Networking Media - Module 3

                                                      Module Overview 

3.1 Copper Media

3.1.1 Atoms and electrons

3.1.2 Voltage

3.1.3 Resistance and impedance

3.1.4 Current

3.1.5 Circuits

3.1.6 Cable specifications

3.1.7 Coaxial cable

3.1.8 STP cable

3.1.9 UTP cable

3.2 Optical Media

3.2.1 The electromagnetic spectrum

3.2.2 Ray model of light

3.2.3 Reflection

3.2.4 Refraction

3.2.5 Total internal reflection

3.2.6 Multimode fiber

3.2.7 Single-mode fiber

3.2.8 Other optical components

3.2.9 Signals and noise in optical fibers

3.2.10 Installation, care, and testing of optical fiber

3.3 Wireless Media

3.3.1 Wireless LAN organizations and standards

3.3.2 Wireless devices and topologies

3.3.3 How wireless LANs communicate

3.3.4 Authentication and association

3.3.5 The radio wave and microwave spectrums

3.3.6 Signals and noise on a WLAN

3.3.7 Wireless security

 

 Module: Summary 

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

 

3.1  Copper Media 

3.1.1  Atoms and electrons 

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:

1. Electrons – Particles with a negative charge that orbit the nucleus
2. Protons – Particles with a positive charge
3. 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.

 3.1  Copper Media 

 3.1.2  Voltage 

 

This page discusses voltage.

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.

In the Lab Activity, students will measure voltage.

The next page describes resistance and impedance.

 

 

  3.1  Copper Media 

  3.1.3  Resistance and impedance 

This page explains the concepts of resistance and impedance.

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 (O). 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.

3.1  Copper Media 

3.1.4  Current 

This page provides a detailed explanation of current.

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.

3.1  Copper Media 

3.1.5  Circuits 

This page explains circuits.

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.

3.1  Copper Media 

3.1.6  Cable specifications 

 

This page discusses cable specifications and expectations.

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

1. 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.
2. Will the transmissions be digital or analog? Digital or baseband transmission and analog or broadband transmission require different types of cable. 
3. 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 Ethernet specification 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 Ethernet specification used in that network.

3.1  Copper Media 

3.1.7  Coaxial cable 

This page provides detailed information about coaxial cable.

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.

 3.1  Copper Media 

 3.1.8  STP cable 

This page provides detailed information about STP cable.

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.

3.1  Copper Media 

3.1.9  UTP cable 

This page provides detailed information about UTP cable.

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 5e is the cable most frequently recommended and implemented in installations. However, analyst predictions and independent polls indicate that Category 6 cable will supersede Category 5e 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 to have a slower data rate than other types of cable. This is no longer true. In fact, today, twisted pair is considered to have the fastest data rate of any 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.

The next three Lab Activities will provides hands-on experience with straight-through, rollover, and crossover cable construction.

In the final Lab Activity, students will research cable costs.

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

 3.2  Optical Media 

 3.2.1  The electromagnetic spectrum 

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.

3.2  Optical Media 

3.2.2  Ray model of light 

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.

3.2  Optical Media 

3.2.3  Reflection 

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 Interactive Media Activity demonstrates the laws of reflection.

The next page describes refraction.

3.2  Optical Media 

3.2.4  Refraction 

 

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 less than the index of refraction of the diamond.

3.2  Optical Media 

3.2.5  Total internal reflection 

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.

 3.2  Optical Media 

 3.2.6  Multimode fiber 

This page will introduce multimode fiber.

The part of an optical fiber through which light rays travel is called the core of the fiber.  Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. Likewise, once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called "multimode" fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. 

Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B.  The second fiber carries data from device B to device A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. Copper twisted-pair uses a wire pair to transmit and a wire pair to receive. Fiber-optic circuits use one fiber strand to transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are attached.

