Curs PC si internet cap 4 Basics of Electricity

Overview The function of the physical layer is to transmit data by defining the electrical specifications between the source and destination. Once it reaches a building, electricity is carried to workstations, servers, and network devices via wires concealed in walls, floors, and ceilings. Data, which can consist of such things as text, pictures, audio, or video, travels through the wires and is represented by the presence of either electrical pulses on copper conducting wires or light pulses in optical fibers. In this chapter, you will learn about the basic theory of electricity. This will provide a foundation upon which networking at the physical layer of the OSI model can be understood. You will also lean how data is transmitted through physical media, such as cables and connectors. Lastly, you will learn about the different factors that affect data transmission such as, alternating current (AC) power line noise. 4.1 Basics of Electricity 4.1.1 A helium atom Instructor Note Why teach electronics in a networking class? Simply put, most of the devices and processes involved in networking are electronic. While it is impossible to properly teach electronics in one chapter, we believe it essential to introduce some basic vocabulary. For some students, covering electronics will engage their prior experiences; for others, it may provide a point of focus for their future networking studies [many part-time jobs for younger students involve cable installation]. Any discussion of copper-based media, for example, must involve some discussion of the media's electrical properties.. By performing some electronics labs, the students have an enjoyable, hands-on, effective way of learning such basic concepts as continuity and circuits. Anyone installing cable must have an awareness of conducting paths, short circuits, and open circuits. Also, much of Layer 1 and Layer 2 assumes a knowledge of electronics, Finally, the extensive use of frame, packet, and segment format diagrams (where the fields of these PDUs are discussed) is based, on a fundamental level, on the voltage versus time diagrams of an oscilloscope or logic analyzer displaying bit patterns. We view the CNAP as educating the future networking professionals of the world, not simply training them to pass a given test. We believe an educated networking professional understands some basic electronics [for example, most of the landmark texts in networking - for example, Computer Networks (by Tannenbaum) and Computer Networks (by Peterson and Davie) -- have substantial discussions of networking signals.There are two specific purposes to this target indicator. First, to relate what the student is about to learn (electrical signals) to prior knowledge from science classes. Secondly, to establish the foundation for understanding that electricity is comprised of electrons. All matter is composed of atoms. "The Periodic Table of Elements" , lists all known types of atoms and their properties. The names of the parts of the atom are: nucleus - the center part of the atom, formed by protons and neutrons protons - particles have a positive charge , and along with neutrons, form the nucleus neutrons - particles have no charge (neutral), and along with protons, form the nucleus electrons - particles have a negative charge, and orbit the nucleus To help you understand the electrical properties of elements/materials, locate "helium" on the periodic chart. It has an atomic number of 2, which means that it has 2 protons and 2 electrons. It has an atomic weight of 4. By subtracting the atomic number (2) from the atomic weight (4) you learn that helium also has 2 neutrons . Example:Atomic number of helium = 2 2 protons+ 2 electrons 4 = atomic weight- 2 = atomic number 2 = neutronsThe Danish physicist, Niels Bohr, developed a simplified model to illustrate atoms. This illustration shows the model for a helium atom. Notice the scale of the parts. If the protons and neutrons of this atom were the size of a soccer ball, in the middle of a soccer field, the only thing smaller than the ball would be the electrons. They would be the size of cherries, and would be orbiting near the outer-most seats of the stadium. The only thing larger would be the space inside the atom, which would be the size of the soccer field. Web Links Helium 4.1 Basics of Electricity 4.1.2 Creating stable atoms Instructor Note The purpose of this target indicator is to make plausible the notion that it is the electrons which can "come loose" from atoms, thus explaining electrical conduction in solids. Of course, an entire unit could be taught on this topic, but the basic notion of electrical forces is what is essential. Again, if this relates to prior learning, the students will be ahead of the game. One of the laws of nature, called 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. A force is a pushing or pulling motion. In the case of opposite and like charges, the force increases as the charges move closer to each other.Examine Bohr's model of the helium atom. If Coulomb's law is true, and if Bohr's model describes helium atoms as stable, then there must be other laws of nature at work. How can they both be true?Coulomb's Law - Opposite charges attract. Bohr's model - Protons are positive charges, and electrons are negative charges. Question 1: Why don't the electrons fly in towards the protons?Coulomb's Law - Like charges repel. Bohr's model - Protons are positive charges. There is more than 1 proton in the nucleus. Question 2: Why don't the protons fly away from each other?The answers to these questions is that there are other laws of nature that must be considered. Following are the answers to each of the above questions.Answer 1: The electrons stay in orbit, even though they are attracted by the protons, because they have just enough velocity to keep orbiting, just like the moon around the Earth, and to not let themselves be pulled into the nucleus.Answer 2: The 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.The protons and neutrons are bound together by a very powerful force; however, the electrons are bound to their orbit around the nucleus by a weaker force. Electrons in certain atoms can be pulled free from the atom, and made to flow. This is electricity - a "free flow of electrons". 4.1 Basics of Electricity 4.1.3 Static electricity Instructor Note There are two purposes to this target indicator. First, to introduce flowing electrons (currents) it makes sense to first consider electrons at rest, and then discuss what might cause them to move. An entirely different reason is to introduce the notion of electrostatic discharge (ESD), which can damage networking devices. Demonstrate to the students the proper handling of printed circuit boards so as to avoid ESD problems -- especially handling the cards at the non-conducting edges, and also the use of an ESD mat and grounding strip as a standard part of electronics assembly. If you have access to a Van de Graff Generator, there are many static electricity demonstrations that can be performed with great drama in the classroom. If you lack a Van de Graff, a balloon works well on fairly dry days. Take the balloon, rub it furiously, and then it can be made to stick to various objects (like a white board or the teacher). Another demonstration can be done using a rubbed comb to pick up small pieces of paper. The goal is to make plausible the idea of charged particles and electrons. Loosened electrons that stay in one place, without moving and with 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). Electrostatic discharge, though usually harmless to people, can create serious problems for sensitive electronic equipment, unless dealt with properly.If you should walk across a carpet, in a cool and dry room, a spark could jump from your fingertips to the next