Content Overview Copper cable is used in almost every LAN. Many different types of copper cable are available, with each type having advantages and disadvantages. Proper selection of cabling is key to efficient network operation. Because copper carries information using electrical current, it is important to understand some basics of electricity when planning and installing a network. Optical fiber is the most frequently used medium for the longer, high bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Using optical media, light is used 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 itself. In fact, the glass used in fiber-optic cable is a very good electrical insulator. Physical connectivity allowed an increase in productivity by allowing the sharing of printers, servers, and software. Traditional networked systems require that the workstation remains stationary permitting moves only within the limits of the media and office area. The introduction of wireless technology removes these restraints and brings true portability to the computing world. 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 installing a new network or when upgrading an existing 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. Students completing this module should be able to:
Content 3.1 Copper Media 3.1.1 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: To help explain the electrical properties of elements/materials, locate helium (He) on the periodic table. Helium has an atomic number of 2, which means that helium has 2 protons and 2 electrons. It has an atomic weight of 4. By subtracting the atomic number (2) from the atomic weight (4), it is learned that helium also has 2 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 an adult (#5) soccer ball in the middle of a soccer field, the only thing smaller than the ball would be the electrons. The electrons would be the size of cherries and would be orbiting near the outer-most seats of the stadium. In other words, the overall volume of this atom, including the electron path, would be about the size of the stadium. The nucleus of the atom where the protons and neutrons exist would be the size of the soccer ball. 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. 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 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? 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. Lab Activity Lab Exercise: Safe handling and Use of a MultimeterIn this lab, the student will learn how to use and handle a multimeter correctly. Web Links Electrical Properties of Materials http://www.science-campus.com/ electrical_theory/fundamentals/ electrical_prop_mats.html
Content 3.1 Copper Media 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