Reflection / Refraction / Total internal reflection

Reflection
3.2.3 This page provides an overview of reflection.


When a ray of light (the incident ray) strikes the shiny surface of a flat piece of glass, some of the light energy in the ray is reflected. The angle between the incident ray and a line perpendicular to the surface of the glass at the point where the incident ray strikes the glass is called the angle of incidence. The perpendicular line is called the normal. It is not a light ray but a tool to allow the measurement of angles. The angle between the reflected ray and the normal is called the angle of reflection. The Law of Reflection states that the angle of reflection of a light ray is equal to the angle of incidence. In other words, the angle at which a light ray strikes a reflective surface determines the angle that the ray will reflect off the surface.

The next page describes refraction.

Refraction
3.2.4 This page provides an overview of refraction.


When a light strikes the interface between two transparent materials, the light divides into two parts. Part of the light ray is reflected back into the first substance, with the angle of reflection equaling the angle of incidence. The remaining energy in the light ray crosses the interface and enters into the second substance.

If the incident ray strikes the glass surface at an exact 90-degree angle, the ray goes straight into the glass. The ray is not bent. However, if the incident ray is not at an exact 90-degree angle to the surface, then the transmitted ray that enters the glass is bent. The bending of the entering ray is called refraction. How much the ray is refracted depends on the index of refraction of the two transparent materials. If the light ray travels from a substance whose index of refraction is smaller, into a substance where the index of refraction is larger, the refracted ray is bent towards the normal. If the light ray travels from a substance where the index of refraction is larger into a substance where the index of refraction is smaller, the refracted ray is bent away from the normal.

Consider a light ray moving at an angle other than 90 degrees through the boundary between glass and a diamond. The glass has an index of refraction of about 1.523. The diamond has an index of refraction of about 2.419. Therefore, the ray that continues into the diamond will be bent towards the normal. When that light ray crosses the boundary between the diamond and the air at some angle other than 90 degrees, it will be bent away from the normal. The reason for this is that air has a lower index of refraction, about 1.000 than the index of refraction of the diamond.

The next page discusses the concept of total internal refraction.

Total internal reflection
3.2.5 This page explains total internal refraction as it relates to optical media.


A light ray that is being turned on and off to send data (1s and 0s) into an optical fiber must stay inside the fiber until it reaches the far end. The ray must not refract into the material wrapped around the outside of the fiber. The refraction would cause the loss of part of the light energy of the ray. A design must be achieved for the fiber that will make the outside surface of the fiber act like a mirror to the light ray moving through the fiber. If any light ray that tries to move out through the side of the fiber were reflected back into the fiber at an angle that sends it towards the far end of the fiber, this would be a good "pipe" or "wave guide" for the light waves.

The laws of reflection and refraction illustrate how to design a fiber that guides the light waves through the fiber with a minimum energy loss. The following two conditions must be met for the light rays in a fiber to be reflected back into the fiber without any loss due to refraction:

• The core of the optical fiber has to have a larger index of refraction (n) than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding.

• The angle of incidence of the light ray is greater than the critical angle for the core and its cladding.

When both of these conditions are met, the entire incident light in the fiber is reflected back inside the fiber. This is called total internal reflection, which is the foundation upon which optical fiber is constructed. Total internal reflection causes the light rays in the fiber to bounce off the core-cladding boundary and continue its journey towards the far end of the fiber. The light will follow a zigzag path through the core of the fiber.

A fiber that meets the first condition can be easily created. In addition, the angle of incidence of the light rays that enter the core can be controlled. Restricting the following two factors controls the angle of incidence:

• The numerical aperture of the fiber – The numerical aperture of a core is the range of angles of incident light rays entering the fiber that will be completely reflected.

• Modes – The paths which a light ray can follow when traveling down a fiber.

By controlling both conditions, the fiber run will have total internal reflection. This gives a light wave guide that can be used for data communications.

The next page will describe multimode fiber.

