Summary of Module 4

Summary
Data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage patterns. Waves are energy traveling from one place to another, and are created by disturbances. All waves have similar attributes such as amplitude, period, and frequency. Sine waves are periodic, continuously varying functions. Analog signals look like sine waves. Square waves are periodic functions whose values remain constant for a period of time and then change abruptly. Digital signals look like square waves.


Exponents are used to represent very large or very small numbers. The base of a number raised to a positive exponent is equal to the base multiplied by itself exponent times. For example, 103 = 10x10x10 = 1000. Logarithms are similar to exponents. A logarithm to the base of 10 of a number equals the exponent to which 10 would have to be raised in order to equal the number. For example, log10 1000 = 3 because 103 = 1000.

Decibels are measurements of a gain or loss in the power of a signal. Negative values represent losses and positive values represent gains. Time and frequency analysis can both be used to graph the voltage or power of a signal.

Undesirable signals in a communications system are called noise. Noise originates from other cables, radio frequency interference (RFI), and electromagnetic interference (EMI). Noise may affect all signal frequencies or a subset of frequencies.

Analog bandwidth is the frequency range that is associated with certain analog transmission, such as television or FM radio. Digital bandwidth measures how much information can flow from one place to another in a given amount of time. Its units are in various multiples of bits per second.

On copper cable, data signals are represented by voltage levels that correspond to binary ones and zeros. In order for the LAN to operate properly, the receiving device must be able to accurately interpret the bit signal. Proper cable installation according to standards increases LAN reliability and performance.

Signal degradation is due to various factors such as attenuation, impedance mismatch, noise, and several types of crosstalk. Attenuation is the decrease in signal amplitude over the length of a link. Impedance is a measurement of resistance to the electrical signal. Cables and the connectors used on them must have similar impedance values or some of the data signal may be reflected back from a connector. This is referred to as impedance mismatch or impedance discontinuity. Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. Crosstalk involves the transmission of signals from one wire to a nearby wire. There are three distinct types of crosstalk: Near-end Crosstalk (NEXT), Far-end Crosstalk (FEXT), Power Sum Near-end Crosstalk (PSNEXT).

STP and UTP cable are designed to take advantage of the effects of crosstalk in order to minimize noise. Additionally, STP contains an outer conductive shield and inner foil shields that make it less susceptible to noise. UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.

Fiber-optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros. The strength of a light signal does not diminish like the strength of an electrical signal does over an identical run length. Optical signals are not affected by electrical noise, and optical fiber does not need to be grounded. Therefore, optical fiber is often used between buildings and between floors within a building.

The TIA/EIA-568-B standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs. Optical fiber must also be tested according to networking standards. Category 6 cable must meet more rigorous frequency testing standards than Category 5 cable.