Measuring Electricity 119 Voltage Because electrons and protons have opposite charges, they are attracted to each other with a force similar to the attractive force of the north and south poles of two magnets. When the charges are separated, this separation creates an attractive force or pressure field between the charges. This force is voltage. The force that is created pulls toward the opposite charge and pushes away from the like charge. This process occurs in a battery, where chemical action causes electrons to be freed from the battery’s negative terminal and to travel to the opposite, or positive, terminal through an external circuit— not through the battery itself. The separation of charges results in voltage. Voltage can also be created by friction (static electricity), by magnetism (electric generator), or by solar energy. Voltage is represented by the letter V. The unit of measurement for voltage is the volt, and it is also represented with the letter V (for example, 12 V = 12 volts). Two kinds of voltage exist: ■ Direct-current (DC) voltage—A battery is an example of a DC voltage source. The movement of electrons in a DC circuit is always in the same direction, from negative to positive. ■ Alternating-current (AC) voltage—In an AC circuit, the positive and negative ter- minals of the AC voltage source regularly change to negative and positive and back again, as shown in Figure 3-3. This change makes the direction of electron movement change, or alternate, with respect to time. Figure 3-3 Alternating Current Lab Activity Safe Handling and Use of a Multimeter In this lab, you learn how to use or handle a multimeter correctly. Lab Activity Voltage Measurements In this lab, you demonstrate the ability to measure voltage with the multimeter. 1102.book Page 119 Tuesday, May 20, 2003 2:53 PM 120 Chapter 3: Networking Media Current Electrical current is the flow of charges that is created when electrons move. When voltage (electrical pressure) is applied and a path for the current exists, electrons move from the negative terminal (which repels them), along the path, to the positive terminal (which attracts them). Current is represented by the letter I. The unit of measurement for current is the ampere, and it is represented by the letter A, or by the abbreviation amp. An amp is defined as the number of charges per second that pass by a point along a path. It can be thought of as the amount of electron traffic that is flowing through a circuit; the more electrons that pass by any given point in a circuit, the higher the current. Current that results from DC voltage always flows in the same direction, from negative to positive. Current that results from AC voltage flows in one direction, then changes direction, and then alternates back to the original direction, and so on. Wattage If amperage or current can be thought of as the amount or volume of electron traffic that is flowing, then voltage can be thought of as the speed of the electron traffic. The combination of amperage (quantity of electrons past a given point) and voltage (pres- sure or speed of electrons) equals wattage or electrical power. A watt (W) is the basic unit of electrical power or work done by electricity. Wattage equals voltage times amper- age (W = V × I). Electrical devices such as light bulbs, motors, and computer power supplies are rated in terms of watts, which is how much power they consume or pro- duce. It is the current or amperage in an electrical circuit that really does the work. As an example, static electricity has very high voltage, so much that it can jump a gap of an inch or more. However, it has very low amperage and as a result can create a shock but not injure someone. The starter motor in an automobile operates at a relatively low 12 volts but requires very high amperage to generate enough energy to turn over the engine. Lightning has very high voltage and high amperage and can cause severe damage or injury. Resistance and Impedance Conductors exchange electrons very easily, so it does not take much voltage to cause electrons to move through them. Conversely, the electrons in insulators are bound to their orbits much more tightly, so they oppose the movement of electrons. Resistance is the property of a material that resists electron movement. Conductors have low resistance, and insulators have high resistance. 