Ray model of light

Ray model of light
3.2.2 This page describes the properties of light rays.


When electromagnetic waves travel out from a source, they travel in straight lines. These straight lines pointing out from the source are called rays.

Think of light rays as narrow beams of light like those produced by lasers. In the vacuum of empty space, light travels continuously in a straight line at 300,000 kilometers per second. However, light travels at different, slower speeds through other materials like air, water, and glass. When a light ray called the incident ray, crosses the boundary from one material to another, some of the light energy in the ray will be reflected back. That is why you can see yourself in window glass. The light that is reflected back is called the reflected ray.

The light energy in the incident ray that is not reflected will enter the glass. The entering ray will be bent at an angle from its original path. This ray is called the refracted ray. How much the incident light ray is bent depends on the angle at which the incident ray strikes the surface of the glass and the different rates of speed at which light travels through the two substances.

The bending of light rays at the boundary of two substances is the reason why light rays are able to travel through an optical fiber even if the fiber curves in a circle.

The optical density of the glass determines how much the rays of light in the glass bends. Optical density refers to how much a light ray slows down when it passes through a substance. The greater the optical density of a material, the more it slows light down from its speed in a vacuum. The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium. Therefore, the measure of the optical density of a material is the index of refraction of that material. A material with a large index of refraction is more optically dense and slows down more light than a material with a smaller index of refraction.

For a substance like glass, the Index of Refraction, or the optical density, can be made larger by adding chemicals to the glass. Making the glass very pure can make the index of refraction smaller. The next lessons will provide further information about reflection and refraction, and their relation to the design and function of optical fiber.

The Interactive Media Activity demonstrates how light travels.

The next page discusses reflection.

Optical Media / The electromagnetic spectrum

The electromagnetic spectrum
3.2.1 This page introduces the electromagnetic spectrum.


The light used in optical fiber networks is one type of electromagnetic energy. When an electric charge moves back and forth, or accelerates, a type of energy called electromagnetic energy is produced. This energy in the form of waves can travel through a vacuum, the air, and through some materials like glass. An important property of any energy wave is the wavelength.

Radio, microwaves, radar, visible light, x-rays, and gamma rays seem to be very different things. However, they are all types of electromagnetic energy. If all the types of electromagnetic waves are arranged in order from the longest wavelength down to the shortest wavelength, a continuum called the electromagnetic spectrum is created.

The wavelength of an electromagnetic wave is determined by how frequently the electric charge that generates the wave moves back and forth. If the charge moves back and forth slowly, the wavelength it generates is a long wavelength. Visualize the movement of the electric charge as like that of a stick in a pool of water. If the stick is moved back and forth slowly, it will generate ripples in the water with a long wavelength between the tops of the ripples. If the stick is moved back and forth more rapidly, the ripples will have a shorter wavelength.

Because electromagnetic waves are all generated in the same way, they share many of the same properties. The waves all travel at the same rate of speed though a vacuum. The rate is approximately 300,000 kilometers per second or 186,283 miles per second. This is also the speed of light.

Human eyes were designed to only sense electromagnetic energy with wavelengths between 700 nanometers and 400 nanometers (nm). A nanometer is one billionth of a meter (0.000000001 meter) in length. Electromagnetic energy with wavelengths between 700 and 400 nm is called visible light. The longer wavelengths of light that are around 700 nm are seen as the color red. The shortest wavelengths that are around 400 nm appear as the color violet. This part of the electromagnetic spectrum is seen as the colors in a rainbow.

Wavelengths that are not visible to the human eye are used to transmit data over optical fiber. These wavelengths are slightly longer than red light and are called infrared light. Infrared light is used in TV remote controls. The wavelength of the light in optical fiber is either 850 nm, 1310 nm, or 1550 nm. These wavelengths were selected because they travel through optical fiber better than other wavelengths.

The next page will discuss the ray model of light.

