1 1 Requirements for the internet connection




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Coulomb's Law – Opposite charges attract and like charges repel.

  • Bohr’s model – Protons are positive charges and electrons are negative charges. There is more than 1 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.

    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, 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 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). A discussion on conductors follows later in this chapter.

    ESD, though usually harmless to people, 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 electrostatic discharge. Use caution when working inside a computer, router, and so on.

    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.

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

    3.1.2 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 itself. 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 electric generator. The last way that voltage can be created is by light, or solar cell.

    Voltage is related to the electrical fields emanating from the charges associated with particles such as protons, electrons, etc. 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). Volt is defined as the amount of work, per unit charge, needed to separate the charges.

    3.1.3 Resistance and impedance

    The materials through which current flows offer varying amounts of opposition, or 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 associated with the flow of electrons. The three characteristics comprise impedance, which is similar to and includes resistance.

    The term attenuation is important when learning about networks. Attenuation refers to the resistance to the flow of electrons and 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 (). The symbol comes from the Greek letter , omega.

    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 conductors, usually just called 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, resulting in a current 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), because they have electrons that are easily freed. 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 the number of protons in the nucleus of the atom. The human body is made of approximately 70% water with ions, which means that the human body is a conductor.

    Semiconductors 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 which 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.

    3.1.4 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, or electrical pressure, 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 (Amp). Amp is defined as the number of charges per second that pass by a point along a path.

    If amperage or current can be thought of as the amount or volume of electron traffic that is flowing, then voltage can be thought of as the speed of the electron traffic. The combination of amperage and voltage equals wattage. Electrical devices such as light bulbs, motors and computer power supplies are rated in terms of watts. A watt is how much power a device consumes or produces.

    It is the current or amperage in an electrical circuit that really does the work. As an example, static electricity has very high voltage, so much that it can jump a gap of an inch or more. However, it has very low amperage and as a result can create a shock but not 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 do severe damage or injury.

    3.1.5 Circuits

    Current flows 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. Current consists of electrons flowing away from negative terminals and towards positive terminals. Knowing these facts allows people to control a 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 three prongs serves as the ground, or zero volts. The ground provides a conducting path for the electrons to flow to the earth because the resistance traveling through the body would be greater than the resistance flowing directly to the ground.

    Ground typically means the zero 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 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 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) = 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). Alternating current (AC) and voltages vary over time by changing their polarity, or direction. 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 electrons from flowing. 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, causing the electrons to release energy in the form of light. The circuits involved in networking use a much more complex version of this very 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. Remember, electrical current follows the path of least resistance. Figure shows part of the electrical circuit that brings power to a home or office.

    3.1.6 Cable specifications

    Cables have different specifications and expectations pertaining to performance:

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

    • What kind of transmission is being considered? Will the transmissions be digital or will they be analog-based? Digital or baseband transmission and analog-based or broadband transmission are the two choices.

    • How far can a signal travel through a particular type of cable before attenuation of that signal becomes a concern? In other words, will the signal become so degraded that the recipient device might not be able to accurately receive and interpret the signal by the time the signal reaches that device? The distance the signal travels through the cable directly affects attenuation of the signal. Degradation of the signal is directly related to the distance the signal travels and the type of cable used.

    Some examples of Ethernet specifications which relate to cable type include:

    • 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 represents the capability of the cable to allow the signal to travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE5 is often referred to as Thicknet. Thicknet is actually a type of network, while 10BASE5 is the cabling 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, represents the capability of the cable to allow the signal to travel for approximately 200 meters, before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE2 is often referred to as Thinnet. Thinnet is actually a type of network, while 10BASE2 is the cabling used in that network.

    3.1.7 Coaxial cable

    Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. 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 reduces the amount of outside electro-magnetic 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, and unshielded twisted pair, UTP, 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, including cable television.

    When working with cable, it is important to consider its size. As the thickness of the cable increases, so does the difficulty in working with it. Remember that cable must be pulled through existing 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, because it has a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, 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 almost never used anymore, except for 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 and special care must be taken to ensure a solid electrical connection at both ends resulting in proper grounding. 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 transmittal on the networking media. For this reason thinnet is no longer commonly used nor supported by latest standards (100 Mbps and higher) for Ethernet networks.

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