Until the connectors are attached, there is no need for shielding, because no light escapes when it is inside a fiber. This means there are no crosstalk issues with fiber. It is very common to see multiple fiber pairs encased in the same cable. This allows a single cable to be run between data closets, floors, or buildings. One cable can contain 2 to 48 or more separate fibers. With copper, one UTP cable would have to be pulled for each circuit. Fiber can carry many more bits per second and carry them farther than copper can.

Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.

The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass for its core. This glass has a lower index of refraction towards the outer edge of the core. Therefore, the outer area of the core is less optically dense than the center and light can go faster in the outer part of the core. This design is used because a light ray following a mode that goes straight down the center of the core does not have as far to travel as a ray following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together. Then the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse. 

Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. Standard multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1µ).

Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. There are two basic cable designs. They are the loose-tube and the tight-buffered cable designs.  Most of the fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. The most practical difference between the two designs is the applications for which they are used. Loose-tube cable is primarily used for outside-building installations, while tight-buffered cable is used inside buildings.

The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests.

The final element is the outer jacket. The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color.

Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs) are two types of light source usually used with multimode fiber. Use one or the other. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6,560 ft).

The next page describes single-mode fiber.

  3.2  Optical Media 

  3.2.7  Single-mode fiber 

This page will introduce single-mode fiber.

Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode fiber is usually yellow. The major difference between multimode and single-mode fiber is that single-mode allows only one mode of light to propagate through the smaller, fiber-optic core. The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of the single-mode fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter.

An infrared laser is used as the light source in single-mode fiber. The ray of light it generates enters the core at a 90-degree angle. As a result, the data carrying light ray pulses in single-mode fiber are essentially transmitted in a straight line right down the middle of the core.  This greatly increases both the speed and the distance that data can be transmitted.

Because of its design, single-mode fiber is capable of higher rates of data transmission (bandwidth) and greater cable run distances than multimode fiber. Single-mode fiber can carry LAN data up to 3000 meters. Although this distance is considered a standard, newer technologies have increased this distance and will be discussed in a later module. Multimode is only capable of carrying up to 2000 meters. Lasers and single-mode fibers are more expensive than LEDs and multimode fiber. Because of these characteristics, single-mode fiber is often used for inter-building connectivity.

Warming: The laser light used with single-mode has a longer wavelength than can be seen. The laser is so strong that it can seriously damage eyes. Never look at the near end of a fiber that is connected to a device at the far end. Never look into the transmit port on a NIC, switch, or router. Remember to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Be very careful.

Figure  compares the relative sizes of the core and cladding for both types of fiber optic in different sectional views. The much smaller and more refined fiber core in single-mode fiber is the reason single-mode has a higher bandwidth and cable run distance than multimode fiber. However, it entails more manufacturing costs.

The next page introduces some components that are used with optical fiber.

3.2  Optical Media 

3.2.8  Other optical components 

 

This page explains how optical devices are used to transmit data.

Most of the data sent over a LAN is in the form of electrical signals. However, optical fiber links use light to send data. Something is needed to convert the electricity to light and at the other end of the fiber convert the light back to electricity. This means that a transmitter and a receiver are required.

The transmitter receives data to be transmitted from switches and routers. This data is in the form of electrical signals. The transmitter converts the electronic signals into their equivalent light pulses. There are two types of light sources used to encode and transmit the data through the cable:

• A light emitting diode (LED) producing infrared light with wavelengths of either 850 nm or 1310 nm. These are used with multimode fiber in LANs. Lenses are used to focus the infrared light on the end of the fiber.

·         Light amplification by stimulated emission radiation (LASER) a light source producing a thin beam of intense infrared light usually with wavelengths of 1310nm or 1550 nm. Lasers are used with single-mode fiber over the longer distances involved in WANs or campus backbones. Extra care should be exercised to prevent eye injury.

Each of these light sources can be lighted and darkened very quickly to send data (1s and 0s) at a high number of bits per second.