STP cable / UTP cable

STP cable

3.1.8 STP cable combines the techniques of cancellation, shielded, and twisted wires. Each pair of wires is wrapped in metallic foil. The two pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-ohm cable. As specified for use in Token Ring network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable such as electromagnetic interference (EMI) and radio frequency interference (RFI). STP cable shares many of the advantages and disadvantages of UTP cable. STP provides more protection from all types of external interference. However, STP is more expensive and difficult to install than UTP.

A new hybrid of UTP is Screened UTP (ScTP), which is also known as foil screened twisted pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. ScTP, like UTP, is also 100-ohm cable. Many cable installers and manufacturers may use the term STP to describe ScTP cabling. It is important to understand that most references made to STP today actually refer to four-pair shielded cabling. It is highly unlikely that true STP cable will be used during a cable installation job.

The metallic shielding materials in STP and ScTP need to be grounded at both ends. If improperly grounded or if there are any discontinuities in the entire length of the shielding material, STP and ScTP can become susceptible to major noise problems. They are susceptible because they allow the shield to act like an antenna that picks up unwanted signals. However, this effect works both ways. Not only does the shield prevent incoming electromagnetic waves from causing noise on data wires, but it also minimizes the outgoing radiated electromagnetic waves. These waves could cause noise in other devices. STP and ScTP cable cannot be run as far as other networking media, such as coaxial cable or optical fiber, without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. The shielding materials make terminations more difficult and susceptible to poor workmanship. However, STP and ScTP still have a role, especially in Europe or installations where there is extensive EMI and RFI near the cabling.

The following page discusses UTP cable.

UTP cable
3.1.9 UTP is a four-pair wire medium used in a variety of networks. Each of the eight copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.

TIA/EIA-568-B.2 contains specifications that govern cable performance. It involves the connection of two cables, one for voice and one for data, to each outlet. The cable for voice must be four-pair UTP. Category 5 is the cable most frequently recommended and implemented in installations. However, analyst predictions and independent polls indicate that Category 6 cable will supersede Category 5 cable in network installations. The fact that Category 6 link and channel requirements are backward compatible to Category 5e makes it very easy for customers to choose Category 6 and supersede Category 5e in their networks. Applications that work over Category 5e will work over Category 6.

UTP cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. However, the real advantage is the size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when a network is installed in an older building. When UTP cable is installed with an RJ-45 connector, potential sources of network noise are greatly reduced and a good solid connection is almost guaranteed.

There are some disadvantages of twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables.

Twisted pair cabling was once considered slower at transmitting data than other types of cable. This is no longer true. In fact, today, twisted pair is considered the fastest copper-based media.

For communication to occur the signal that is transmitted by the source needs to be understood by the destination. This is true from both a software and physical perspective. The transmitted signal needs to be properly received by the circuit connection designed to receive signals. The transmit pin of the source needs to ultimately connect to the receiving pin of the destination. The following are the types of cable connections used between internetwork devices.

In Figure , a LAN switch is connected to a computer. The cable that connects from the switch port to the computer NIC port is called a straight-through cable.

In Figure , two switches are connected together. The cable that connects from one switch port to another switch port is called a crossover cable.

In Figure , the cable that connects the RJ-45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable.

The cables are defined by the type of connections, or pinouts, from one end to the other end of the cable. See Figures , , and . A technician can compare both ends of the same cable by placing them next to each other, provided the cable has not yet been placed in a wall. The technician observes the colors of the two RJ-45 connections by placing both ends with the clip placed into the hand and the top of both ends of the cable pointing away from the technician. A straight-through cable should have both ends with identical color patterns. While comparing the ends of a cross-over cable, the color of pins #1 and #2 will appear on the other end at pins #3 and #6, and vice-versa. This occurs because the transmit and receive pins are in different locations. On a rollover cable, the color combination from left to right on one end should be exactly opposite to the color combination on the other end.

In the first Lab Activity, a simple communication system is designed, built, and tested.

In the next Lab Activity, students will use a cable tester to determine if a straight-through or crossover cable is good or bad.