object that you touch. This would cause you to feel a small electric shock. You know from experience that an electrostatic discharge can be uncomfortable, but is quite harmless. However, when a computer experiences an ESD, the result can be disastrous. A static discharge can randomly damage computer chips and/or data. Web Links "Static Electricity" means "High Voltage" 4.1 Basics of Electricity 4.1.4 Electrical current including insulators, conductors, and semiconductors Instructor Note The purpose of this target indicator is that students realize that the networking devices and components with which they will be working are based on very precise control of electron flow using a combination of conductors (usually copper conducting paths), semiconductors (usually Silicon-based Integrated Circuits), and insulators (usually plastic or rubber to form plugs, connectors, cable jackets). There are many demonstrations possible with a digital multimeter. You can show that the electrical resistance of objects gives us a measure of whether a material is a conductor (low resistance), semiconductor (moderate resistance), or insulator (high resistance). You could demonstrate a low voltage series circuit with a 6 V lantern battery, a low voltage light bulb, a pencil, some plastic, some alligator clip leads, and a piece of copper. Make a complete circuit with the copper piece and the bulb burns brightly. Make a complete circuit with a short piece of pencil graphite and the bulb burns dimly. Make a complete circuit with a piece of plastic and the bulb does not glow. Atoms, or groups of atoms called molecules, can be referred to as materials. Materials are classified as belonging to one of three groups, depending on how easily electricity, or free electrons, flows through them. Electrical Insulators Electrical insulators, or insulators, are materials that allow electrons to flow through them with great difficulty, or not at all. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas. These materials have very stable chemical structures, with orbiting electrons tightly bound within the atoms. Electrical ConductorsElectrical conductors, or conductors, are materials that allow electrons to flow through them with great ease. They flow easily because 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, causing a current to flow. The periodic table categorizes some groups of atoms by listing them 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 behave similarly, when interacting with other atoms and molecules. The best conductors are metals, such as copper (Cu), silver (Ag), and gold (Au). All of these metals are located in one column of the periodic chart, and have electrons that are easily freed, making them excellent materials for carrying a current.Other conductors include solder (a mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an atom that has more electrons, or fewer electrons, than a neutral atom. The human body is made of approximately 70% water with ions, which means that it, too, is a conductor. Electrical SemiconductorsSemiconductors are materials where the amount of electricity they conduct can 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). The most important semiconductor, the one that makes the best microscopic-sized electronic circuits, is silicon (Si). 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.Whether materials are classified as insulators, conductors, or semiconductors, it is the knowledge of how each one controls the flow of electrons, and of how they work together in various combinations, that is the basis for all electronic devices. Web Links Forms of Energy 4.1 Basics of Electricity 4.1.5 Electrical measurement terms Instructor Note The purpose of this target indicator is to help the student start refining their use of electronics terms. Have students make a chart and complete it. These are the terms that describe networking media.VoltageVoltage, sometimes referred to as electromotive force (EMF), is 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 battery's negative terminal, and to travel to the opposite, or positive, terminal. The separation of charges results in voltage. Voltage can also be created by friction (static electricity), by magnetism (electric generator), or by light (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), and is defined as the amount of work, per unit charge, needed to separate the charges.CurrentElectrical current, or current, is the flow of charges that is created when electrons move. In electrical circuits, current is caused by a flow of free electrons. When voltage (electrical pressure) is applied, and there is a path for the current, electrons move from the negative terminal (which repels them), along the path, to the positive terminal (which attracts them).Current is represented by the letter "I". The unit of measurement for current is ampere (amp), and is defined as the number of charges per second that pass by a point along a path.ResistanceMaterials through which current flows, offer varying amounts of opposition, or resistance, to the movement of the electrons. Materials that offer very little, or no, resistance, are called conductors. Those 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.Resistance is represented by the letter "R". The unit of measurement for resistance is the ohm (W ). The symbol comes from the Greek capital letter "W" - omega.Alternating Current (AC)This is one of the two ways in which current flows. Alternating current (AC) and voltages vary with time, by changing their polarity, or direction. AC flows in one direction, then reverses its direction, and repeats the process. AC voltage is positive at one terminal, and negative at the other, then it reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously.Direct Current (DC)This is the other one, of the two ways, in which current flows. Direct current (DC) always flow 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.ImpedanceImpedance is the total opposition to current flow (due to AC and DC voltages). The term resistance is generally used when referring to DC voltages. Impedance is the general term, and is the measure of how the flow of electrons is resisted, or impeded.Impedance is represented by the letter "Z". Its unit of measurement, like that for resistance, is the ohm (W ).Voltage, Current, Resistance RelationshipCurrents only flow in closed loops called circuits. These circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. Knowing these facts allows people to control a flow of current. GroundThe term ground can be difficult concept to understand, completely, because people use the term for many different purposes. Ground can refer to the place on the earth that touches your house (probably via the buried water pipes), eventually making an indirect connection to your electric outlets. When you use an electric appliance that has a plug with three prongs, the third prong is the ground. It gives the electrons an extra conducting path to flow to the earth, rather than through your body. Ground can also mean the reference point, or the 0 volts level, when making electrical measurements. Voltage is created by the separation of charges, which means that voltage measurements must be made between two points. A multimeter (which measures voltage, current, and resistance) has two wires for that reason. The black wire is referred to as the ground, or reference ground. A negative terminal on a battery is also referred to as 0 volts, or reference ground. Note: A multimeter is test equipment used for measuring voltage, current, resistance, and possibly other electrical quantities and displaying the value in numeric form. Web Links Electricity Fundamentals 4.1 Basics of Electricity 4.1.6 Analogy for voltage, resistance, and current Instructor Note The purpose of this target indicator is to give the students a powerful analogy for thinking about electric currents -- the voltages that cause currents, and the resistance that controls currents. Using a 2-Liter bottle with a stopped hole in the bottom, a funnel to collect the water and a siphon-pump to push the water back into the top of the 2 liter bottle, you can demonstrate how the water level and the openness of the tap effect the flow of water. The water analogy helps to explain concepts of electricity. The higher the water, and the greater the pressure, the more the water will flow. The water current depends on how much the tap (valve) has been opened. Similarly, the higher the voltage, and the greater the electrical pressure, the more current will be produced. The electric current then encounters resistance which like the water tap reduces the flow. If it is on an AC circuit, then the amount of current will depend on how much impedance (resistance) is present. The pump is like a battery. It provides pressure to keep the flow moving. 