1. 1 Electronics & Signals Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

2. 2 Overview n Focus is basic theory of electricity. n Provides foundation for understanding the physical layer of the OSI model. u How data is transmitted through physical media, such as cables and connectors. u Different factors that affect data transmission such as AC power line noise.

3. 3 Static Electricity n Charges can be separated by friction, e.g. by shuffling you feet across a carpet. n Very high voltages (thousands of volts) can be generated, referred to as static electricity. n When you reach for a metal object, a spark occurs - this is current flow, as the high voltage pushes the free electrons to the metal object. n This is ESD or electro-static discharge. u can randomly damage computer chips and/or data.

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5. 5 Electrical Terms (1) n Voltage: electrical force, or pressure on charges. u Represented by the letter "V” or "E”. u unit of voltage is volt (v). n Current: flow of charges (free electrons). u Represented by the letter "I". u unit of current is ampere (amp). n Resistance: opposition to the movement of electrons. u Represented by the letter "R".  unit of resistance is the ohm, .

6. 6 Electrical Terms (2) n Alternating Current (AC) or AC voltage. u varies with time, periodically changes direction or polarity. n Direct Current (DC) or DC voltage u always flow in the same direction; or always has the same polarity. n Impedance u total opposition to current flow (for AC and DC). u represented by the letter "Z".  unit of impedance is the ohm (  ).

7. 7 Electrical Terms (3) n Voltage, Current, Resistance Relationship u Currents flow in closed loops called circuits. u Voltage causes current flow. u Resistance and impedance oppose current flow. n Ground u Earth potential at your location. u Zero reference level.

8. 8 Water Analogy

9. 9 Grounding Network Equipment n AC power is supplied in homes, schools, etc through a 3 prong plug. n Top 2 connectors are the power. n Other connector is safety ground (earth ground). n Any exposed metal is connected to safety ground. n Computer motherboard’s ground plane is connected to the chassis and safety ground. n Ground helps dissipate static electricity.

10. 10 Safety Ground n Purpose - to prevent exposed metal parts from becoming energized should a wiring fault occur in device. n In this case, current will flow through the ground connection, and activate protective devices such as circuit breakers to disconnect the power.

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12. 12 Analog Signals

13. 13 Digital Signals

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15. 15 Signals on the Physical Media n Typically electronic digital signals are represented: u Binary 1 as +5v; Binary 0 as 0v. n Zero level is signal reference ground. u Ground plane on printed circuit boards, connected to chassis, which is connected to safety ground or earth ground. n Six things can happen to 1 bit: u propagation, attenuation, reflection, noise, timing problem, and collisions.

16. 16 Network Signal Propagation n Propagation means travel. n It takes some time for a signal to travel through the network, from source to destination. u Propagation delay.

17. 17 Network Attenuation n Attenuation is the loss of signal strength. n Can resolve problem of attenuation: u by choice of networking media. u Using structures with low amounts of attenuation. F use of repeaters (There are repeaters for electrical, optical, and wireless bits).

18. 18 Network Reflection n Reflection occurs when voltage pulses, or bits, hit a discontinuity and some energy is ‘reflected’. n Reflection can cause interference with later bits. n For optimal network performance, it is important that the impedance of the network media matches the NICs.

19. 19 Impedance Matching n For optimal signal transfer (max power transfer), we need matched impedance. n Cables have a characteristic impedance: u UTP - 100 ohms. u STP - 100 ohms / 150 ohms. u Coax - 50 ohms / 75 ohms. n Need to match impedances of the cables to minimize signal loss.

20. 20 Noise n Noise is unwanted additions to voltage, optical, or electromagnetic signals. Digital Signal with Noise

21. 21 Sources of Noise n NEXT - near end crosstalk. u from signals on other wires in the cable. u Usually due to untwisting at ends of cable. n Thermal Noise u due to the random motion of electrons, usually relatively small compared to our signals. n AC Power and reference ground noises. n EMI / RFI u caused by lighting, electrical motors, and radio systems.

22. 22 AC Power Line Noise n Electricity supplied by the power company is AC voltage. n AC voltage creates noise, due to a changing magnetic field around the power lines. n AC noise, a form of EMI, can produce errors in your data.

23. 23 Avoiding Problem of Electrical Noise n Single power transformer for your LAN. u Short neutral and ground lines. u Restrict noise creating devices like motors, heater, etc. n Use separate power panels for each work area. u Minimizes neutral and ground leads (connected together at power panel).

24. 24 Interference n EMI - electromagnetic interference, from magnetic fields from motors, heavy equipment. n RFI - radio frequency interference, from transmitting equipment. n Two techniques in dealing with EMI and RFI are shielding and cancellation. u shielding (a barrier to any interfering signals) increases size of the conductors and its costs. u cancellation is the preferred technique to deal with undesirable interference.

25. 25 Cancellation n Currents in circuit creates magnetic fields. n Fields cancel if currents are in opposite directions. n Cancellation is enhanced by twisting. n This process is also referred to as “self- shielding”.

26. 26 Dispersion, jitter, and latency n Dispersion - signal broadens in time, affects the timing of a bit. u can be fixed by proper cable design, limiting cable lengths, and using the proper impedance.

27. 27 Jitter n If the clock on source host is not synchronized with the destination, you get timing jitter. u bits will arrive a little earlier and later than expected. n Jitter can be fixed by a series of complicated clock synchronization, including hardware and software, or protocol synchronization. n Jitter can cause errors as the receiving computer tries to reassemble packets into a message

28. 28 Latency n Latency is delay. u Signals take some time to travel thru the network, giving propagation delay. u Devices have latency, time it takes to process the signal. u It takes time to transmit a signal, giving transmission delay. n Problem of latency is solved by careful use of internetworking devices, different encoding strategies, and various layer protocols.

29. 29 Collisions n Collision occurs when two bits from two different communicating computers are on a shared- medium at the same time. u The bits are corrupted, "destroyed". n Excessive collisions can slow the network down or bring it to a halt. n One way is to deal with collisions is to have rules, such as backoff in Ethernet. n Another way is to try to prevent collisions, as with tokens in token-ring and FDDI.

30. 30 Encoding Network Signals n To send a message over a long distance, need to address two problems: u how to express the message (encoding or modulation);. F Encoding means converting binary data into a form that can be sent on a physical communications link. F Modulation means using the binary data to manipulate a wave. u how to transport the message (carrier).

31. 31 Encoding Network Signals (2) n Computers use three technologies, encoding messages as: u voltages on copper media. u pulses of guided light on optical fibers. u modulated, radiated electromagnetic waves.

32. 32 Modulation and Encoding n Encoding means converting 1s and 0s into a form for transmission: u an electrical pulse on a wire, a light pulse on an optical fiber, or a pulse of electromagnetic waves. n Two methods of enncoding are: u NRZ encoding - non-return to zero - based on the signal level. u Manchester encoding - based on the transitions..

33. 33 Encoding Schemes

34. 34 Modulation n Taking a wave and changing, or modulating it so that it carries information.

35. 35 Summary n Basic electricity - voltage, current, resistance, and impedance. n AC and DC. n Electrical grounds provide a reference from which to measure voltage. They also provide a safety mechanism to prevent hazardous shocks. n Signals in a network: u propagation, attenuation, reflection, Noise, Dispersion, jitter, and latency, Collision. n Encoding and Modulation.

36. 36 The End