This page concludes this lesson. The next lesson will discuss optical media. The first page will describe the electromagnetic spectrum.

Circuits / Cable specifications / Coaxial cable

Circuits
This page explains circuits.


3.1.5 Current flows in closed loops called circuits. These circuits must be made of conductive materials and must have sources of voltage. Voltage causes current to flow. Resistance and impedance oppose it. Current consists of electrons that flow away from negative terminals and toward positive terminals. These facts allow people to control the flow of current.

Electricity will naturally flow to the earth if there is a path. Current also flows along the path of least resistance. If a human body provides the path of least resistance, the current will flow through it. When an electric appliance has a plug with three prongs, one of the prongs acts as the ground, or 0 volts. The ground provides a conductive path for the electrons to flow to the earth. The resistance of the body would be greater than the resistance of the ground.

Ground typically means the 0-volts level in reference to electrical measurements. Voltage is created by the separation of charges, which means that voltage measurements must be made between two points.

A water analogy can help explain the concept of electricity. The higher the water and the greater the pressure, the more the water will flow. The water current also depends on the size of the space it must flow through. Similarly, the higher the voltage and the greater the electrical pressure, the more current will be produced. The electric current then encounters resistance that, like the water tap, reduces the flow. If the electric current is in an AC circuit, then the amount of current will depend on how much impedance is present. If the electric current is in a DC circuit, then the amount of current will depend on how much resistance is present. The pump is like a battery. It provides pressure to keep the flow moving.

The relationship among voltage, resistance, and current is voltage (V) equals current (I) multiplied by resistance (R). In other words, V=I*R. This is Ohm’s law, named after the scientist who explored these issues.

Two ways in which current flows are alternating current (AC) and direct current (DC). AC voltages change their polarity, or direction, over time. AC flows in one direction, then reverses its direction and flows in the other direction, and then repeats the process. AC voltage is positive at one terminal, and negative at the other. Then the AC voltage reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously.

DC always flows in the same direction and DC voltages always have the same polarity. One terminal is always positive, and the other is always negative. They do not change or reverse.

An oscilloscope is an electronic device used to measure electrical signals relative to time. An oscilloscope graphs the electrical waves, pulses, and patterns. An oscilloscope has an x-axis that represents time, and a y-axis that represents voltage. There are usually two y-axis voltage inputs so that two waves can be observed and measured at the same time.

Power lines carry electricity in the form of AC because it can be delivered efficiently over large distances. DC can be found in flashlight batteries, car batteries, and as power for the microchips on the motherboard of a computer, where it only needs to go a short distance.

Electrons flow in closed circuits, or complete loops. Figure shows a simple circuit. The chemical processes in the battery cause charges to build up. This provides a voltage, or electrical pressure, that enables electrons to flow through various devices. The lines represent a conductor, which is usually copper wire. Think of a switch as two ends of a single wire that can be opened or broken to prevent the flow of electrons. When the two ends are closed, fixed, or shorted, electrons are allowed to flow. Finally, a light bulb provides resistance to the flow of electrons, which causes the electrons to release energy in the form of light. The circuits in networks use a much more complex version of this simple circuit.

For AC and DC electrical systems, the flow of electrons is always from a negatively charged source to a positively charged source. However, for the controlled flow of electrons to occur, a complete circuit is required. Figure shows part of the electrical circuit that brings power to a home or office.

The Lab Activity explores the basic properties of series circuits.

The next page covers cable specifications.

Cable specifications

3.1.6 Cables have different specifications and expectations. Important considerations related to performance are as follows:

• What speeds for data transmission can be achieved? The speed of bit transmission through the cable is extremely important. The speed of transmission is affected by the kind of conduit used.

• Will the transmissions be digital or analog? Digital or baseband transmission and analog or broadband transmission require different types of cable.

