Computer networks networking media

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.)