Chapter 4 - Electricity

Foundation of Physical Layer

Physical Layer Function

• Transmit data by defining electrical specifications between source and destination

• Electricity carried to workstations, servers, network devices via wires

• Data travels through wires and is represented by presence of absence of electrical pulses or light pulses

Why Learn Electronics

• Most of the devices and processes involved in networking are electronic

• Anyone installing cable must have an awareness of conducting paths, short circuits, and open circuits

• Extensive use of frame, packet, and segment format diagrams is based on voltage versus time diagrams of oscilloscope or logic analyzer

Everything You Wanted To Know About Atoms

• Nucleus – center

• Proton – particles that have positive charge

• Neutrons – particles that have no charge; form nucleus when combined with protons

• Electrons – particles that have a negative charge and orbit the nucleus

Did You Know That

• Electrons can “come loose” from atoms– This explains electrical conduction in solids

• Opposite charges react to each other– They are attracted to each other– The force increases as charges move closer

Questions

• Why don’t electrons fly in to the center?– They stay in orbit because they have just enough

velocity to keep orbiting

• Why don’t protons fly apart?– A nuclear force acts as a kind of glue to hold them

together

• What causes electricity?– Electrons can be pulled from from the atom and pulled

free from the atom and made to flow

ESD – Electrostatic Discharge

• Static Electricity – electrons at rest – loose but stay in one place– Can jump if given opportunity

• Harmless to people

• Dangerous to computers & computing equipment– Randomly damage computer chips or data

Computing Devices/Current Flow

• Control electrons using– Conductors – copper paths– Semiconductors– Insulators

• Plastic or rubber

Materials

• Insulators - Allow electrons to flow through them with great difficulty, or not at all (high resistance)– Glass, plastic, wood, air

• Conductors - allow electrons to flow through them with great ease– Copper, silver, gold

• Semiconductors - amount of electricity they conduct can be precisely controlled– Silicon, carbon, gallium arsenide

Terms

• Voltage (Electromagnetic Force) V– electrical force, or pressure, that occurs when electrons

and protons are separated.

• Electrical Current I– flow of charges that is created when electrons move

• Resistance R– varying amounts of opposition to flow of electrons

– Measured in Ohms W

Current

• The measurement of electron flow in electrical circuits

• AC (Alternating)– Polarity changes; terminals reverse polarity or direction

• DC (Direct)– Always flows in same direction; one terminal is always

positive and the other is always negative

• Impedance– total opposition to current flow Z

• Measured in Ohms W

Circuits

• Currents only flow in closed loops (complete path) called circuits– Must be composed of conducting materials– Must have source of voltage (power, e.g.

battery) and load or resistance

Grounds

• Reference Point or 0 voltage level

• Place on earth where current goes into ground

• Safety ground wire connected to chassis (exposed metal case)– prevent metal parts from becoming energized

with a hazardous voltage resulting from a wiring fault

Protection Devices

• Circuit Breakers– Interrupt the circuit and stop the flow of electrons

• Ground Fault Circuit Interrupters– Same as Circuit Breakers

• Surge Suppressors– Protect against spikes

• UPS (Uninterrupted Power Supply)– Take over when there is power outage

Using the Multimeter

• Measures voltage, resistance, and continuity• Set meter to DC when measuring

– Batteries– Solar cells– DC generators– Computer power supplies

• Set meter to AC when measuring– Wall sockets

• 120 V in USA and 220 V around the world

• Remember – line voltage can kill

Analog Signal

• Is wavy

• Has a continuously varying voltage-versus-time graph

• Is typical of things in nature

• Has been widely used in tele-communications for over 100 years

• Measured by amplitude and time

Digital Signal

• Has discrete, or jumpy, voltage-versus-time graphs

• Is typical of technology, rather than nature• Fixed amplitude• Can be approximated with square wave with

seemingly instantaneous transitions between high and low

• Square wave can be built using right combination of many sine waves (Fourier)

Binary Digit

• Building Block of data communication system– Could be +5 V for 1 and 0 V for 0

– Low or no light for 0 and high intensity for 1

– Short wave burst for 0 and long wave burst for 1

• Signal reference ground must be close to computer’s digital circuits– Designed into circuit boards

• Remember – 8 bits = 1 byte

Bit Events

• Propagation

• Attenuation

• Reflection

• Noise

• Timing Problems

• Collision

Propagation

• A lump of energy, representing 1 bit, travels from one place to another

• Speed depends on the actual material used in the medium, the geometry (structure) of the medium, and the frequency of the pulses.