1102.book Page 120 Tuesday, May 20, 2003 2:53 PM Measuring Electricity 121 Resistance is represented by the letter R. The unit of measurement for resistance is the ohm, and it is represented by the Greek letter, omega (Ω), because omega sounds like ohm. The term resistance is generally used when referring to DC circuits. The resistance to the movement of electrons in an AC circuit is called impedance. Impedance is repre- sented by the letter Z. Like resistance, its unit of measurement is the ohm, represented by Ω. Circuits Electrons move best through conductive materials. Although air in a dry climate can be conductive, as noticed through shocks of static electricity, electrons cannot jump across air from a battery to an unconnected, nearby piece of copper wire. Current, or electron movement, occurs only in circuits that form complete loops. These circuits are known as closed circuits. Figure 3-4 shows a simple circuit, typical of a lantern-style flashlight. The switch is like two ends of a single wire that can be opened (or broken) and then closed (or shorted) to prevent or allow current. Figure 3-4 Serial Circuit—Flashlight Lab Activity Resistance Measurements In this lab, you demonstrate the ability to measure resistance and continuity with the multimeter. 6V Lantern + 6V Lantern + 1102.book Page 121 Tuesday, May 20, 2003 2:53 PM 122 Chapter 3: Networking Media The top of Figure 3-4 illustrates the flashlight with its switch turned off. The chemical processes in the battery cause charges to be separated, which provides voltage. How- ever, because is no complete path for electron movement exists, there is no current, and the bulb will not be lit. As shown in the bottom of Figure 3-4, the switch is turned on, and a complete path of conductive wire for current exists. The bulb provides resistance to the flow of electrons, causing the current to release energy in the form of light. The circuits involved in networking use the same concepts as this very simple circuit, but networking circuits are much more complex. When you are learning a new concept, it is often helpful to relate the concept to a familiar example. The circuit discussed pre- viously can be compared to a water circuit, as illustrated in Figure 3-5. The pressure that causes water flow comes from the weight of the water in the tank. The tap can be compared with the switch in the previous example. When the tap is turned off, it blocks water from moving. When the tap is turned on, it allows water to move and also pro- vides resistance to the flow of water, because a small tap will allow a lesser flow than a large tap. Finally, the pipe provides a closed path for the flow of water to cycle back into the tank. Figure 3-5 Water Circuit Analogy for Flowing Electrons Lab Activity Communications Circuits In this lab, you build series circuits and explore their basic properties. 1102.book Page 122 Tuesday, May 20, 2003 2:53 PM Copper Media 123 Copper Media Copper is the most common medium for signal wiring. Copper wires are the components of a cable that carry the signals from the source computer to the destination computer. Copper has several important properties that make it well suited for electronic cabling: ■ Conductivity—Copper is perhaps best known for its ability to conduct electric current. Copper is also an excellent conductor of heat. This property makes it useful in cooking utensils, radiators, and refrigerators. ■ Corrosion resistance—Copper does not rust and is fairly resistant to corrosion; the copper corrodes as copper oxide at a somewhat slower pace than other metals. ■ Ductility—Copper possesses great ductility, the ability to be drawn into thin wires without breaking. For example, copper rod that is 1 centimeter (cm) in diameter can be heated, rolled, and drawn into a wire that is thinner than a human hair. ■ Malleability—Pure copper is highly malleable (easy to shape). It does not crack when hammered, stamped, forged, or spun into unusual shapes. Copper can be worked (shaped) when it is hot or cold. ■ Strength—Cold-rolled copper has a tensile strength 3500 to 4900 kilograms per square centimeter. Copper keeps its strength and toughness up to about 400° Fahrenheit (F) (204° Celsius [C]). This section focuses on two types of copper cable used for networks: ■ Twisted-pair—Twisted-pair cables are composed of one or more pairs of copper wires. Most data and voice networks use twisted-pair cabling. ■ Coaxial—Coaxial cable has one center conductor of either solid or stranded copper wire. Coaxial cable, once the choice for local-area network (LAN) cabling, is now used primarily for video connections, high-speed connections such as T3 (or E3) lines, and cable television. American Wire Gauge System The diameter of cable wires or conductors is commonly measured using the American wire gauge (AWG) system. AWG is a U.S. standard for measuring the diameter of pri- marily copper and aluminum cable. Typical residential wiring is AWG 12 or 14. The conductor or wire size used in the UTP in most telephone local loops (from the central office to a home or residence) is between 19 and 26 AWG. Most newer telephone wire is from 22 to 26 gauge with 24 gauge being the most common. The lower the gauge number the thicker the wire. Thicker wire has less resistance and can carry more cur- rent resulting in a better signal over longer distances. A wire with an AWG size of 24 would be 1/24th of an inch in diameter. 