• How far can a signal travel before attenuation becomes a concern? If the signal is degraded, network devices might not be able to receive and interpret the signal. The distance the signal travels through the cable affects attenuation of the signal. Degradation is directly related to the distance the signal travels and the type of cable used.

The following Ethernet specifications relate to cable type:

• 10BASE-T
• 10BASE5
• 10BASE2

10BASE-T refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The T stands for twisted pair.

10BASE5 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 5 indicates that a signal can travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to interpret the signal. 10BASE5 is often referred to as Thicknet. Thicknet is a type of network and 10BASE5 is the cable used in that network.

10BASE2 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 2, in 10BASE2, refers to the approximate maximum segment length being 200 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. The maximum segment length is actually 185 meters. 10BASE2 is often referred to as Thinnet. Thinnet is a type of network and 10BASE2 is the cable used in that network.

The next page describes coaxial cable.

Coaxial cable

3.1.7 Coaxial cable consists of a copper conductor surrounded by a layer of flexible insulation. The center conductor can also be made of tin plated aluminium cable allowing for the cable to be manufactured inexpensively. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield also reduces the amount of outside electromagnetic interference. Covering this shield is the cable jacket.

For LANs, coaxial cable offers several advantages. It can be run longer distances than shielded twisted pair, STP, unshielded twisted pair, UTP, and screened twisted pair, ScTP, cable without the need for repeaters. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable and the technology is well known. It has been used for many years for many types of data communication such as cable television.

It is important to consider the size of a cable. As the thickness increases, it becomes more difficult to work with a cable. Remember that cable must be pulled through conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable since it has greater transmission lengths and noise rejection characteristics. This type of coaxial cable is frequently referred to as Thicknet. This type of cable can be too rigid to install easily in some situations. Generally, the more difficult the network media is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is rarely used anymore aside from special purpose installations.

In the past, Thinnet coaxial cable with an outside diameter of only 0.35 cm was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since Thinnet was easier to install, it was also cheaper to install. This led some people to refer to it as Cheapernet. The outer copper or metallic braid in coaxial cable comprises half the electric circuit. A solid electrical connection at both ends is important to properly ground the cable. Poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmission. For this reason Thinnet is no longer commonly used nor supported by latest standards, 100 Mbps and higher, for Ethernet networks.

The following page describes STP cable.

Copper Media / Atoms and electrons / Voltage / Resistance and impedance / Current

Atoms and electrons
3.1.1 This lesson discusses the copper media used in networking. Since all matter is composed of atoms, this page begins with a detailed explanation of atoms and electrons.


All matter is composed of atoms. The Periodic Table of Elements lists all known types of atoms and their properties. The atom is comprised of three basic particles:

• Electrons – Particles with a negative charge that orbit the nucleus
• Protons – Particles with a positive charge
• Neutrons – Neutral particles with no charge

The protons and neutrons are combined together in a small group called a nucleus.

To better understand the electrical properties of different elements, locate helium (He) on the periodic table. Helium has an atomic number of 2, which means that helium has two protons and two electrons. It has an atomic weight of 4. If the atomic number of 2 is subtracted from the atomic weight of 4, the result shows that helium also has two neutrons.

The Danish physicist, Niels Bohr, developed a simplified model to illustrate the atom. This illustration shows the model for a helium atom. If the protons and neutrons of an atom were the size of adult soccer balls in the middle of a soccer field, the only thing smaller than the balls would be the electrons. The electrons would be the size of cherries that would be in orbit near the outer-most seats of the stadium. The overall volume of this atom would be about the size of the stadium. The nucleus would be the size of the soccer balls.

Coulomb's Electric Force Law states that opposite charges react to each other with a force that causes them to be attracted to each other. Like charges react to each other with a force that causes them to repel each other. In the case of opposite and like charges, the force increases as the charges move closer to each other. The force is inversely proportional to the square of the separation distance. When particles get extremely close together, nuclear force overrides the repulsive electrical force and keeps the nucleus together. That is why a nucleus does not fly apart.

