1. High Voltage!!

2. Parts of An Atom • Proton • Neutron • Electron

3. Flowing Electrons • Electrons are negatively charged • Protons are positively charged • Opposite charges attract • Velocity of electrons keep them in orbit around nucleus • Electrons pulled free from the atom is what we call electricity!

4. “Dynamic” Electricity • Electricity can be viewed as a dynamic process. • Dynamic means “changing.” • Electrons are changing—moving from one atom to another. • This flowing of electrons is called an “electrical current.”

5. Static Electricity • “Static” means stationary or unchanging. • Electrons have been “loosened” from the atom and stay in one place. • The electrons have “voltage” but lack a “current.” • A conductor supplies the current—or path—for static electricity to discharge.

6. ESD • Electrostatic Discharge (ESD) is the process of static electrons jumping to a conductor. • Simple experiment: • Rub your shoes on a carpet (this will cause a voltage to build up around your body) • Touch a metal door knob (the metal is a conductor providing a path for the “flow of electrons”—high voltage electricity!!)

7. Conductors • Conductors have a large number of loosely attached electrons. • These electrons can easily be freed from the nucleus of the atom when voltage is applied. • See this web page for a demonstration: • Free the Electron!

8. Examples of Conductors • Metals • Gold • Silver • Copper (Cat 5 Cable) • Water • Humans!!

9. Insulators • Material with a high resistance to electrical current. • Electron orbits are very close to the nucleus. • Examples: • Plastic • Glass • Wood • Air and other gases

10. Semiconductors • With semiconductor materials, the flow of electrons can be precisely controlled. • Examples: • Carbon • Germanium • And Silicon!! • Because silicon is widely available (sand), it is the material we use for computer chips.

11. Networking Uses All Three!! • We use conductors to provide a path for the electrical current. • For example, copper wire in our cables. • We use insulators to keep the flow of electrons going in one direction. • For example, the plastic sheathing on cables. • We use semiconductors to precisely control the flow of electrons. • For example, computer chips use silicon.

12. Measuring Electricity • Voltage—force or pressure caused by the separation of electrons and protons. • Unit of measurement: Volts (V) • Current—the path provided for the free flow of electrons in an electrical circuit. • Unit of measurement: Ampere (amp) • Resistance—impedance or opposition to the flow of electrons: conductor=low resistance; insulators=high resistance. • Unit of measurement: ohms (Ω)

13. e e e e e Current and Voltage V Low Voltage and Low Current V V V V Low Voltage and High Current

14. e e e e e V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V Current and Voltage V V V V V V V V V High Voltage and Low Current High Voltage and High Current

15. Two Types of Current • Alternating Current (AC)—electrical current flows in both directions; positive and negative terminals continuously trade places (polarity) • Example: Electricity provided by CPL • Direct Current (DC)—electrical current flows in one direction; negative to positive • Example: Electricity provided by batteries

16. Source or Battery Complete Path Resistance Three Required Partsof an Electrical Circuit

17. Safety Ground Wire • Safety Ground Wire prevents electrons from energizing metal parts of the computer. • Without grounding, severe shock and fires can occur. • Safety grounds are connected to the exposed metal parts of the computer’s chassis.

18. Multimeter Basics • A Multimeter is used to measure: • Voltage • Resistance • Continuity (level of resistance) • When using a Multimeter, you must properly set it to either AC or DC, depending on the voltage you’re trying to measure.

19. Analog vs. Digital Signals • Analog signals have a continuously varying voltage-versus-time graph

20. Analog vs. Digital Signals • Digital signals have a square wave with instant transitions from low to high voltage states (0 to 1).

21. Networks Use Digital Signaling • Bits are represented by either no voltage (0) or +3 to +6 Volts (1). • A Signal Reference Ground attached close to a computer’s digital circuits establishes the baseline for no voltage. • Bits must arrive at the destination undistorted in order to be properly interpreted. • What six things can distort a bit?