1102.book Page 123 Tuesday, May 20, 2003 2:53 PM 124 Chapter 3: Networking Media Twisted-Pair Cable Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. The wire pairs are twisted for two reasons. First, when a wire is carrying a current, that current creates a magnetic field around the wire. This field can interfere with sig- nals on nearby wires. To combat this, pairs of wires carry signals in opposite directions, so that the two magnetic fields also occur in opposite directions and cancel each other out. This process is known as cancellation. Twisting the pairs holds the two wires closer together and helps to ensure effective cancellation within the cable. Second, network data is sent using two wires in a twisted pair. One copy of the data is sent on each wire, and the two copies are mirror images of each other. These signals are called differential signals. If the two wires are twisted together, noise seen on one wire is also be seen on the other wire. When the data is received, one copy is inverted, and the two signals are then compared. In this manner the receiver can filter out noise because the noise signals cancel each other. Two basic types of twisted-pair cable exist: shielded twisted-pair (STP) and unshielded twisted-pair (UTP). The following sections discuss UTP and STP cable in more detail. Shielded Twisted-Pair Cable Shielded twisted-pair (STP) cable contains four pairs of thin, copper wires covered in color-coded plastic insulation that are twisted together. Each pair is wrapped in metallic foil, and then the four pairs are collectively wrapped in another layer of metallic braid or foil. This layer is wrapped with a plastic outer jacket. Figure 3-6 illustrates an example of STP. Figure 3-6 Shielded Twisted-Pair Cable Outer Jacket Overall Shield Pair Shields Twisted Pair RJ-45 Connector Color-Coded Plastic Insulation 1102.book Page 124 Tuesday, May 20, 2003 2:53 PM Copper Media 125 Screened twisted-pair (ScTP), also known as foil twisted-pair (FTP), is a variation of STP. ScTP is essentially STP with just one layer of foil shielding around the set of all four-wire pairs, as shown in Figure 3-7. The shielding in both STP and ScTP reduces unwanted electrical noise. This noise reduction provides a major advantage of STP over unshielded cable. Figure 3-7 Screened Twisted-Pair Cable However, shielded cable is more difficult to install than unshielded cable because the metallic shielding needs to be grounded. If improperly installed, STP and ScTP become very susceptible to noise problems because an ungrounded shield acts like an antenna, picking up unwanted signals. STP and ScTP cable cannot be run as far as coaxial and fiber-optic cable without the use of repeaters. The insulation and shielding consider- ably increase the size, weight, and cost of the cable. Despite these disadvantages, shielded copper cable is still used as networking media today, especially in Europe. The following summarizes the features of STP cable: ■ Speed and throughput—10 to 100 Mbps ■ Average cost per node—Moderately expensive ■ Media and connector size—Medium to large ■ Maximum cable length—100 meters (m) (short) Unshielded Twisted-Pair Cable Unshielded twisted-pair (UTP) cable is a common networking media. It consists of four pairs of thin, copper wires covered in color-coded plastic insulation that are twisted together, as shown in Figure 3-8. The wire pairs are then covered with a plastic outer jacket. The connector used on a UTP cable is called a registered jack 45 (RJ-45) connector, as shown in Figure 3-9. Outer Jacket Overall Shield Twisted Pair RJ-45 Connector Color-Coded Plastic Insulation 1102.book Page 125 Tuesday, May 20, 2003 2:53 PM 126 Chapter 3: Networking Media Figure 3-8 Unshielded Twisted-Pair Cable Figure 3-9 RJ-45 Connector UTP cable has many advantages. It has a small diameter and does not require ground- ing, so it is the easiest type of cable to install. Its size provides an additional advantage because more UTP cable can fit in a given area than other copper media. It is also the least expensive type of networking media, and the connector is the easiest to build. It supports the same data speeds as other copper media. The primary disadvantage to UTP is that it is more susceptible to electrical noise and interference than any other type of networking media. Because it has no shielding, it relies solely on the cancellation and differential signals