4.1 Basics of Electricity 4.1.7 Graphing AC and DC voltage Instructor Note The purpose of this target indicator is to have students attain a deep understanding of voltage versus time diagrams. The importance of this target indicator cannot be overstated. Virtually every concept in networking -- from bit streams, to frame format diagrams, to packet and fragment diagrams, to segments -- is based, ultimately, on how digital signals vary with time. Electrical waves, pulses and signals, unlike water waves or pulses on a string -- cannot be seen or felt directly; measuring devices such as oscilloscopes and logic analyzers are our eyes. Emphasize the labeling of axes any time a graph is drawn. A particularly poignant analogy will be the EKG machines; most students will have seen them on TV. Graph a heartbeat -- it's a voltage pulse that varies with time in the heart pulse. Similarly the brain waves and in general nervous system is electrical, so the medical analogy may help. Particularly relevant technologies similar to the oscilloscope are the television picture tube and the computer monitor, both of which are cathode ray tubes like the oscilloscope. Describe how both of these devices "draw" a picture with a beam that scans horizontally and is varied vertically.If you have access to oscilloscope and function generators, spend a class studying sine and square waves. Even if you have one oscilloscope, demonstrate it to the class. Use a microphone as the scope input and let the students see how their voice makes a voltage versus time graph on the oscilloscope display.If you do not have access to any oscilloscopes, you can make a sand pendulum which gives the basic idea. Suspend a styrofoam or paper cup from both sides and hang it like a pendulum. Make a small hole in the bottom of the cup and fill it with sand. As the cup-pendulum swings back and forth, it will trace out a straight line on a piece of paper. Now slowly move the paper along perpendicular to the direction of the swinging pendulum and you will trace out a beautiful sine wave. An oscilloscope is an important, and sophisticated electronic device used to study electrical signals. Because it is possible to control electricity precisely, deliberate electrical patterns called waves can be created. An oscilloscope graphs the electrical waves, pulses, and patterns. It 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.Electricity is brought to your home, school, and office by power lines. The power lines carry electricity in the form of alternating current (AC). Another type of current, called direct current (DC) can be found in flashlight batteries, car batteries, and as power for the microchips on the motherboard of a computer. It is important to understand the difference between these two types of current flow. 4.1 Basics of Electricity 4.1.8 Constructing a simple series electrical current Instructor Note The purpose of this target indicator is for students to construct, with their hands and in their mind, a simple series electric circuit. Throughout networking there are references to ground loop circuit, circuit versus packet switching, virtual circuits, in addition to all of the real circuits formed by networking media and networking devices. Of course, a thorough understanding of all of the circuitry involved in networking would require an undergraduate degree in electrical engineering; but here our goals are humble: students attaining the idea of a simple series circuit. At least they will have this idea to build on when testing for layer 1 connectivity, or upper layer protocols and their real and virtual connections. Electrons flow only in circuits that are closed, or complete, loops. The diagram in the main graphic shows a simple circuit, typical of a lantern-style flashlight. The chemical processes in the battery cause charges to be separated, which provides a voltage, or electrical pressure, enabling electrons to flow through various devices. The lines represent a conductor, usually, copper wire. You can think of a switch as two ends of a single wire that can be opened (or broken), and then closed (also known as fixed or shorted), to prevent or to allow, electrons to flow. Finally, the bulb provides resistance to the flow of electrons, causing the electrons to release energy in the form of light. The circuits involved in networking use the same concepts as this very simple circuit, but are much more complex. 4.1 Basics of Electricity 4.1.9 Purpose of grounding networking equipment Instructor Note The purpose of this target indicator is to emphasize the importance of grounding for networks. The concept of ground is difficult for the beginning electronics student, and worthy of some review. Of particular importance are signal reference ground (the zero Volts line, or reference line, for determining the polarity of our networking signals) and power-line earth ground. Differences between signal reference ground and earth ground, or between the power-line grounding of two different networking devices, can cause noise problems as well as dangerous shock conditions. Grounding connections and practices vary around the world; feel free to teach whatever standards apply to you locally.After covering this target indicator, a quiz game - such as "Jeopardy" -- might be in order. The Categories could be Voltage versus time graphs; electronic materials; electric circuits; the water analogy; and grounding. Students can select a category and a level of difficulty and attempt to answer your review questions, earning "points" for themselves and their team. 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. Generally speaking, electrical current follows the path of least resistance. Because metals such as copper provide little resistance, they are frequently used as conductors for electrical current. Conversely, materials such as glass, rubber, and plastic provide more resistance. Therefore, they do not make good electrical conductors. Instead, these materials are frequently used as insulators. They are used on conductors to prevent shock, fires, and short circuits. Electrical power is usually delivered to a pole-mounted transformer. The transformer reduces the high voltages, used in the transmission, to the 120 or 240 volts used by typical consumer electrical appliances. Figure shows a familiar object, electricity as supplied through wall outlets in the US (other nations have different wall outlet configurations). The top two connectors supply power. The round connector on the bottom protects people and equipment from shocks and short circuits. This connector is called the safety ground connection. In electrical equipment where this is used, the safety ground wire is connected to any exposed metal part of the equipment. The motherboards and computing circuits in computing equipment are electrically connected to the chassis. This also connects