At the other end of the optical fiber from the transmitter is the receiver. The receiver functions something like the photoelectric cell in a solar powered calculator. When light strikes the receiver, it produces electricity. The first job of the receiver is to detect a light pulse that arrives from the fiber. Then the receiver converts the light pulse back into the original electrical signal that first entered the transmitter at the far end of the fiber. Now the signal is again in the form of voltage changes. The signal is ready to be sent over copper wire into any receiving electronic device such as a computer, switch, or router. The semiconductor devices that are usually used as receivers with fiber-optic links are called p-intrinsic-n diodes (PIN photodiodes).

PIN photodiodes are manufactured to be sensitive to 850, 1310, or 1550 nm of light that are generated by the transmitter at the far end of the fiber. When struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces an electric current of the proper voltage for the network. It instantly stops producing the voltage when no light strikes the PIN photodiode. This generates the voltage changes that represent the data 1s and 0s on a copper cable.

Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC). On single-mode fiber, the Straight Tip (ST) connector is frequently used. 

In addition to the transmitters, receivers, connectors, and fibers that are always required on an optical network, repeaters and fiber patch panels are often seen.

Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings. The restored signals can then be sent on along the journey to the receiver at the far end of the fiber.

Fiber patch panels similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers with various available fiber runs, or cable links. 

The Lab Activity will teach students about the price of different types of fiber cables.

The next page will discuss data loss in optical fiber.

3.2  Optical Media 

3.2.9  Signals and noise in optical fibers 

 

This page explains some factors that reduce signal strength in optical media.

Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end. The cladding is covered by a buffer and an outer jacket that stops light from entering or leaving the cable.

Furthermore, the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber. This means that fiber does not have the problem with crosstalk that copper media does. In fact, the quality of fiber-optic links is so good that the recent standards for gigabit and ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and wide-area networks (WANs).

Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems. When light travels through fiber, some of the light energy is lost. The farther a light signal travels through a fiber, the more the signal loses strength. This attenuation of the signal is due to several factors involving the nature of fiber itself. The most important factor is scattering. The scattering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy.

Absorption is another cause of light energy loss. When a light ray strikes some types of chemical impurities in a fiber, the impurities absorb part of the energy. This light energy is converted to a small amount of heat energy. Absorption makes the light signal a little dimmer.

Another factor that causes attenuation of the light signal is manufacturing irregularities or roughness in the core-to-cladding boundary. Power is lost from the light signal because of the less than perfect total internal reflection in that rough area of the fiber. Any microscopic imperfections in the thickness or symmetry of the fiber will cut down on total internal reflection and the cladding will absorb some light energy.

Dispersion of a light flash also limits transmission distances on a fiber. Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber.

Graded index multimode fiber is designed to compensate for the different distances the various modes of light have to travel in the large diameter core. Single-mode fiber does not have the problem of multiple paths that the light signal can follow. However, chromatic dispersion is a characteristic of both multimode and single-mode fiber. When wavelengths of light travel at slightly different speeds through glass than do other wavelengths, chromatic dispersion is caused. That is why a prism separates the wavelengths of light. Ideally, an LED or Laser light source would emit light of just one frequency. Then chromatic dispersion would not be a problem.

Unfortunately, lasers, and especially LEDs generate a range of wavelengths so chromatic dispersion limits the distance that can be transmitted on a fiber. If a signal is transmitted too far, what started as a bright pulse of light energy will be spread out, separated, and dim when it reaches the receiver. The receiver will not be able to distinguish a one from a zero.

The next page will discuss the installation, care, and testing of optical fiber.

 3.2  Optical Media 

 3.2.10  Installation, care, and testing of optical fiber 

 

This page will teach students how to troubleshoot optical fiber.

A major cause of too much attenuation in fiber-optic cable is improper installation. If the fiber is stretched or curved too tightly, it can cause tiny cracks in the core that will scatter the light rays. Bending the fiber in too tight a curve can change the incident angle of light rays striking the core-to-cladding boundary. Then the incident angle of the ray will become less than the critical angle for total internal reflection. Instead of reflecting around the bend, some light rays will refract into the cladding and be lost. 