to reduce the effects of noise. The other main disadvantage is that its maximum run length is less than that allowed for coaxial and fiber-optic cables. Twisted Pair RJ-45 Connector Color-Coded Plastic Insulation Outer Jacket 1102.book Page 126 Tuesday, May 20, 2003 2:53 PM Copper Media 127 Although UTP was once considered to be slower at transmitting data than other types of cable, this is no longer true. In fact, UTP is considered the fastest copper-based medium today. The following summarizes the features of UTP cable: ■ Speed and throughput—10 to 1000 Mbps ■ Average cost per node—Least expensive ■ Media and connector size—Small ■ Maximum cable length—100 m (short) Commonly used types of UTP cabling are as follows: ■ Category 1 (CAT 1)—Used for telephone communications. Not suitable for transmitting data. ■ Category 2 (CAT 2)—Capable of transmitting data at speeds up to 4 Mbps. ■ Category 3 (CAT 3)—Used in 10BASET Ethernet networks. Can transmit data at speeds up to 10 Mbps. ■ Category 4 (CAT 4)—Used in Token Ring networks. Can transmit data at speeds up to 16 Mbps. ■ Category 5 (CAT 5)—Can transmit data at speeds up to 100 Mbps. Used in Fast Ethernet networks. ■ Category 5e (CAT 5e)—Used in networks running at speeds up to 1000 Mbps (1 Gbps). Used in Gigabit Ethernet (GigE) networks. ■ Category 6 (CAT 6)—The specification for CAT 6 is new, was released on February 3, 2003, and is currently available for installation and use. Used in Gigabit Ethernet (GigE) networks. Typically, CAT 5 and higher network cable consists of four pairs of 24 AWG multi- strand copper wires. Older cabling installations run CAT 3 for voice and CAT 5 for data. Most new installations run a minimum of CAT 5e for voice and data. Although CAT 5e costs a little more it is worth it in the long run. When comparing UTP and STP, keep the following points in mind: ■ The speed of both types of cable is usually satisfactory for local-area distances. ■ These are the least-expensive media for data communication. UTP is less expen- sive than STP. ■ Because most buildings are already wired with UTP, many transmission standards are adapted to use it to avoid costly rewiring with an alternative cable type. You must take care to ensure that the category level of the cable is adequate to handle the bandwidth desired. As an example, a building wired with CAT 3 cable cannot support Fast Ethernet, which requires at least CAT 5. 1102.book Page 127 Tuesday, May 20, 2003 2:53 PM 128 Chapter 3: Networking Media Coaxial Cable Coaxial cable, as shown in Figure 3-10, consists of four main parts: ■ Copper conductor ■ Plastic insulation ■ Braided copper shielding ■ Outer jacket At the center of the cable is a solid copper conductor. Surrounding that conductor is a layer of flexible plastic insulation. A woven copper braid or metallic foil is wrapped around the insulation. This layer acts as the second wire in the cable. It also acts as a shield for the inner conductor and helps reduce the amount of outside interference. Covering this shield is the outer cable jacket. The connector used on coaxial cable is called a BNC, short for British Naval Connector or Bayonet Neill Concelman, connector. Figure 3-10 Coaxial Cable Coaxial cable was a popular choice with LANs in the past. It offered several advantages. It can be run with fewer boosts from repeaters for longer distances between network nodes than either STP or UTP cable. Although more expensive than UTP, coaxial cable is less expensive than fiber-optic cable. The technology is well known, because it has been used for many years in various types of data communication. For example, coaxial cable is commonly used in homes to deliver cable television signals and high-speed Internet access. For cable TV, RG-59 is commonly used inside the home and has a center conductor of 20 AWG. RG-6 is most often used from the street pedestal to the home due to heavier shielding and a larger center conductor of 18 AWG. RG-11 cable is heavier still with a center conductor of 14 AWG and is used to bring cable into neighborhoods. 1102.book Page 128 Tuesday, May 20, 2003 2:53 PM . the ability to measure resistance and continuity with the multimeter. 6V Lantern + 6V Lantern + 11 02. book Page 12 1 Tuesday, May 20 , 20 03 2: 53 PM 12 2 Chapter 3: Networking Media The top of Figure. longer distances. A wire with an AWG size of 24 would be 1/ 24 th of an inch in diameter. 11 02. book Page 12 3 Tuesday, May 20 , 20 03 2: 53 PM 12 4 Chapter 3: Networking Media Twisted-Pair Cable Twisted-pair. requires at least CAT 5. 11 02. book Page 12 7 Tuesday, May 20 , 20 03 2: 53 PM 12 8 Chapter 3: Networking Media Coaxial Cable Coaxial cable, as shown in Figure 3 -10 , consists of four main parts: ■ Copper conductor ■