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High Voltage!!. Parts of An Atom. Proton Neutron Electron. 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!.

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High Voltage!!

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High Voltage!!


Parts of An Atom

  • Proton

  • Neutron

  • Electron


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!


“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.”


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.


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!!)


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!


Examples of Conductors

  • Metals

    • Gold

    • Silver

    • Copper (Cat 5 Cable)

  • Water

  • Humans!!


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


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.


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.


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)


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Low Voltage and Low Current

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Low Voltage and High Current


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Current and Voltage

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High Voltage and Low Current

High Voltage and High Current


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


Source or Battery

Complete Path

Resistance

Three Required Partsof an Electrical Circuit


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.


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.


Analog vs. Digital Signals

  • Analog signals have a continuously varying voltage-versus-time graph


Analog vs. Digital Signals

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


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?


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


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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.


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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.


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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


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


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


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)


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.


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.


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.


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


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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


Bits Are Distorted By...

  • Propagation

  • Attenuation

  • Reflection

  • Noise

  • Timing Problems

  • Collisions


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.


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)


Good Luck

on the

Test!!


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