Examine the Bohr model of the helium atom. If Coulomb's law is true and the Bohr model describes helium atoms as stable, then there must be other laws of nature at work. Review both theories to see how they conflict with each other:

• Coulomb's law – Opposite charges attract and like charges repel.

• The Bohr model – Protons have positive charges and electrons have negative charges. There is more than one proton in the nucleus.

Electrons stay in orbit, even though the protons attract the electrons. The electrons have just enough velocity to keep orbiting and not be pulled into the nucleus, just like the moon around the Earth.

Protons do not fly apart from each other because of a nuclear force that is associated with neutrons. The nuclear force is an incredibly strong force that acts as a kind of glue to hold the protons together.

Electrons are bound to their orbit around the nucleus by a weaker force than nuclear force. Electrons in certain atoms, such as metals, can be pulled free from the atom and made to flow. This sea of electrons, loosely bound to the atoms, is what makes electricity possible. Electricity is a free flow of electrons.

Loosened electrons that do not move and have a negative charge are called static electricity. If these static electrons have an opportunity to jump to a conductor, this can lead to electrostatic discharge (ESD). Conductors will be discussed later in this module.

ESD is usually harmless to people. However, ESD can create serious problems for sensitive electronic equipment. A static discharge can randomly damage computer chips, data, or both. The logical circuitry of computer chips is extremely sensitive to ESD. Students should take safety precautions before they work inside computers, routers, and similar devices.

Atoms, or groups of atoms called molecules, can be referred to as materials. Materials are classified into three groups based on how easily free electrons flow through them.

The basis for all electronic devices is the knowledge of how insulators, conductors, and semiconductors control the flow of electrons and work together.

The Lab Activity reviews the proper way to handle a multimeter.

The next page introduces voltage.

Voltage
3.1.2 Voltage is sometimes referred to as electromotive force (EMF). EMF is related to an electrical force, or pressure, that occurs when electrons and protons are separated. The force that is created pushes toward the opposite charge and away from the like charge. This process occurs in a battery, where chemical action causes electrons to be freed from the negative terminal of the battery. The electrons then travel to the opposite, or positive, terminal through an external circuit. The electrons do not travel through the battery. Remember that the flow of electricity is really the flow of electrons. Voltage can also be created in three other ways. The first is by friction, or static electricity. The second way is by magnetism, or an electric generator. The last way that voltage can be created is by light, or a solar cell.


Voltage is represented by the letter V, and sometimes by the letter E, for electromotive force. The unit of measurement for voltage is volt (V). A volt is defined as the amount of work, per unit charge, that is needed to separate the charges.

The next page describes resistance and impedance.

This page explains the concepts of resistance and impedance.

Resistance and impedance
3.1.3 The materials through which current flows vary in their resistance to the movement of the electrons. The materials that offer very little or no resistance are called conductors. Those materials that do not allow the current to flow, or severely restrict its flow, are called insulators. The amount of resistance depends on the chemical composition of the materials.

All materials that conduct electricity have a measure of resistance to the flow of electrons through them. These materials also have other effects called capacitance and inductance that relate to the flow of electrons. Impedance includes resistance, capacitance, and inductance and is similar to the concept of resistance.

Attenuation is important in relation to networks. Attenuation refers to the resistance to the flow of electrons and explains why a signal becomes degraded as it travels along the conduit.

The letter R represents resistance. The unit of measurement for resistance is the ohm (Ω). The symbol comes from the Greek letter omega.

Electrical insulators are materials that are most resistant to the flow of electrons through them. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas. These materials have very stable chemical structures and the electrons are tightly bound within the atoms.

Electrical conductors are materials that allow electrons to flow through them easily. The outermost electrons are bound very loosely to the nucleus and are easily freed. At room temperature, these materials have a large number of free electrons that can provide conduction. The introduction of voltage causes the free electrons to move, which results in a current flow.

