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Transcript
Electronics & Signals Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology [email protected] http://www.cse.dlit.edu.tw/~andres Overview The basic theory of electricity Learn how data is transmitted through physical media Learn about the different factors that affect data transmission Alternating line noise current (AC) power Basics of Electricity A helium atom Creating stable atoms Static electricity Electrical current Electrical measurement terms Analogy for voltage, resistance, and current Graphing AC and DC voltage Constructing a simple series electrical current Purpose of grounding networking equipment A helium atom nucleus - the center part of the atom, formed by protons and neutrons protons - particles have a positive charge , and along with neutrons, form the nucleus neutrons - particles have no charge (neutral), and along with protons, form the nucleus electrons - particles have a negative charge, and orbit the nucleus Helium Atom Atomic number of 2 Has 2 protons and 2 electrons. It has an atomic weight of 4 Subtract the atomic number (2) from the atomic weight (4) you learn that helium also has 2 neutrons Danish physicist, Niels Bohr If the protons and neutrons of this atom were the size of a soccer ball, in the middle of a soccer field, the electrons would be the size of cherries Creating stable atoms Coulomb's Electric Force Law 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 Free flow of electrons The protons and neutrons are bound together by a very powerful force The electrons are bound to their orbit around the nucleus by a weaker force Electrons in certain atoms can be pulled free from the atom, and made to flow. This is electricity - a "free flow of electrons" Static electricity Loosened electrons that stay in one place, without moving and with a negative charge Electrostatic discharge (ESD) These static electrons have an opportunity to jump to a conductor harmless to people problems for sensitive electronic equipment Electrical current Insulators Conductors Semiconductors Electrical insulators Materials that allow electrons to flow through them with great difficulty, or not at all. Electrical conductors Materials that allow electrons to flow through them with great ease The outermost electrons are bound very loosely to the nucleus, and are easily freed. The introduction of voltage causes the free electrons to move, causing a current to flow Semiconductors Materials where the amount of electricity they conduct can be precisely controlled Analogy for voltage, resistance, and current A simple series electrical current Electrons flow only in circuits that are closed, or complete, loops The chemical processes in the battery cause charges to be separated, which provides a voltage, or electrical pressure, enabling electrons to flow through various devices. Oscilloscope x-axis that represents time 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. Basics of Digital Multimeters Perform voltage, resistance, and continuity measurements Resistance: Short circuit DC/AC Voltage Ground The place on the earth that touches your house The third prong is the ground Gives the electrons an extra conducting path to flow to the earth, rather than through your body. The reference point, or the 0 volts level Purpose of grounding Electrical current follows the path of least resistance Pole-mounted transformer Reduces the high voltages, used in the transmission, to the 120 or 240 volts used by typical consumer electrical appliances Safety ground connection In electrical equipment, the safety ground wire is connected to any exposed metal part of the equipment The motherboards and computing circuits in computing equipment are electrically connected to the chassis This also connects them to the safety grounding wire, which is used to dissipate static electricity Prevent such metal parts from becoming energized with a hazardous voltage resulting from a wiring fault inside the device Basics of Signals and Noise Comparing analog and digital signals Using analog signals to build digital signals Representing one bit on a physical medium Network signal propagation Network attenuation Network reflection Noise Dispersion, jitter, and latency Collision Messages in terms of bits Signal Refers to a desired electrical voltage, light pattern, or modulated electromagnetic wave Analog signal is wavy has a continuously varying voltageversus-time graph is typical of things in nature has been widely used in telecommunications for over 100 years Important characteristics Amplitude (A) - its height and depth Period (T) - length of time to complete 1 cycle Frequency (f) - f = 1/T Digital signal Has discrete, or jumpy, voltage-versustime graphs Is typical of technology, rather than nature Have a fixed amplitude but their pulse width and frequency can be changed Representing one bit 0 volts for binary 0, and +5 volts for binary 1 Signal reference ground is an important concept Optical signals binary 0: low-light, or no-light intensity binary 1: higher-light intensity Wireless signals binary 0: short burst of waves binary 1: longer burst of waves Six things that can happen to 1 bit Propagation Attenuation Reflection Noise Timing problem Collisions