them to the safety grounding wire, which is used to dissipate static electricity.The purpose of connecting the safety ground to exposed metal parts of the computing equipment is to prevent such metal parts from becoming energized with a hazardous voltage resulting from a wiring fault inside the device.An accidental connection between the hot wire and the chassis is an example of a wiring fault that could occur in a network device. If such a fault were to occur, the safety ground wire connected to the device would serve as a low resistance path to the earth ground. The safety ground connection provides a lower resistance path than your body. When properly installed, the low resistance path, provided by the safety ground wire, offers sufficiently low resistance and current carrying capacity to prevent the build up of hazardously high voltages. The circuit links directly to the hot connection to the earth. Whenever an electrical current is passed through this path into the ground, it causes protective devices such as circuit breakers and Ground Fault Circuit Interrupters (GFCIs) to activate. By interrupting the circuit, circuit breakers and GFCIs stop the flow of electrons, and reduce the hazard of electrical shock. The circuit breakers protect you and your house wiring, but further protection - often in the form of surge suppressors and Uninterrupted Power Supplies (UPS) - are required to protect computing and networking equipment. 4.2 Basics of Digital Multimeters 4.2.1 Safe handling and use of the multimeter Instructor Note The purpose of this target indicator - a lab activity -- is to orient the students to the multimeters. The multimeter will give the student the perfect opportunity to use newly acquired electrical vocabulary, while making interesting and network-relevant measurements. In this lab you will learn how to use a multimeter. The multimeter can perform voltage, resistance, and continuity measurements, which are important in networking. You can learn about the multimeter from two different sources - the hard copy (paper) manual, and the online (manufacturer's Web site) version of the manual. Lab Activity In this lab, you will learn how to use or handle a multimeter correctly. Web Links Using a Multimeter 4.2 Basics of Digital Multimeters 4.2.2 Using a multimeter to make resistance measurements Instructor Note There are two purposes for this target indicator - a lab activity - is to study the electrical properties of materials by measuring their resistance, and to study the conductive properties of passive network components (cables, jacks, connectors) using the continuity measurement of the multimeter. In this lab you will use a multimeter to measure the resistance and continuity of objects. The unit of measurement for both, is the ohm (W ). Continuity refers to the level of resistance of a path. If a path is intentionally made into a low-resistance path, for use by two connected electrical devices, then the path has what is called continuity. If a path is unintentionally made into a low-resistance path, then it is called a short circuit. With either measurement, the multimeter emits a high-pitched sound when it detects a low-resistance path. You will perform measurements on the following: CAT 5 cable terminated CAT 5 cable terminated coaxial cable telephone wire CAT 5 jacks switches wall outlets Lab Activity In this lab, you will demonstrate your ability to measure resistance and continuity with the multimeter. Web Links Using a Multimeter 4.2 Basics of Digital Multimeters 4.2.3 Using a multimeter to make voltage measurements Instructor Note The purpose of this target indicator - a lab activity -- is to study voltage sources. Students should be encouraged to respect line voltage, since it cannot be seen but only detected using proper measuring instruments. If you have any classroom management difficulties, do NOT allow the student to do the wall socket measurement. Cisco and Fluke deny any responsibility for improperly used multimeters. If you have any doubts as to whether students can safely and maturely use the meters to measure the wall socket voltage, simply skip that measurement or demonstrate it to the entire class. A variety of batteries will provide DC voltages to measure and a variety of generators will generate pulsing DC or AC. Even an electric motor spun by hand will generate a measurable voltage on the multimeter, and if they quickly change the direction of spinning, the polarity will flip -- thus producing low voltage AC. In this lab you will use the multimeter to measure voltage. There are two types of voltage measurements. For your personal safety, and to protect the meter, it is important that you understand the difference. The two types are DC and AC. DC VoltageThe meter must be set to DC when measuring DC voltages. This includes the following:batteries outputs of computer power supplies solar cells DC generators AC VoltageThe meter must be set to AC when measuring AC voltages. If you are measuring a wall socket, you must assume that line voltage is present. Line voltage is 120 V AC in the US, and 220 V AC in most other places around the world. Line voltage can kill you! You must remember to be very careful to use the correct setting on the multimeter. Lab Activity In this lab, you will demonstrate your ability to measure voltage SAFELY with the multimeter. Web Links Using a Multimeter 4.2 Basics of Digital Multimeters 4.2.5 Constructing a simple electrical communication system Instructor Note The purpose of this target indicator - a lab activity -- is three-fold: to consolidate students knowledge of conducting paths, circuits, and Cat 5 media; to raise issues that any data communications system will face; and to have fun while learning. First, students will have been using electrical vocabulary, developing their multimeter skills, and getting familiar with CAT 5 UTP. This lab requires them to build several series circuits using the Cat 5 medium.Secondly, the students will run into issues from all 7 layers. They must decide what ideas, or range of ideas, they want to communicate and what "network" services are required (Layer 7). They must decide on a form of data representation, for example ASCII or Morse code (Layer 6). The must decide on how to open and close sessions (layer 5). The must decide upon a window size and whether data will be delivered reliably (with acknowledgements and re-transmissions) or unreliably. Since this is a point to point link, they need not worry about layer 3 addressing, but if they had multiple stations then they might consider address information. They definitely have to decide upon a frame format (Layer 2) and of course decide the signal and media specifications (Layer 1) which will govern their communications link. Feel free to adapt the basic lab ideas to your students needs and interests. The diagram shows part of the circuits that allow Ethernet NICs to communicate with each other. This should give you a hint on how to approach your challenge in this lab, which is to design, build, and demonstrate, a simple electrical communication system. Lab Activity In this lab, you will design, build, and test a simple, complete, fast, and reliable communication system, using common materials. 4.3 Basics of Signals and Noise in Communications Systems 4.3.2 Using digital signals to build analog signals Instructor Note The purpose of this target indicator is to show Fourier synthesis -- how square waves can be built for sine waves. Since the interaction of waves is a well-understood phenomenon (waves propagate, attenuate, reflect, disperse, and collide), knowing how one wave behaves can inform our understanding of square pulses made of waves. Jean Baptiste Fourier is responsible for one of the greatest mathematical discoveries. He proved that a special sum of sine waves, of harmonically related frequencies, which are multiples of some basic frequency, could be added together to create any wave pattern. This is how voice recognition devices and heart pacemakers work. Complex waves can be built out of simple waves. A square wave, or a square pulse, can be built by using the right combination of sine waves. The main graphic shows how the square wave (digital signal) can be built with sine waves (analog signals). This is important to remember as you examine what happens to a digital pulse as it travels along networking media. 