The periodic table categorizes some groups of atoms in the form of columns. The atoms in each column belong to particular chemical families. Although they may have different numbers of protons, neutrons, and electrons, their outermost electrons have similar orbits and interactions with other atoms and molecules. The best conductors are metals such as copper (Cu), silver (Ag), and gold (Au). These metals have electrons that are easily freed. Other conductors include solder, which is a mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an atom that has a different number of electrons than the number of protons in the nucleus. The human body is made of approximately 70 percent water with ions, which means that it is a conductor.

Semiconductors are materials that allow the amount of electricity they conduct to be precisely controlled. These materials are listed together in one column of the periodic chart. Examples include carbon (C), germanium (Ge), and the alloy gallium arsenide (GaAs). Silicon (Si) is the most important semiconductor because it makes the best microscopic-sized electronic circuits.

Silicon is very common and can be found in sand, glass, and many types of rocks. The region around San Jose, California is known as Silicon Valley because the computer industry, which depends on silicon microchips, started in that area.

The next page describes electrical current

Current

3.1.4 Electrical current is the flow of charges created when electrons move. In electrical circuits, the current is caused by a flow of free electrons. When voltage is applied and there is a path for the current, electrons move from the negative terminal along the path to the positive terminal. The negative terminal repels the electrons and the positive terminal attracts the electrons. The letter I represents current. The unit of measurement for current is Ampere (A). An ampere is defined as the number of charges per second that pass by a point along a path.

Current can be thought of as the amount or volume of electron traffic that flows. Voltage can be thought of as the speed of the electron traffic. The combination of amperage and voltage equals wattage. Electrical devices such as light bulbs, motors, and computer power supplies are rated in terms of watts. Wattage indicates how much power a device consumes or produces.

It is the current or amperage in an electrical circuit that really does the work. For example, static electricity has such a high voltage 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 permanent injury. 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.

The next page discusses circuits.

Module 3: Networking Media / Overview

Overview
Copper cable is used in almost every LAN. Many different types of copper cable are available. Each type has advantages and disadvantages. Proper selection of cabling is key to efficient network operation. Since copper uses electrical currents to transmit information, it is important to understand some basics of electricity.


Optical fiber is the most frequently used medium for the longer, high bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Optical media uses light to transmit data through thin glass or plastic fiber. Electrical signals cause a fiber-optic transmitter to generate the light signals sent down the fiber. The receiving host receives the light signals and converts them to electrical signals at the far end of the fiber. However, there is no electricity in the fiber-optic cable. In fact, the glass used in fiber-optic cable is a very good electrical insulator.

Physical connectivity allows users to share printers, servers, and software, which can increase productivity. Traditional networked systems require the workstations to remain stationary and permit moves only within the limits of the media and office area.

The introduction of wireless technology removes these restraints and brings true portability to computer networks. Currently, wireless technology does not provide the high-speed transfers, security, or uptime reliability of cabled networks. However, flexibility of wireless has justified the trade off.

Administrators often consider wireless when they install or upgrade a network. A simple wireless network could be working just a few minutes after the workstations are turned on. Connectivity to the Internet is provided through a wired connection, router, cable, or DSL modem and a wireless access point that acts as a hub for the wireless nodes. In a residential or small office environment these devices may be combined into a single unit.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.

Students who complete this module should be able to perform the following tasks:

• Discuss the electrical properties of matter
• Define voltage, resistance, impedance, current, and circuits
• Describe the specifications and performances of different types of cable
• Describe coaxial cable and its advantages and disadvantages compared to other types of cable
• Describe STP cable and its uses
• Describe UTP cable and its uses
• Discuss the characteristics of straight-through, crossover, and rollover cables and where each is used
• Explain the basics of fiber-optic cable
• Describe how fiber-optic cables can carry light signals over long distances
• Describe multimode and single-mode fiber
• Describe how fiber is installed
• Describe the type of connectors and equipment used with fiber-optic cable
• Explain how fiber is tested to ensure that it will function properly
• Discuss safety issues related to fiber optics