Noise Unwanted additions to voltage, optical, or electromagnetic signals Keep the signal-to-noise (S/N) ratio as high as possible Corrupt a bit turning a binary 1 into a binary 0, or a 0 into a 1 NEXT-A and NEXT-B Near-end crosstalk (NEXT) Electrical noise on the cable originates from signals on other wires in the cable When two wires are near each other and untwisted, energy from one wire can wind up in an adjacent wire Cause noise at both ends of a terminated cable Thermal Noise Thermal noise, due to the random motion of electrons, is unavoidable but usually relatively small compared to our signals. AC Power/Reference Ground Noise Ideally the signal reference ground should be completely isolated from the electrical ground Isolation would keep AC power leakage and voltage spikes off the signal reference ground AC Power/Reference Ground Noise But the chassis of a computing device serves as the signal reference ground, and as the AC power line ground Problems with the power ground can lead to interference with the data system EMI/RFI Electromagnetic interference (EMI) Radio frequency interference (RFI) Lighting, electrical motors, and radio systems Each wire in a cable can act like an antenna Noices to other media Optical fiber is immune to NEXT and AC power/reference ground noise Wireless systems are particularly prone to EMI/RFI Solution to NEXT The problem of NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and the use of quality twisted pair cables Solution to Thermal noice There is nothing that can be done about thermal noise, other than to give the signals a large enough amplitude so that it doesn't matter Solution to AC/reference Work closely with your electrical contractor and power company Ask that separate power distribution panels, known as breaker boxes, be installed for each office area Get the best and shortest electrical ground Solution to EMI/RFI - 1 Shielding A metal braid or foil surrounds each wire pair or group of wire pairs. A barrier to any interfering signals Increases the diameter of the cable and the cost Solution to EMI/RFI - 2 Cancellation:more commonly used technique The magnetic fields of two wires are the exact opposite of each other The two magnetic fields will cancel each other out Cancel out any outside magnetic fields Twisting the wires can enhance this cancellation effect Dispersion, jitter, and latency Affect the same thing - the timing of a bit Dispersion The signal broadens in time 1 bit can start to interfere with the next bit and confuse it with the bits before and after it. Solution Be fixed by proper cable design, limiting cable lengths, and finding the proper impedance. In optical fibers: Laser light Wireless communications Be minimized by the frequencies used to transmit Jitter The clock on the source host is not synchronized with the destination Bits will arrive a little earlier and later than expected Solution Be fixed by a series of complicated clock synchronizations, including hardware and software, or protocol synchronizations Latency (delay) Einstein’s theory of relativity states, "nothing can travel faster than the speed of light in a vacuum (3.0 x 108 meters/second)." If the bit goes through any devices, the transistors and electronics introduce more latency Solution The careful use of internetworking devices, different encoding strategies, and various layer protocols. The Speed Light in vacuum: 3.0 x 108 m/s Networking signals on copper media they travel in the range of 1.9x108 m/s to 2.4x108 m/s Networking signals on optical fiber travel at approximately 2.0x108 m/s Collision Two bits from two different communicating computers are on a shared-medium at the same time The voltages of the two binary signals are added, and cause a third voltage level Excessive collisions can slow the network down or bring it to a halt. Solution to Collision A set of rules for dealing with them when they occur (Ethernet) Prevent collisions (FDDI) Basics of Encoding Networking Signals Historical examples of encoding Modulation and encoding Analog and Digital Communication Analog Transmission Analog Data Analog Signal (Analog modulation) Digital Transmission Digital Signal (Pulse modulation, quantization) Digital Data Digital Signal (Coding) Analog Signal (Digital modulation) Amplifier, Repeater, Modem, Codec Analog signal Amplifier Analog signal Digital signal Repeater Digital signal Digital data Modem Analog signal Analog data Digital data Codec Digital signal Encoding & Modulation How to express the message The method to use to transport the message (carrier) Encoding Convert binary data into a form that can travel on a physical communications link Modulation Use the binary data to manipulate a wave Modulation and encoding Encoding means converting 1s and 0s into something real and physical, such as: an electrical pulse on a wire a light pulse on an optical fiber a pulse of electromagnetic waves into space TTL/Manchester Encoding Manchester encoding is more complex, but is more immune to noise and is better at remaining synchronized Modulation Taking a wave and changing, or modulating it so that it carries information. AM (amplitude modulation) FM (frequency modulation) PM (phase modulation) Summary Electricity Signals and noise Encoding and modulation