4.3 Basics of Signals and Noise in Communications Systems 4.3.3 Representing one bit on a physical medium Instructor Note The purpose of this target indicator is to show one bit encoded as a voltage on a copper-based medium. While networks send many bits, understanding what happens to even one single bit is crucial to an understanding of networks -- every packet is ultimately a sequence of bits, all of which undergo a range of phenomena. If you have access to a dual-trace oscilloscope, you can display a square pulse put onto coaxial (or Cat 5) cable. Data networks have become increasingly dependent on digital (binary, two-state) systems. The basic building block of information is 1 binary digit, known as the bit or pulse. One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. This may be as simple as 0 volts for binary 0, and +5 volts for binary 1, or a more complex encoding. Signal reference ground is an important concept relating to all networking media that use voltages to carry messages. In order to function correctly, a signal reference ground must be close to a computer's digital circuits. Engineers have accomplished this by designing ground planes into circuit boards. The computer cabinets are used as the common point of connection for the circuit board ground planes to establish the signal reference ground. Signal reference ground establishes the 0 volts line in the signal graphics.With optical signals, binary 0 would be encoded as a low-light, or no-light, intensity (darkness). Binary 1 would be encoded as a higher-light intensity (brightness), or other more complex patterns. With wireless signals, binary 0 might be a short burst of waves; binary 1 might be a longer burst of waves, or another more complex pattern.You will examine six things that can happen to 1 bit: propagation attenuation reflection noise timing problem collisions 4.3 Basics of Signals and Noise in Communications Systems 4.3.4 Network signal propagation Instructor Note The purpose of this target indicator is to show that electrical signals take time to travel; nothing happens instantaneously. Especially as networks increase in speed to Megabits and Gigabits per second, the fundamental time units are microseconds and nanoseconds. Thus even "small" time delays or distortions become fundamental to the functioning of the network. If you have access to a slinky, induce a wave and time its propagation.Using sound waves down a long hall, you can notice a slight delay in the time it takes the sound to travel the length of the hall. Propagation means travel. When a NIC puts out a voltage or light pulse onto a physical medium, that square pulse made up of waves travels along the medium (propagates). Propagation means that a lump of energy, representing 1 bit, travels from one place to another. The speed at which it propagates depends on the actual material used in the medium, the geometry (structure) of the medium, and the frequency of the pulses. The time it takes the bit to travel from one end of the medium and back again is referred to as the round trip time, (RTT). Assuming no other delays, the time it takes the bit to travel down the medium to the far end is RTT/2. The fact that the bit takes a small amount of time to travel along the medium does not normally cause network problems. However, with the ever-increasing data transmission rates of today's networks sometimes you must account for the amount of time it takes the signal to travel. There are two extreme situations to consider. Either the bit takes "0" time to travel, meaning it travels instantaneously, or it takes "forever" to travel. The first case is wrong according to Albert Einstein, whose "Theory of Relativity" says no information can travel faster than the speed of light in a vacuum. This means that the bit takes at least a small amount of time to travel. The second case is also wrong, because with the right equipment, you can actually time the pulse. Lack of knowledge of propagation time is a problem, because you might assume the bit arrives at some destination either too soon, or too late. If the propagation time is too long, you should re-evaluate how the rest of the network will deal with this delay. If the propagation delay is too short, you may have to slow down the bits, or save them temporarily (known as buffering), so that the rest of the networking equipment can catch up with the bit. 4.3 Basics of Signals and Noise in Communications Systems 4.3.5 Network attenuation Instructor Note The purpose of this target indicator is to make the idea of attenuation of signals plausible. Using sound waves down a long hall, you can show how sound signals are attenuated.If you have access to a dual trace oscilloscope and function generator, measure the signal at two points along the long cable and you can see the attenuation. Attenuation is the loss of signal strength, for example, when cables exceed a maximum length. This means that a 1 bit voltage signal loses amplitude as energy passes from the signal to the cable. While choosing materials carefully, (e.g. using copper instead of carbon), and geometry (the shape and positioning of the wires) can reduce electrical attenuation, some loss is always unavoidable when electrical resistance is present. Attenuation also happens to optical signals; the optical fiber absorbs and scatters some of the light energy as the light pulse, 1 bit, travels down the fiber. This can be minimized by the wavelength, or color, of the light that you choose. This can also be minimized by whether or not you use single mode or multi-mode fiber, and by the actual glass that is used for the fiber. Even with these choices, some signal loss is unavoidable. Attenuation also happens to radio waves and microwaves, as they are absorbed and scattered by specific molecules in the atmosphere. Attenuation can effect a network since it limits the length of network cabling over which you can send a message. If the cable is too long or too attenuating, 1 bit sent from the source can look like a 0 bit by the time it gets to the destination.You can resolve this problem through the networking media that you choose, and by choosing structures that are designed to have low amounts of attenuation. One way to fix the problem is to change the medium. A second way is to use a repeater after a certain distance. There are repeaters for electrical, optical, and wireless bits. 4.3 Basics of Signals and Noise in Communications Systems 4.3.6 Network reflection Instructor Note The purpose of this target indicator is to make the idea of wave and pulse reflection plausible to students. If you have access to a dual trace oscilloscope and function generator, measure the signal at two points along the long cable and you can see the reflection of both an open and a short circuit at the end of the coaxial cable or twisted pair.If you have access to a slinky, you can send a longitudinal wave down the slinky and watch as some of it reflects.If you have access to a rope, you can send a pulse down the rope and see a reflection off a the fixed end of the rope. To understand reflection, imagine having a slinky or a jump rope stretched out with a friend holding the other end. Now, imagine sending them a pulse or a 1 bit message. If you watch carefully, you will see that a small wave (pulse) returns (reflects) to you. Reflection occurs in electrical