To prevent fiber bends that are too sharp, fiber is usually pulled through a type of installed pipe called interducting. The interducting is much stiffer than fiber and cannot be bent so sharply that the fiber inside the interducting has too tight a curve. The interducting protects the fiber, makes it easier to pull the fiber, and ensures that the bending radius (curve limit) of the fiber is not exceeded.

When the fiber has been pulled, the ends of the fiber must be cleaved (cut) and properly polished to ensure that the ends are smooth.  A microscope or test instrument with a built in magnifier is used to examine the end of the fiber and verify that it is properly polished and shaped. Then the connector is carefully attached to the fiber end. Improperly installed connectors, improper splices, or the splicing of two cables with different core sizes will dramatically reduce the strength of a light signal. 

Once the fiber-optic cable and connectors have been installed, the connectors and the ends of the fibers must be kept spotlessly clean. The ends of the fibers should be covered with protective covers to prevent damage to the fiber ends. When these covers are removed prior to connecting the fiber to a port on a switch or a router, the fiber ends must be cleaned. Clean the fiber ends with lint free lens tissue moistened with pure isopropyl alcohol. The fiber ports on a switch or router should also be kept covered when not in use and cleaned with lens tissue and isopropyl alcohol before a connection is made. Dirty ends on a fiber will cause a big drop in the amount of light that reaches the receiver.

Scattering, absorption, dispersion, improper installation, and dirty fiber ends diminish the strength of the light signal and are referred to as fiber noise. Before using a fiber-optic cable, it must be tested to ensure that enough light actually reaches the receiver for it to detect the zeros and ones in the signal.

When a fiber-optic link is being planned, the amount of signal power loss that can be tolerated must be calculated. This is referred to as the optical link loss budget. Imagine a monthly financial budget. After all of the expenses are subtracted from initial income, enough money must be left to get through the month.

The decibel (dB) is the unit used to measure the amount of power loss. It tells what percent of the power that leaves the transmitter actually enters the receiver.

Testing fiber links is extremely important and records of the results of these tests must be kept. Several types of fiber-optic test equipment are used. Two of the most important instruments are Optical Loss Meters and Optical Time Domain Reflectometers (OTDRs).

These meters both test optical cable to ensure that the cable meets the TIA standards for fiber. They also test to verify that the link power loss does not fall below the optical link loss budget. OTDRs can provide much additional detailed diagnostic information about a fiber link. They can be used to trouble shoot a link when problems occur.

This page concludes this lesson. The next lesson will discuss wireless media. The first page will discuss Wireless LAN organizations and standards.

 Web Links

 3.3  Wireless Media 

 3.3.1  Wireless LAN organizations and standards 

 

This page will introduce the regulations and standards that apply to wireless technology. These standards ensure that deployed networks are interoperable and in compliance.

Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC).

A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS). DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps. The next standard approved was 802.11b, which increased transmission capabilities to 11 Mbps. Even though DSSS WLANs were able to interoperate with the Frequency Hopping Spread Spectrum (FHSS) WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task was simply to create a standard that matched the manufacturer’s solution.

802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps. All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates for DSSS only. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points.

802.11b devices achieve the higher data throughput rate by using a different coding technique from 802.11, allowing for a greater amount of data to be transferred in the same time frame. The majority of 802.11b devices still fail to match the 11 Mbps bandwidth and generally function in the 2 to 4 Mbps range.

802.11a covers WLAN devices operating in the 5 GHZ transmission band. Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ. 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps. In production networks, a more standard rating is 20-26 Mbps.

802.11g provides the same bandwidth as 802.11a but with backwards compatibility for 802.11b devices using Orthogonal Frequency Division Multiplexing (OFDM) modulation technology and operating in the 2.4 GHZ transmission band. Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate.

The next page explains the devices and topologies used in wireless networks.

3.3  Wireless Media 

3.3.2  Wireless devices and topologies 

This page describes the devices and related topologies for a wireless network.