• Round Trip Time - time it takes the bit to travel from one end of the medium and back again

• Extremes– 0 time to travel– Forever to travel

• May have to buffer to accommodate differences

Attenuation

• Loss of signal strength, possibly due to distance traveled– Material and geometry can reduce attenuation

• Optical Signals– Minimize by color or wavelength used– Or by single or multi-mode fiber– Or by type of glass filament used

• Radio WavesCan be absorbed or scattered by atmospheric molecules

Solving Attenuation Problems

• Select media carefully

• Choose structures with low rates of attenuation

• Use a repeater after a certain distance

Reflection

• Results from impedance mismatch

• Small part of pulse returns to you

• Energy reflected can interfere with bits following in the data stream

• Correct impedance can solve reflection and interference problems

Noise

• AC Power and reference ground – big problem– AC line noise is all around us– Power line noise can cause network problems

• Problems with the power ground can lead to interference with the data system

• Long neutral and ground wires can act as an antenna for electrical noise. It is this noise that interferes with the digital signals (bits)

Noise Sources

• Video monitor

• Hard disk drive

• Electric motor

• Other Wires

So Why Twisted Pairs?

• Crosstalk is a form of electrical noise that results from signals from other wires

• Solution is Twisting the pairs of wires – see curriculum for details on how it works

EMI/RFI

• External Noises

• Lighting, electrical motors, and radio systems

• Each wire in a cable can act like an antenna

• Most LANs use frequencies in the 1-100 megahertz (MHz) frequency region– So do radio, TV, and appliances

Noise Caveats

• Optical fiber is immune to NEXT and AC power/reference ground noise

• Wireless systems are particularly prone to EMI/RFI

• The problem of NEXT can be addressed by termination technology, strict adherence to standard termination procedures, and UTP

• Can install a power transformer to serve only the area covered by the LAN

Jitter, Dispersion, and Latency

• All affect the timing of a bit– Dispersion - signal broadens in time

• can be fixed by proper cable design, limiting cable lengths, and finding the proper impedance

– Jitter – clock on host and destination are not synchronized

– Latency – aka Delay of network signals• bit takes at least a small amount of time to get to where it’s

going

• Network devices add more latency

Why Timing is Important

• Network speeds today 1 Mbps to 155 Mbps

• Speed will be 1 Gbps– Dispersion can cause 0 to be mistaken for 1– Jitter can cause problems as messages are

assembled by destination computer– When bits are late, network devices can get

overwhelmed

Collision

• Two bits from two different communicating computers are on a shared-medium at the same time– Two voltages are added & cause a higher voltage level

(not allowed in binary system)• Bits are destroyed

• Excessive collisions can slow down a network

• Require a set of rules to deal with event or

• Allow only one computer to transmit at a time (token)

Encoding

• Converting binary data into a form that can travel on a physical communications link– voltages on various forms of copper wire– pulses of guided light on optical fibers– modulated, radiated electromagnetic waves.

• Modulation - using the binary data to manipulate a wave.

Encoding Schemes

• NRZ encoding and Manchester encoding. – NRZ (non-return to 0) encoding is the simplest

• high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 V for binary 0

– Manchester - more complex, but is more immune to noise and is better at remaining synchronized

• results in 1 being encoded as a low-to-high transition and 0 being encoded as a high-to-low transition

Encoding Messages

• As voltages on copper; Manchester and NRZI encoding are popular on copper-based networks

• As guided light; Manchester and 4B/5B encoding are popular on fiber based networks

• As radiated EM waves; a wide variety of encoding schemes are used on wireless networks