signals. When voltage pulses, or bits, hit a discontinuity some energy can be reflected. If not carefully controlled, this energy can interfere with later bits. Remember, while you are focused on only 1 bit at a time right now, in real networks you will want to send millions and billions of bits every second, thus requiring you to keep track of this reflected pulse energy. Depending on the cabling and connections that the network uses, reflections may or may not be a problem. Reflection also occurs with optical signals. Optical signals reflect whenever they hit a discontinuity in the glass fiber, such as when a connector is plugged into a device. You can see this effect at night if you look out a window. You can see your reflection in the window even though the window is not a mirror. Some of the light that is reflected off your body reflects in the window. This also happens with radio waves and microwaves as they encounter different layers in the atmosphere. This may cause problems on your network. For optimal network performance, it is important that the network media have a specific impedance in order to match the electrical components in the NICs. Unless the network media have the correct impedance, the signal will suffer some reflection and interference will be created. Then multiple reflecting pulses can occur. Whether the system is electrical, optical, or wireless, impedance mismatches cause reflections. If enough energy is reflected, the binary, two-state system can become confused by all the extra energy bouncing around. You can resolve this by ensuring that all networking components are carefully impedance matched. 4.3 Basics of Signals and Noise in Communications Systems 4.3.7 Noise Instructor Note The purpose of this target indicator is to make the idea of electrical noise plausible to students. If you have access to a dual-trace oscilloscope and function generator, you can run the cable near some notorious noise source (electric motors, fluorescent lighting, power cables) and see what noise the signal acquires.If you rub a nail across a file near an AM radio, you can "hear" electromagnetic interference. Noise is unwanted additions to voltage, optical, or electromagnetic signals. No electrical signal is without noise, however, it is important to keep the signal-to-noise (S/N) ratio as high as possible. In other words, each bit receives additional unwanted signals from various sources. Too much noise can corrupt a bit turning a binary 1 into a binary 0, or a 0 into a 1, destroying the message. Figure shows five sources of noise that can affect a bit on a wire.NEXT-A and NEXT-B When electrical noise on the cable originates from signals on other wires in the cable, this is known as crosstalk. NEXT stands for near-end crosstalk. When two wires are near each other and untwisted, energy from one wire can wind up in an adjacent wire and vice versa. This can cause noise at both ends of a terminated cable. There are actually many forms of cross-talk which must be considered when building networks. NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and use of quality twisted pair cables. Thermal Noise Thermal noise, due to the random motion of electrons, is unavoidable but usually relatively small compared to our signals.AC Power/Reference Ground NoiseAC Power and reference ground noises are crucial problems in networking. AC line noise creates problems in our homes, schools, and offices. Electricity is carried to appliances and machines by wires concealed in walls, floors, and ceilings. Consequently, inside these buildings AC power line noise is all around us. If not properly prevented, power line noise can cause problems for a network. Ideally the signal reference ground should be completely isolated from the electrical ground. Isolation would keep AC power leakage and voltage spikes off the signal reference ground. But the chassis of a computing device serves as the signal reference ground, and as the AC power line ground. Since there is a link between the signal reference ground and the power ground, problems with the power ground can lead to interference with the data system. Such interference can be difficult to detect and trace. Usually, it stems from the fact that electrical contractors and installers don't care about the length of the neutral and ground wires that lead to each electrical outlet. Unfortunately, when these wires are long, they can act as an antenna for electrical noise. It is this noise that interferes with the digital signals (bits) a computer must be able to recognize and process.You will discover that AC line noise coming from a nearby video monitor or hard disk drive can be enough to create errors in a computer system. It does this by interfering (changing the shape and voltage level) with the desired signals and preventing a computer's logic gates from detecting the leading and trailing edges of the square waves. This problem can be further compounded when a computer has a poor ground connection.EMI/RFI External sources of electrical impulses that can attack the quality of electrical signals on the cable include lighting, electrical motors, and radio systems. These types of interference are referred to as electromagnetic interference (EMI), and radio frequency interference (RFI). Each wire in a cable can act like an antenna. When this happens, the wire actually absorbs electrical signals from other wires in the cable and from electrical sources outside the cable. If the resulting electrical noise reaches a high enough level, it can become difficult for NICs to discriminate the noise from the data signal. This is particularly a problem because most LANs use frequencies in the 1-100 megahertz (MHz) frequency region, which happens to be where FM Radio signals, TV signals, and lots of appliances have their operating frequencies as well.To understand how electrical noise, regardless of the source, impacts digital signals, imagine that you want to send data, represented by the binary number 1011001001101, over the network. Your computer converts the binary number to a digital signal. Figure shows what the digital signal for 1011001001101 looks like. The digital signal travels through the networking media to the destination. The destination happens to be near an electrical outlet that is fed by both long neutral and long ground wires. These wires act as possible antennas for electrical noise. Figure shows what electrical noise looks like. Because the destination computer's chassis is used for both the earth ground and the signal reference ground, the noise generated interferes with the digital signal that the computer receives. Figure shows what happens to the signal when it is combined with this electrical noise. Instead of reading the signal as 1011001001101, the computer reads the signal as 1011000101101, making the data unreliable (corrupted). Unlike Copper wire, optical and wireless systems experience some of these forms of noise but are immune to others. For example, optical fiber is immune to NEXT and AC power/reference ground noise, and wireless systems are particularly prone to EMI/RFI. The focus here has been on noise in copper-based wiring systems. The problem of NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and the use of quality twisted pair cables. There is nothing that can be done about thermal noise, other than to give the signals a large enough amplitude so that it doesn't matter. In order to avoid the problem of AC/reference ground as described above, it is important to work closely with your electrical contractor and power company. This will enable you to get the best and shortest electrical ground. One way to do this is to investigate the cost of installing a single power transformer, dedicated to your LAN installation area. If you can afford this option, you can control the attachment of other devices to