22. Bits Are Distorted By... Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing Problems Timing Problems Collisions Collisions Let’s look at each in more detail

23. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

24. Propagation • Propagation means travel • A bit takes at least a small amount of time to travel (propagate) down the wire. • If the receiving device cannot handle the speed of the arriving bits, data will be lost. • To avoid data loss, the computer either... • Buffers the arriving bits into memory for later processing, or • Sends a message to the source to slow down the speed of propagation.

25. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

26. Attenuation • Attenuation is the loss of signal strength. • The signal degrades or losses amplitude as it travels (propagates) along the medium • Loss of amplitude means that the receiving device can no longer distinguish a 1 bit from a 0 bit. • Attenuation is prevented by: • Not exceeding a medium’s distance requirement (100 meters for Cat 5 cable) • By using repeaters that “amplify” the signal

27. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

28. Reflection • Reflection refers to reflected energy resulting from an impedance mismatch between the NIC and network media. • Impedance is the resistance to the flow of current in a circuit provided by the insulating material. • When impedance is mismatched, the digital signal can “bounce back” (reflect) causing it to be distorted as bits run into each other.

29. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

30. Noise • Noise is unwanted additions to the signal • Noise is unavoidable • Too much noise can corrupt a bit turning a binary 1 into a binary 0, or a 0 into a 1, thus destroying the message. • There are five kinds of noise: • NEXT A; Thermal Noise; Impulse/Reference Ground Noise; EMI/RFI; & NEXT B

31. Noise • Our signaling is usually strong enough to override the effects of thermal noise. • Reference Ground Noise can usually only be solved by an electrical contractor. • Noise threats we can control directly include: • NEXT (Near End Cross Talk) whether at the source (A) or the destination (B) • EMI/RFI

32. NEXT Noise • Near End Cross Talk (NEXT) originates from other wires in the same cable. • Crosstalk is avoided by a network technician using proper installation procedures including: • Strict adherence to RJ-45 termination procedures (Chapter 5); • Using high quality twisted pair cabling

33. EMI/RFI Noise • EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference) attack the quality of electrical signals on the cable. • Sources of EMI/RFI include: • Fluorescent lighting (EMI) • Electrical motors (EMI) • Radio systems (RFI)

34. Digital Signal EMI Distorted Signal EMI/RFI Noise Example • Source computer sends out a digital signal. • Along the path, the signal encounters EMI noise. • The digital signal and EMI combine to distort the signal.

35. EMI/RFI Noise • Two ways to prevent EMI/RFI Noise: • Through shielding the wires in the cable with a metal braid or foil. (Increases cost and diameter of the cable) • Through cancellation the wires are twisted together in pairs to provide self-shielding within the network media.

36. Canceling EMI/RFI Noise • UTP Cat 5 has eight wires twisted into four pairs. • In each pair, one wire is sending data and the other is receiving. • As the electrons flow down the wire, they create a small, circular magnetic field around the wire.

37. Canceling EMI/RFI Noise • Since the two wires are close together, their opposing magnetic fields cancel each other. • They also cancel out outside magnetic fields (EMI/RFI). • Twisting of the wires enhances cancellation

38. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

39. Timing Problems • Dispersion—similar to attenuation; is the broadening of a signal as it travels down the media. • Jitter—caused by unsynchronized clocking signals between source and destination. This means bits will arrive later or earlier than expected. • Latency—is the delay of a network signal caused by: • Time it takes a bit to travel to its destination • Devices the bit travels through

40. Bits Are Distorted By... • Propagation • Attenuation • Reflection • Noise • Timing Problems • Collisions

41. Collisions • Collisions occur in broadcast topologies where devices share access to the network media. • A collision happens when two devices attempt to communicate on the shared-medium at the same time. • Collisions destroy data requiring the source to retransmit. • The prevention of collisions will be discussed in more detail later in the semester.

42. Final Topic: Encoding • Encoding is the process of converting binary data into a form that can travel on a physical communications link. • For our purposes, you only need to know the two types of encoding schemes most commonly used: • Manchester • NRZ (non-return to zero)

43. Good Luck on the Test!!