The Danish physicist, Niels Bohr, developed a simplified model to illustrate the atom.
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.
Loosened electrons that do not move and have 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). Conductors will be discussed later in this module.
ESD is usually harmless to people. However, ESD 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 ESD. Students should take safety precautions before they work inside computers, routers, and similar devices.
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. 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 an electric generator. The last way that voltage can be created is by light, or a 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). A volt is defined as the amount of work, per unit charge, that is needed to separate the charges.
The materials through which current flows vary in their 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 that relate to the flow of electrons. Impedance includes resistance, capacitance, and inductance and is similar to the concept of resistance.
Attenuation is important in relation to networks. Attenuation refers to the resistance to the flow of electrons and explains 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 (Ω). The symbol comes from the Greek letter omega.
Electrical insulators are materials that are most resistant to the flow of electrons through them. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas. These materials have very stable chemical structures and the electrons are tightly bound within the atoms.
Electrical conductors are materials that allow electrons to flow through them easily. 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, which results in a current flow.
The periodic table categorizes some groups of atoms 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 interactions with other atoms and molecules. The best conductors are metals such as copper (Cu), silver (Ag), and gold (Au). These metals have electrons that are easily freed. Other conductors include solder, which is a mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an atom that has a different number of electrons than the number of protons in the nucleus. The human body is made of approximately 70 percent water with ions, which means that it is a conductor.
Semiconductors are materials that allow the amount of electricity they conduct to 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). Silicon (Si) is the most important semiconductor because it makes the best microscopic-sized electronic circuits.
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.
The unit of measurement for current is Ampere (A).
The relationship among voltage, resistance, and current is voltage (V) equals current (I) multiplied by resistance (R). In other words, V=I*R. This is Ohms law, named after the scientist who explored these issues.
Two ways in which current flows are alternating current (AC) and direct current (DC). AC voltages change their polarity, or direction, over time. 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 the flow of electrons. 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, which causes the electrons to release energy in the form of light. The circuits in networks use a much more complex version of this 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. Figure shows part of the electrical circuit that brings power to a home or office.
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 indicates that a signal can travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to interpret the signal. 10BASE5 is often referred to as Thicknet. Thicknet is a type of network and 10BASE5 is the Ethernet specification 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, refers to the approximate maximum segment length being 200 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. The maximum segment length is actually 185 meters. 10BASE2 is often referred to as Thinnet. Thinnet is a type of network and 10BASE2 is the Ethernet specification used in that network.
Difference between parallel and series circuit
Coaxial cable consists of a copper conductor surrounded by a layer of flexible insulation. The center conductor can also be made of tin plated aluminium cable allowing for the cable to be manufactured inexpensively. 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 also reduces the amount of outside electromagnetic 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, unshielded twisted pair, UTP, and screened twisted pair, ScTP, 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 such as cable television.
It is important to consider the size of a cable. As the thickness increases, it becomes more difficult to work with a cable. Remember that cable must be pulled through 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 since it has greater transmission lengths and noise rejection characteristics. This type of coaxial cable is frequently referred to as Thicknet. 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 rarely used anymore aside from 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. A solid electrical connection at both ends is important to properly ground the cable. 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 transmission. For this reason Thinnet is no longer commonly used nor supported by latest standards, 100 Mbps and higher, for Ethernet networks.
STP cable combines the techniques of cancellation, shielded, and twisted wires. Each pair of wires is wrapped in metallic foil. The two pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-ohm cable. As specified for use in Token Ring network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable such as electromagnetic interference (EMI) and radio frequency interference (RFI). STP cable shares many of the advantages and disadvantages of UTP cable. STP provides more protection from all types of external interference. However, STP is more expensive and difficult to install than UTP.
A new hybrid of UTP is Screened UTP (ScTP), which is also known as foil screened twisted pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. ScTP, like UTP, is also 100-ohm cable. Many cable installers and manufacturers may use the term STP to describe ScTP cabling. It is important to understand that most references made to STP today actually refer to four-pair shielded cabling. It is highly unlikely that true STP cable will be used during a cable installation job.
The metallic shielding materials in STP and ScTP need to be grounded at both ends. If improperly grounded or if there are any discontinuities in the entire length of the shielding material, STP and ScTP can become susceptible to major noise problems. They are susceptible because they allow the shield to act like an antenna that picks up unwanted signals. However, this effect works both ways. Not only does the shield prevent incoming electromagnetic waves from causing noise on data wires, but it also minimizes the outgoing radiated electromagnetic waves. These waves could cause noise in other devices. STP and ScTP cable cannot be run as far as other networking media, such as coaxial cable or optical fiber, without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. The shielding materials make terminations more difficult and susceptible to poor workmanship. However, STP and ScTP still have a role, especially in Europe or installations where there is extensive EMI and RFI near the cabling.
UTP is a four-pair wire medium used in a variety of networks. Each of the eight copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.
TIA/EIA-568-B.2 contains specifications that govern cable performance. It involves the connection of two cables, one for voice and one for data, to each outlet. The cable for voice must be four-pair UTP. Category 5e is the cable most frequently recommended and implemented in installations. However, analyst predictions and independent polls indicate that Category 6 cable will supersede Category 5e cable in network installations. The fact that Category 6 link and channel requirements are backward compatible to Category 5e makes it very easy for customers to choose Category 6 and supersede Category 5e in their networks. Applications that work over Category 5e will work over Category 6.
UTP cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. However, the real advantage is the size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when a network is installed in an older building. When UTP cable is installed with an RJ-45 connector, potential sources of network noise are greatly reduced and a good solid connection is almost guaranteed.
There are some disadvantages of twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables.
Twisted pair cabling was once considered to have a slower data rate than other types of cable. This is no longer true. In fact, today, twisted pair is considered to have the fastest data rate of any copper-based media.
For communication to occur the signal that is transmitted by the source needs to be understood by the destination. This is true from both a software and physical perspective. The transmitted signal needs to be properly received by the circuit connection designed to receive signals. The transmit pin of the source needs to ultimately connect to the receiving pin of the destination. The following are the types of cable connections used between internetwork devices.