your power circuit. Restricting how and where devices, such as motors or high-current electrical heaters, are attached can eliminate much of the electrical noise generated by them. When working with your electrical contractor, you should ask that separate power distribution panels, known as breaker boxes, be installed for each office area. Since the neutral wires and ground wires from each outlet come together in the breaker box, taking this step will increase your chances of shortening the length of the signal ground. While installing individual power distribution panels for every cluster of computers can increase the up-front cost of the power wiring, it reduces the length of the ground wires, and limits several kinds of signal-burying electrical noise.There are a number of ways to limit EMI and RFI. One way is to increase the size of the conductor wires. Another way is to improve the type of insulating material used. However, such changes increase the size and cost of the cable faster than they improve its quality. Therefore, it is more typical for network designers to specify a cable of good quality, and to provide specifications for the maximum recommended cable length between nodes. Two techniques that cable designers have used successfully in dealing with EMI and RFI are shielding and cancellation. In cable that employs shielding, a metal braid or foil surrounds each wire pair or group of wire pairs. This shielding acts as a barrier to any interfering signals. However, as with increasing the size of the conductors, using braid or foil covering increases the diameter of the cable and the cost as well. Therefore, cancellation is the more commonly used technique to protect the wire from undesirable interference.When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. The direction of these magnetic lines of force is determined by the direction in which the current flows along the wire. If two wires are part of the same electrical circuit, electrons flow from the negative voltage source to the destination along one wire. Then the electrons flow from the destination to the positive voltage source along the other wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields will cancel each other out. They also will cancel out any outside magnetic fields as well. Twisting the wires can enhance this cancellation effect. By using cancellation in combination with twisting of wires, cable designers can provide an effective method of providing self-shielding for wire pairs within the network media. Web Links Tests on Conducted Electrical Noise 4.3 Basics of Signals and Noise in Communications Systems 4.3.8 Dispersion, jitter, and latency Instructor Note The purpose of this target indicator is to make the idea that timing matters in computer data networks plausible to students. If you have access to a dual-trace oscilloscope and function generator, you can show the pristine pulse and then the broadened pulse if a long enough cable segment is used. While dispersion, jitter, and latency are actually three different things that can happen to a bit, they are grouped together because they each affect the same thing - the timing of a bit. Since you are trying to understand what problems might occur as millions and billions of bits travel on a medium in one second, timing matters a lot. Dispersion is when the signal broadens in time. It caused by is the type of media involved. If serious enough, 1 bit can start to interfere with the next bit and confuse it with the bits before and after it. Since you want to send billions of bits per second, you must be careful not to allow the signals to spread out. Dispersion can be fixed by proper cable design, limiting cable lengths, and finding the proper impedance. In optical fibers, dispersion can be controlled by using laser light of a very specific wavelength. For wireless communications, dispersion can be minimized by the frequencies used to transmit. All digital systems are clocked, meaning clock pulses cause everything to happen. Clock pulses cause a CPU to calculate, data to store in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, you will get timing jitter. This means that bits will arrive a little earlier and later than expected. Jitter can be fixed by a series of complicated clock synchronizations, including hardware and software, or protocol synchronizations. Latency, also known as delay, has two main causes. First, Einsteins theory of relativity states, "nothing can travel faster than the speed of light in a vacuum (3.0 x 108 meters/second)." Wireless networking signals travel at slightly less than the speed of light in vacuum. Networking signals on copper media travel in the range of 1.9x10^8 m/s to 2.4x10^8 m/s. Networking signals on optical fiber they travel at approximately 2.0x10^8 m/s. So to travel a distance, a bit takes at least a small amount of time to get to where its going. Second, if the bit goes through any devices, the transistors and electronics introduce more latency. The solution to the problem of latency is the careful use of internetworking devices, different encoding strategies, and various layer protocols.Modern networks typically work at speeds from 1 Mbps-155 Mbps and greater. Soon they will work at 1 Gbps or 1 billion bits in one second. If bits are broadened by dispersion, then 1s can be mistaken for 0s and 0s for 1s. If groups of bits get routed differently and there is no attention paid to timing, the jitter can cause errors as the receiving computer tries to reassemble packets into a message. If groups of bits are late, the networking devices and other destination computers might get hopelessly lost and overwhelmed by a billion bits per second. 4.3 Basics of Signals and Noise in Communications Systems 4.3.9 Collision Instructor Note The purpose of this target indicator is to make the idea of collisions plausible to students. If you have access to a dual-trace oscilloscope and function generator, place two signals on the media, synchronize them, and watch as the voltage level is twice what it should be for binary zero. A collision occurs when two bits from two different communicating computers are on a shared-medium at the same time. In the case of copper media, the voltages of the two binary signals are added, and cause a third voltage level. This voltage variation is not allowed in a binary system, which only understands two voltage levels. The bits are corrupted "destroyed". Some technologies, such as Ethernet, deal with a certain quantity of collisions to negotiate whose turn it is to transmit on the shared media when communicating between hosts. In some instances collisions are a natural part of the functioning of a network. However, excessive collisions can slow the network down or bring it to a halt. Therefore, a lot of network design goes into minimizing and localizing collisions. There are many ways to deal with collisions. One way is to detect them and simply have a set of rules for dealing with them when they occur, as in Ethernet. Another way is to try to prevent collisions by only allowing one computer on a shared media environment to transmit at a time. This requires that a computer have a special bit pattern called a token to transmit, as in token -ring and FDDI. 4.4 Basics of Encoding Networking Signals 4.4.1 Historical examples of encoding Instructor Note The purpose of this target indicator is to provide rich historical analogies for long distance data communications. If you study each historical instance carefully, you will notice some aspect of these now out-dated means of communication has survived in modern data networks. Whenever you want to send a message over a long distance, there are two problems you must solve - how to express the message (encoding or modulation); and which method to use to transport the message (carrier).Throughout history there have been a variety