Content Overview Networking media is literally and physically the backbone of a network. Inferior quality of network cabling results in network failures and unreliable performance. Copper, optical fiber, and wireless networking media all require testing to determine the quality. These tests involve certain electrical and mathematical concepts and terms, such as signal, wave, frequency, and noise. Understanding this vocabulary is helpful when learning about networking, cabling, and cable testing. The goal of the first lesson in this module is to provide some basic definitions so that the cable testing concepts presented in the second lesson will be better understood. The second lesson of this module describes the issues relating to the testing of media used for physical layer connectivity in local-area networks (LANs). In order for the LAN to function properly, the physical layer medium must meet the industry standard specifications. Attenuation (signal deterioration) and noise (signal interference) cause problems in networks because the data is not recognizable when it is received. Proper attachment of cable connectors and proper cable installation are important. If standards are followed in these areas, attenuation and noise levels are minimized. After cable has been installed, it must be tested with quality cable testers to verify that the specifications of the TIA/EIA standards are met. This module also describes the various important tests that are performed. Students completing this module should be able to:
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.1 Waves A wave is energy traveling from one place to another. There are many types of waves, but all can be described with similar vocabulary. It is helpful to think of waves as disturbances. A bucket of water that is completely still does not have waves, because there are no disturbances. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide. Ocean waves can be described in terms of their height, or amplitude, which could be measured in meters. They can also be described in terms of how frequently the waves reach the shore, using period and frequency. The period of the waves is the amount of time between each wave, measured in seconds. The frequency is the number of waves that reach the shore each second, measured in Hertz. One Hertz is equal to one wave per second, or one cycle per second. Experiment with these concepts by adjusting the amplitude and frequency in Figure . Networking professionals are specifically interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves. The amplitude of an electrical signal still represents height, but it is measured in volts instead of meters. The period is the amount of time to complete one cycle, measured in seconds. The frequency is the number of complete cycles per second, measured in Hertz. If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are important in electrical signals because they determine the value of the data being transmitted. Web Links Waves Table of Contents http://www.glenbrook.k12.il.us/gbssci/phys/Class/ waves/ wavestoc.html
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.2 Sine waves and square waves Sine waves, or sinusoids, are graphs of mathematical functions. Sine waves have certain characteristics. Sine waves are periodic, which means that they repeat the same pattern at regular intervals. Sine waves are continuously varying, which means that no two adjacent points on the graph have the same value. Sine waves are graphical representations of many natural occurrences that change regularly over time. Some examples of these occurrences are the distance from the earth to the sun, the distance from the ground while riding a Ferris wheel, and the time of day that the sun rises. Since sine waves are continuously varying, they are examples of analog waves. Square waves, like sine waves, are periodic. However, square wave graphs do not continuously vary with time. The wave holds one value for some time, and then suddenly changes to a different value. This value is held for some time, and then quickly changes back to the original value. Square waves represent digital signals, or pulses. Like all waves, square waves can be described in terms of amplitude, period, and frequency. Web Links Sine Waves and Sound http://www.mindspring.com/~scottr/zmusic/ Square Waves http://www.dattalo.com/technical/ theory/ sqwave.html
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.3 Exponents and logarithms In networking, there are three important number systems: Recall that the base of a number system refers to the number of different symbols that can occupy one position. For example, binary numbers have only two different placeholders, 0 and 1. Decimal numbers have 10 different placeholders, the numbers 0-9. Hexadecimal numbers have 16 different placeholders, the numbers 0-9 and the letters A-F. Remember that 10x10 can be written as 102. 102 means ten squared or ten raised to the second power. When written this way, it is said that 10 is the base of the number and 2 is the exponent of the number. 10x10x10 can be written as 103. 103 means ten cubed or ten raised to the third power. The base is still 10, but the exponent is now 3. Use the Media Activity below to practice calculating exponents. Enter x, and y is calculated, or enter y, and x is calculated. The base of a number system also refers to the value of each digit. The least significant digit has a value of base0, or one. The next digit has a value of base1. This is equal to 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers. Numbers with exponents are used to easily represent very large or very small numbers. It is much easier and less error-prone to represent one billion numerically as 109 than as 1000000000. Many calculations involved in cable testing involve numbers that are very large, so exponents are the preferred format. Exponents can be explored in the flash activity. One way to work with the very large and very small numbers that occur in networking is to transform the numbers according to the rule, or mathematical function, known as the logarithm. Logarithms are referenced to the base of the number system being used. For example, base 10 logarithms are often abbreviated log. To take the “log” of a number use a calculator or the flash activity. For example, log (109) equals 9, log (10-3) = -3. You can also take the logarithm of numbers that are not powers of 10, but you cannot take the logarithm of a negative number. While the study of logarithms is beyond the scope of this course, the terminology is used commonly in calculating decibels, a way of measuring signals on copper, optical, and wireless media. Interactive Media Activity Interactivity: Logarithms This activity calculates log base 10 according to values entered for x and y. Interactive Media Activity