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University of the Western Cape Chapter 4: Electronic and Signals Aleksandar Radovanovic University of the Western Cape Parts of an Atom University of the Western Cape 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 Basics of Electricity University of the Western Cape Voltage (V): Electrical pressure due to separation of electrical charge. Unit of measurement: Volt Current (I): Flow of electrons Unit of measurement: Ampere Resistance (R): Property of material that opposes electrical flow. Unit of measurement: Ohm Electrons flow only in circuits that are closed, or complete, loops. Measuring electricity University of the Western Cape A millimeter is a electrical testing tool that can perform measurement of: voltage resistance, continuity (the level of resistance of a path) Some of the networking related measurements: CAT 5 cable terminated coaxial cable telephone wire CAT 5 jacks wall outlets Analog and Digital University of the Western Cape Analog signals: Continuous, “wave” voltageversus-time graphs used in telecommunications for over 100 years Digital signals: has discrete, or jumpy, voltageversus-time graphs Usually has two voltage levels Bits on Media University of the Western Cape One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. The bit takes a small amount of time to travel (propagate) along the medium. The time it takes the bit to travel from one end of the medium and back again is referred to as the round trip time, (RTT). Attenuation University of the Western Cape A 1 bit voltage signal loses amplitude as energy passes from the signal to the cable. Attenuation is the loss of signal strength Attenuation also happens to optical signals; the optical fiber absorbs and scatters some of the light energy. One way to fix the problem is to change the medium. A second way is to use a repeater after a certain distance. Reflection University of the Western Cape When voltage pulses, or bits, hit a discontinuity some energy can be reflected. If not carefully controlled, this energy can interfere with later bits. Can be resolved by ensuring that all networking components are impedance matched. Reflection occurs with optical signals. Optical signals reflect whenever they hit a discontinuity in the glass fiber, such as when a connector is plugged into a device. Dispersion, jitter, and latency University of the Western Cape Dispersion is when the signal broadens in time. Timing jitter happens when source and destination clock loose synchronization. Latency, is a signal delay. To travel a distance, a bit takes at least a small amount of time to get to where it's going. If the bit goes through any devices, the transistors and electronics introduce more latency. Noise University of the Western Cape Noise is unwanted additions to voltage, optical, or electromagnetic signals. 1. NEXT-A and NEXT-B (near-end crosstalk): when electrical noise on the cable originates from signals on other wires in the cable, this is known as crosstalk. Solution: quality cables and termination. 2. Thermal noise, due to the random motion of electrons, is unavoidable but usually relatively small compared to our signals. 3. AC Power/Reference Ground Noise: bad grounding, long wires, faulty electrical devices. Solution: separate LAN power supply. 4. Electromagnetic interference (EMI), and radio frequency interference (RFI). Solution: shielding and cancellation Collision University of the Western Cape A collision occurs when two bits from two different communicating computers are on a shared-medium at the same time. In the case of copper media, the voltages of the two binary signals are added, and cause a third voltage level. This voltage variation is not allowed in a binary system, which only understands two voltage levels. The bits are corrupted "destroyed". Summary University of the Western Cape Electricity is based on the ability of electrons of certain types of atoms to separate, or flow. Electricity flows from negative to positive within electrical circuits The concepts of voltage, current, and resistance, provide a means of measuring electricity which is required to be able to design and manufacture electronic devices. One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. Six things that can happen to 1 bit: propagation attenuation reflection noise timing problems (dispersion, jitter, and latency) collisions