated for data communications). The oldest UTP. Frequently referred to as POTS, or plain old telephone service. Before
1983, this was the standard cable used throughout the North American telephone system. POTS cable still exists in parts of the Public Switched Telephone Network (PSTN). Supports signals
limited to a frequency of 1MHz. Category 2 Four twisted wire pairs (eight wires). Suitable for up to 4Mbps, with a frequency
limitation of 10MHz. Category 3 Four twisted wire pairs (eight wires) with three twists per foot. Acceptable for transmissions up to 16MHz. A popular cable choice since the mid-1980s, but now limited mainly to telecommunication equipment.
Category 4 Four twisted wire pairs (eight wires) and rated for 20MHz.
Category 5 Four twisted wire pairs (eight wires) and rated for 100MHz .
Category 5e Four twisted wire pairs (eight wires) and rated for 100MHz, but capable of handling the disturbance on each pair caused by transmitting on all four pairs at the same time, which is needed for Gigabit Ethernet.
Category 6 Four twisted wire pairs (eight wires) and rated for 250MHz. Became a standard in June 2002.
Note:
Frequently, you will hear Category shortened to Cat. Today, any cable that you
install should be a minimum of Cat 5e. This is a minimum because some cable is now certified to carry a bandwidth signal of 350MHz or beyond. This allows unshielded twisted-pair cables to exceed speeds of 1Gbps, which is fast enough to carry broadcast-quality video over a network. A common saying is that there are three ways to do things: the Right way, the Wrong way, and the IBM way. IBM uses types instead of categories when referring to TP (twistedpair) cabling specifications. Even though a cabling type may seem to correspond
to a cabling category (such as Type 1 and Category 1), the two are not the same; IBM defines its own specifications.
Now that you’ve learned the different types of UTP cables, you will learn how best to connect them to the various pieces of networking equipment using UTP.
*Real World Scenario*
Category 5e Cabling Tips
For example, pulling too hard on Cat 5e cable will stretch the number of twists inside the jacket, rendering the Cat 5e label on the outside of the cable invalid. Also, be certain to connect
and test all four pairs of wire. Although today’s wiring usually uses only two pairs, or four wires, the standard for Gigabit Ethernet over UTP requires that all four pairs, or eight wires, be in good condition.
You should also be aware that a true Cat 5e cabling system uses rated components from end to end, patch cables from workstation to wall panel, cable from wall panel to patch panel, and patch cables from patch panel to hub. If any components are missing or if the lengths do not match the Category 5e specification, you don’t have a Category 5e cabling installation. Also, installers should certify that the entire installation is Category 5e compliant. However, this requires very expensive test equipment that can make the appropriate measurements.
Connecting UTP
Clearly, a BNC connector won’t fit easily on UTP cable, so you need to use an RJ (Registered Jack) connector. You are probably familiar with RJ connectors. Most telephones connect with an RJ-11 connector. The connector used with UTP cable is called RJ-45. The RJ-11 has four wires, or two pairs, and the network connector RJ-45 (also known as an 8P8C connector when referring to the plug instead of the jack) has four pairs, or eight wires, as shown in Figure 1.13.
Exmple:
FIGURE 1 . 1 3 RJ-11 and RJ-45 connectors
Signaling Methods
The amount of a cable’s available bandwidth (overall capacity, such as 10Mbps) that is used by each signal depends on whether the signaling method is baseband or broadband. With baseband, the entire bandwidth of the cable is used for each signal (using one channel). It is typically used with digital signaling. With broadband, on the other hand, the available bandwidth is divided into descrete bands. Multiple signals can then be transmitted within these different bands. Some form of tuning device, or demodulator, is required to choose the specific frequency of interest, as opposed to baseband receiving circuitry, which can be hardwired to a specific frequency. Don’t confuse this broadband with the term that is the opposite of narrowband, which is any bit rate of T1 speeds (1.544Mbps) or slower. That broadband refers to speeds in excess
of T1/E1 rates, such as Broadband-ISDN (B-ISDN), which has been developed under the ATM specifications.
Ethernet Cable Descriptions
Ethernet cable types are described using a code that follows this format: N<Signaling>-X. Generally speaking, N is the signaling rate in megabits per second, and <Signaling> is the signaling type, which is either base or broad (baseband or broadband). X is a unique identifier for a specific Ethernet cabling scheme.
Let’s use a generic example: 10BaseX. The two-digit number 10 indicates that the transmission speed is 10Mb, or 10 megabits. The value X can have different meanings. For example, the
5 in 10Base5 indicates the maximum distance that the signal can travel—500 meters. The 2 in 10Base2 is used the same way, but fudges the truth. The real limitation is 185 meters. Only the
IEEE committee knows for sure what this was about. We can only guess that it’s because 10Base2 seems easier to say than 10Base1.85.