of ways in which the problem of carrying a long distance communication has been solved: runners, riders, horses, optical telescopes, carrier pigeons, and smoke signals. Each method of delivery required a form of encoding. For example, smoke signals announcing that good hunting had just been found might be three short puffs of smoke, carrier pigeon messages that someone had reached a destination safely might be a picture of a smiling face. In more modern times, the creation of Morse code revolutionized communications. Two symbols, the dot and the dash, were used to encode the alphabet. For instance, - - - means SOS, the universal distress signal. Modern telephones, FAX, AM, FM, short wave radio, and TV all encode their signals electronically, typically using the modulation of different waves from different parts of the electromagnetic spectrum. Encoding means converting binary data into a form that can travel on a physical communications link; modulation means using the binary data to manipulate a wave. Computers use three particular technologies, all of which have their counterparts in history. These technologies are: encoding messages as voltages on various forms of copper wire; encoding messages as pulses of guided light on optical fibers; and encoding messages as modulated, radiated electromagnetic waves. 4.4 Basics of Encoding Networking Signals 4.4.2 Modulation and encoding Instructor Note The key function of this target indicator is that both terms are used extensively in networking, both terms are similar, but both terms must be distinguished from each other. Modulation refers to using one signal to vary another. Thus in Amplitude modulation, the signal wave varies the amplitude of the carrier wave. In Frequency modulation, the signal wave varies the frequency of the carrier wave. In phase modulation, the signal wave varies the relative phase of the carrier wave. Encoding is a somewhat broader term. In its most succinct definition, encoding is how binary one and binary zero are represented. We use the term in the broadest sense, meaning how binary one and binary zero are represented physically. This should be made tangible to the students -- data communications encodes binary ones and zeros as voltages onto copper (using various encoding schemes, such as NRZ, Manchester, 4B/5B), data communications encodes light into optical fibers (again, using various schemes like 4B/5B and 8B/10B), and data communications encodes EM waves into free space (using a wide variety of schemes). Again, encoding is how are the mathematical abstractions (binary ones and zeros) represented in something measurable in the physical world.Also the students should come to appreciate that messages have been historically encoded as voltages on copper wires for at least 150 years. Secondly, they should realize that many modern networks still use voltage pulses on copper wires to achieve data communications. Again, an oscilloscope demonstration is very helpful if at all possible.Also the students should come to appreciate that messages have been historically encoded as visible light pulses for thousands of years, albeit at rather low data rates. Secondly, students should realize that many modern data networks use pulsed LED and Laser light on optical fibers and in free space to achieve data communications. A laser pen and an optical fiber patch cable are useful demonstration tools for this target indicator.Also the students should come to appreciate that messages have been historically encoded as electromagnetic waves for about 100 years. Finally, students should realize that many modern data networks use free-space (unbounded) electromagnetic waves to achieve data communication. Such networks are often called wireless networks, and they tend to use the Infrared, Microwave, and Radio Wave parts of the electromagnetic spectrum. An AM/FM radio and an oscilloscope are useful demonstration tools for this target indicator. Encoding means converting 1s and 0s into something real and physical, such as:an electrical pulse on a wire a light pulse on an optical fiber a pulse of electromagnetic waves into space Two methods of accomplishing this are NRZ encoding and Manchester encoding. NRZ (non-return to 0) encoding is the simplest. It is characterized by a high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 V for binary 0). In optical fibers, binary 1 might be a bright LED or laser light, and binary 0 dark or no light. In wireless networks, binary 1 might mean a carrier wave is present, and binary 0 no carrier at all. Manchester encoding is more complex, but is more immune to noise and is better at remaining synchronized. In Manchester encoding, the voltage on copper wire, the brightness of LED or laser light in optical fiber, or the power of an EM wave in wireless has the bits encoded as transitions. Observe that the Manchester encoding results in 0 being encoded as a low-to-high transition and 1 being encoded as a high-to-low transition. Because both 0s and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver.Closely related to encoding is modulation, which specifically means taking a wave and changing, or modulating it so that it carries information. To give you an idea of what modulation is, examine three forms of modifying, or modulating, a carrier wave to encode bits:AM (amplitude modulation) - the modulation, or height, of a carrier sine wave is varied to carry the message FM (frequency modulation) - the frequency, or wiggly-ness, of the carrier wave is varied to carry the message PM (phase modulation) - the phase, or beginning and ending points of a given cycle, of the wave is varied to carry the message Other more complex forms of modulation also exist. The Figure shows three ways binary data can be encoded onto a carrier wave by the process of modulation. Binary 11 (Note: read as one one, not eleven!) can be communicated on a wave by either AM (wave on/wave off), FM (wave wiggles a lot for 1s, a little for 0s), or PM (one type of phase change for 0s, another for 1s).Messages can be encoded in a variety of ways:As voltages on copper; Manchester and NRZI encoding are popular on copper-based networks. As guided light; Manchester and 4B/5B encoding are popular on fiber based networks. As radiated EM waves; a wide variety of encoding schemes (variations on AM, FM, and PM) are used on wireless networks. Web Links Digital Encoding Summary This chapter provided you the basic theory of electricity and the factors which affect data transmission. More specifically, you learned that: electricity is based on the ability of electrons of certain types of atoms to separate, or flow, from the confines of these atoms opposite charges attract and like charges repel. Electricity flows from negative to positive within electrical circuits materials can be classified as either insulators, conductors, or semiconductors, depending on their ability to allow electrons to flow the concepts of voltage, current, resistance, and impedance provide a means of measuring electricity which is required to be able to design and manufacture electronic devices alternating current and direct current are the two types of current. AC is used to provide power to our homes, schools, and work places. DC is used with electrical devices that depend on a battery to function electrical grounds provide a baseline from which to measure voltage. They also are used as a safety mechanism to prevent hazardous shocks all electronic equipment is composed of electrical circuits that regulate the flow of electricity via switches The next chapter discusses the different types of networking media that are used at the physical layer. In addition, it describes how network devices, cable specifications, network topologies, collisions and collision domains can help determine such things as how much data can travel across the network and how fast.