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Computer networks - networking media - mạng máy tính, mạng truyền thông
2008/7/19 1 Module 3: Networking Media 2008/7/19 2 Outline 3.1 Copper Media Atoms and electrons Voltage Resistance and impedance Current Circuits Cable specifications Coaxial cable STP cable UTP cable 3.2 Optical Media The electromagnetic spectrum Ray model of light Reflection Refraction Total internal reflection Multimode fiber Single-mode fiber Other optical components Signals and noise in optical fibers Installation, care, and testing of optical fiber 3.3 Wireless Media Wireless LAN organizations and standards Wireless devices and topologies How wireless LANs communicate Authentication and association The radio wave and microwave spectrums Signals and noise on a WLANs Wireless security 2008/7/19 3 3.1 Copper Media 2008/7/19 4 Atoms and electrons The atom is comprised of: Electrons – Particles with a negative charge that orbit the nucleus Nucleus – The center part of the atom, composed of protons and neutrons Protons – Particles with a positive charge Neutrons – Particles with no charge (neutral) Coulomb's Electric Force Law The 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. The force is inversely proportional to the square of the separation distance. Bohr’s model Protons are positive charges and electrons are negative charges. There is more than 1 proton in the nucleus. 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). 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. 2008/7/19 5 Atoms and electrons (cont.) 2008/7/19 6 Voltage Voltage is sometimes referred to as electromotive force (EMF – lực điện động). EMF is related to an electrical force, or pressure. Voltage can also be created in three other ways. The first is by friction (sự ma sát), or static electricity (tĩnh điện). The second way is by magnetism (từ tính), or electric generator . The last way that voltage can be created is by light, or solar cell. Voltage is represented by the letter V, and sometimes by the letter E. The unit of measurement for voltage is volt (V). 2008/7/19 7 Resistance and impedance The materials through which current flows offer varying amounts of opposition, or resistance to the movement of the electrons. Conductors : The materials that offer very little, or no, resistance. Insulators : Those materials that do not allow the current to flow, or severely restrict its flow. The letter R represents resistance. The unit of measurement for resistance is the ohm. The symbol comes from the Greek letter, omega (Ω). Semiconductors are materials where the amount of electricity they conduct can be precisely controlled. The most important semiconductor which makes the best microscopic-sized (kích thước cực nhỏ) electronic circuits is silicon (Si). Silicon is very common and can be found in sand, glass, and many types of rocks. 2008/7/19 8 Resistance and impedance (cont.) 2008/7/19 9 Current 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 letter “I” represents current. The unit of measurement for current is Ampere (Amp or A). Amp is defined as the number of charges per second that pass by a point along a path. 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. 2008/7/19 10 Current (cont.)