Another 10Base standard is 10Base-T. The T is short for twisted-pair. This is the standard for running 10-Megabit Ethernet over two pairs (four wires) of Category 4, 5e, or 6 UTP. The fourth, and currently final, 10Base is 10Base-FL. The F is short for fiber, while the L stands for link. 10Base-FL is the standard for running 10-Megabit Ethernet over fiber-optic cable to the desktop. Table 1.2, shown a bit later, summarizes this data.
Similarly, there are also standards for 100Base, 1000Base, and 10GBase cabling. Let’s take
A closer look at these standards:
100Base-TX As network applications increased in complexity, so did their bandwidth requirements. Ten-megabit technologies were too slow. Businesses were clamoring for a higher
speed standard so that their data could be transmitted at an acceptable rate of speed. A 100- megabit standard was needed. Thus the 100Base-TX standard was developed.
The 100Base-TX standard is a standard for Ethernet transmission at a data rate of 100Mbps. This Ethernet standard is also known as Fast Ethernet. It uses two UTP pairs (four wires) in a minimum of Category 5 UTP cable.
1000Base-TX 1000Base-TX, most commonly known as Gigabit Ethernet, allows 1000Mbps throughput on standard twisted-pair, copper cable (rated at Category 5e or higher).
1000Base-SX The implementation of Gigabit Ethernet running over multimode fiber-optic cable (instead of copper, twisted-pair cable) and using short wavelength laser.
1000Base-LX The implementation of Gigabit Ethernet over single-mode and multimode fiber using long wavelength laser.
1000Base-CX An implementation of Gigabit Ethernet over balanced, 150ohm copper cabling and uses a special 9-pin connector known as the High Speed Serial Data Connector (HSSDC).
10GBase-SR An implementation of 10 Gigabit Ethernet that uses short wavelength lasers at 850 nanometers(nm) over multimode fiber. It has a maximum transmission distance of between
2 and 300 meters, depending on the size and quality of the fiber.
10GBase-LR An implementation of 10 Gigabit Ethernet that uses long wavelength lasers at 1310 nm over single-mode fiber. It also has a maximum transmission distance between 2 meters and 10 kilometers, depending on the size and quality of the fiber.
10GBase-ER An implementation of 10 Gigabit Ethernet running over single-mode fiber. It uses extra long wavelength lasers at 1550 nm. It has the longest transmission distances possible of the 10-Gigabit technologies: anywhere from 2 meters up to 40 kilometers, depending on the size and quality of the fiber used.
Note:
See the upcoming section, “Fiber-Optic Cable,” in this chapter, for more information
on single-mode and multimode fiber and on fiber in general.
IEEE Standard 1394 (FireWire)
One unique cabling type that is used in a limited sense is IEEE standard 1394, more commonly known as FireWire (or as Sony calls it, i.Link). Developed by Apple Computer, FireWire runs
at 100, 200, 400Mbps (800Mbps in the 1394b standard), but in its standard mode it has a cable length limitation of 15 feet (4.5 meters), which limits it to specialized applications like data
transfer between two computers located in close proximity or data transfer between a computer and another device (like an MP3 player).
FireWire uses two types of connectors: the 6 pin and the 4 pin. The 6-pin connector (as shown in Figure 1.14) is for devices that need to be powered from the computer. FireWire cables with the 6-pin connector contain two pairs (four conductors) of copper wire for carrying data and one pair for powering devices, all within a common, braided metal shield. Cables using the 4-pin connector (Figure 1.15) are for data transfer only, and they contain only the four conductors for data, none for power.
Example:
FIGURE 1 . 1 4 Six-pin FireWire connector (male)
Example:
FIGURE 1 . 1 5 Four-pin FireWire connector (male)
Note:
More information about FireWire and its associated standards can be found at
the 1394 Trade Association website at www.1394ta.org.
Universal Serial Bus (USB)
Over the past few years, computer peripherals have been moving away from parallel or serial connection and to a new type of bus. That bus is the Universal Serial Bus (USB). The built-in serial bus of most motherboards generally offers a maximum of 2 external interfaces for connectivity to a PC, although add-on adapters can take that count up to as many as 16 serial interfaces. USB, on the other hand, can connect a maximum of 127 external devices. Also, USB is a much more flexible peripheral bus than either serial or parallel. USB supports connections to printers, scanners, and many other input devices (such as keyboards, joysticks, and mice).
When connecting USB peripherals, you must connect them either directly to one of the USB ports (as shown in Figure 1.16) on the PC or to a USB hub that is connected to one of those USB ports. Hubs can be chained together to provide multiple USB connections. Although you can connect up to 127 devices (each device has a USB plug, as shown in Figure 1.17), it is impractical in reality. Most computers with USB interfaces will support around 12 USB devices.
Example:
FIGURE 1 . 1 6 A USB port
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