A wireless network may consist of as few as two devices.  -  The nodes could simply be desktop workstations or notebook computers. Equipped with wireless NICs, an ‘ad hoc’ network could be established which compares to a peer-to-peer wired network. Both devices act as servers and clients in this environment. Although it does provide connectivity, security is at a minimum along with throughput. Another problem with this type of network is compatibility. Many times NICs from different manufacturers are not compatible.

To solve the problem of compatibility, an access point (AP) is commonly installed to act as a central hub for the WLAN infrastructure mode.  The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell.  Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell could greatly vary. Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet). To service larger areas, multiple access points may be installed with a degree of overlap. The overlap permits "roaming" between cells.  This is very similar to the services provided by cellular phone companies. Overlap, on multiple AP networks, is critical to allow for movement of devices within the WLAN. Although not addressed in the IEEE standards, a 20-30% overlap is desirable. This rate of overlap will permit roaming between cells, allowing for the disconnect and reconnect activity to occur seamlessly without service interruption.

When a client is activated within the WLAN, it will start "listening" for a compatible device with which to "associate". This is referred to as "scanning" and may be active or passive.

Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response. The authentication and association steps are completed.

Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes.

The first Interactive Media Activity shows the levels of the OSI reference model and the related networking devices.

The second Interactive Media Activity shows the addition of a wireless hub to a wired network.

The next page explains how wireless LANs communicate.

3.3  Wireless Media 

3.3.3  How wireless LANs communicate 

This page explains the communication process of a WLAN.

After establishing connectivity to the WLAN, a node will pass frames in the same manner as on any other 802.x network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, management, and data.  Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500 bytes; however, an Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.

Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.

When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.

Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked. The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.

The next page explains authentication and association.

3.3  Wireless Media 

3.3.4  Authentication and association 

This page describes WLAN authentication and association.

WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. This is a critical point to remember when considering WLAN security, troubleshooting and overall management.

Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process.

Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.

Authentication and Association types

• Unauthenticated and unassociated
• The node is disconnected from the network and not associated to an access point.
• Authenticated and unassociated
• The node has been authenticated on the network but has not yet associated with the access point.
• Authenticated and associated
• The node is connected to the network and able to transmit and receive data through the access point.

Methods of authentication

IEEE 802.11 lists two types of authentication processes.

The first authentication process is the open system. This is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment although the ability of low level network ‘sniffers’ to discover the SSID of the WLAN is high.

The second process is the shared key. This process requires the use of Wired Equivalent Privacy (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof.

The problem of unauthorized entry into WLANs is being addressed by a number of new security solution technologies.

The next page explains radio waves and modulation.

3.3  Wireless Media 

3.3.5  The radio wave and microwave spectrums 

 

This page describes radio waves and modulation.

Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna.  However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air.

These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey.

Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals.

In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal.

The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter.

In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal.

A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal.

The first Interactive Media Activity explains electromagnetic fields and polarization.

The second Interactive Media Activity shows the names, devices, frequencies, and wavelengths of the EM spectrum.

The next page describes problems caused by signals and noise.

3.3  Wireless Media 

3.3.6  Signals and noise on a WLAN 

This page discusses how signals and noise can affect a WLAN.

On a wired Ethernet network, it is usually a simple process to diagnose the cause of interference. When using RF technology many kinds of interference must be taken into consideration.

Narrowband is the opposite of spread spectrum technology. As the name implies narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. Actually diagnosing the cause of narrowband interference can be a costly and time-consuming experience. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive.

All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering. In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.

Generally the RF signal will not be affected by even the most extreme weather conditions. However, fog or very high moisture conditions can and do affect wireless networks. Lightning can also charge the atmosphere and alter the path of a transmitted signal.

The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic dish antenna that concentrates the signal will increase the transmission range.

In a SOHO environment most access points will utilize twin omnidirectional antennae that transmit the signal in all directions thereby reducing the range of communication.

The next page describes WLANs security.

  3.3  Wireless Media 

  3.3.7  Wireless security 

 

This page will explain how wireless security can be achieved.

Where wireless networks exist there is little security. This has been a problem from the earliest days of WLANs. Currently, many administrators are weak in implementing effective security practices.

A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose. Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.

• EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest authentication type, which is very similar to CHAP password protection on a wired network.
• LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP keys, and supports mutual authentication.
• User authentication – Allows only authorized users to connect, send and receive data over the wireless network.
• Encryption – Provides encryption services further protecting the data from intruders.
• Data authentication – Ensures the integrity of the data, authenticating source and destination devices.

VPN technology effectively closes the wireless network since an unrestricted WLAN will automatically forward traffic between nodes that appear to be on the same wireless network. WLANs often extend outside the perimeter of the home or office in which they are installed and without security intruders may infiltrate the network with little effort. Conversely it takes minimal effort on the part of the network administrator to provide low-level security to the WLAN.

This page concludes the lesson. The next page will summarize the main points from the module.

 Summary

  

This page summarizes the topics discussed in this module.

Copper cable carries information using electrical current. The electrical specifications of a cable determines the kind of signal a particular cable can transmit, the speed at which the signal is transmitted and the distance the signal will travel.

An understanding of the following electrical concepts is helpful when working with computer networks:

• Voltage – the pressure that moves electrons through a circuit from one place to another
• Resistance – opposition to the flow of electrons and why a signal becomes degraded as it travels along the conduit
• Current – flow of charges created when electrons move
• Circuits – a closed loop through which an electrical current flows

Circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. A multimeter is used to measure voltage, current, resistance, and other electrical quantities expressed in numeric form.

Coaxial cable, unshielded twisted pair (UTP) and shielded twisted pair (STP) are types of copper cables that can be used in a network to provide different capabilities. Twisted-pair cable can be configured for straight through, crossover, or rollover signaling. These terms refer to the individual wire connections, or pinouts, from one end to the other end of the cable. A straight-through cable is used to connect unlike devices such as a switch and a PC. A crossover cable is used to connect similar devices such as two switches. A rollover cable is used to connect a PC to the console port of a router. Different pinouts are required because the transmit and receive pins are in different locations on each of these devices.

Optical fiber is the most frequently used medium for the longer, high-bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Light energy is used to transmit large amounts of data securely over relatively long distances The light signal carried by a fiber is produced by a transmitter that converts an electrical signal into a light signal. The receiver converts the light that arrives at the far end of the cable back to the original electrical signal.

Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive.

The part of an optical fiber through which light rays travel is called the core of the fiber. Surrounding the core is the cladding. Its function is to reflect the signal back towards the core. Surrounding the cladding is a buffer material that helps shield the core and cladding from damage. A strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar. The final element is the outer jacket that surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants.

The laws of reflection and refraction are used to design fiber media that guides the light waves through the fiber with minimum energy and signal loss. Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Because of its design, single-mode fiber is capable of higher rates of data transmission and greater cable run distances than multimode fiber.

Fiber is described as immune to noise because it is not affected by external noise or noise from other cables. Light confined in one fiber has no way of inducing light in another fiber. Attenuation of a light signal becomes a problem over long cables especially if sections of cable are connected at patch panels or spliced.

Both copper and fiber media require that devices remains stationary permitting moves only within the limits of the media. Wireless technology removes these restraints. Understanding the regulations and standards that apply to wireless technology will ensure that deployed networks will be interoperable and in compliance with IEEE 802.11 standards for WLANs.

A wireless network may consist of as few as two devices. The wireless equivalent of a peer-to-peer network where end-user devices connect directly is referred to as an ad-hoc wireless topology. To solve compatibility problems among devices, an infrastructure mode topology can be set up using an access point (AP) to act as a central hub for the WLAN. Wireless communication uses three types of frames: control, management, and data frames. To avoid collisions on the shared radio frequency media WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

WLAN authentication is a Layer 2 process that authenticates the device, not the user. Association, performed after authentication, permits a client to use the services of the access point to transfer data.