Transcript High Voltage!!

Institute of Technology Sligo - Dept of Computing
High Voltage!!
Paul Flynn
Institute of Technology Sligo - Dept of Computing
Parts of An Atom
electron
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Proton
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Neutron
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Electron
neutron
proton
Institute of Technology Sligo - Dept of Computing
Flowing Electrons
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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!
Institute of Technology Sligo - Dept of Computing
“Dynamic” Electricity
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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.”
Institute of Technology Sligo - Dept of Computing
Static Electricity
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“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.
Institute of Technology Sligo - Dept of Computing
ESD
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Electrostatic Discharge (ESD) is the
process of static electrons jumping to a
conductor.
Simple experiment:
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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!!)
Institute of Technology Sligo - Dept of Computing
Conductors
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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:
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Free the Electron!
Institute of Technology Sligo - Dept of Computing
Examples of Conductors
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Metals
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Gold
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Silver
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Copper (Cat 5 Cable)
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Water
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Humans!!
Institute of Technology Sligo - Dept of Computing
Insulators
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Material with a high resistance to
electrical current.
Electron orbits are very close to the
nucleus.
Examples:
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Plastic
Glass
Wood
Air and other gases
Institute of Technology Sligo - Dept of Computing
Semiconductors
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With semiconductor materials, the flow
of electrons can be precisely controlled.
Examples:
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Carbon
Germanium
And Silicon!!
Because silicon is widely available
(sand), it is the material we use for
computer chips.
Institute of Technology Sligo - Dept of Computing
Networking Uses All Three!!
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We use conductors to provide a path for
the electrical current.
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We use insulators to keep the flow of
electrons going in one direction.
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For example, copper wire in our cables.
For example, the plastic sheathing on
cables.
We use semiconductors to precisely
control the flow of electrons.
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For example, computer chips use silicon.
Institute of Technology Sligo - Dept of Computing
Measuring Electricity
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Voltage—force or pressure caused by
the separation of electrons and protons.
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Current—the path provided for the free
flow of electrons in an electrical circuit.
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Unit of measurement: Volts (V)
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)
Institute of Technology Sligo - Dept of Computing
Two Types of Current
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Alternating Current (AC)—electrical
current flows in both directions; positive
and negative terminals continuously
trade places (polarity)
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Example: Electricity provided by CPL
Direct Current (DC)—electrical current
flows in one direction; negative to
positive
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Example: Electricity provided by batteries
Institute of Technology Sligo - Dept of Computing
Three Required Parts
of an Electrical Circuit
Source or Battery
Complete Path
Resistance
Institute of Technology Sligo - Dept of Computing
Safety Ground Wire
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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.
Institute of Technology Sligo - Dept of Computing
Multimeter Basics
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A Multimeter is used to measure:
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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.
Institute of Technology Sligo - Dept of Computing
Analog vs. Digital Signals
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Analog signals have a continuously
varying voltage-versus-time graph
Institute of Technology Sligo - Dept of Computing
Analog vs. Digital Signals
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Digital signals have a square wave
with instant transitions from low to
high voltage states (0 to 1).
Institute of Technology Sligo - Dept of Computing
Networks Use Digital
Signaling
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Bits are represented by either no voltage
(0) or +3 to +6 Volts (1).
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A Signal Reference Ground attached
close to a computer’s digital circuits
establishes the baseline for no voltage.
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Bits must arrive at the destination
undistorted in order to be properly
interpreted.
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What six things can distort a bit?
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Let’s look at each in more detail
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Propagation
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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...
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Buffers the arriving bits into memory for later
processing, or
Sends a message to the source to slow down the
speed of propagation.
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Attenuation
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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:
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Not exceeding a medium’s distance requirement
(100 meters for Cat 5 cable)
By using repeaters that “amplify” the signal
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Reflection
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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.
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Noise
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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:
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NEXT A; Thermal Noise; Impulse/Reference
Ground Noise; EMI/RFI; & NEXT B
Institute of Technology Sligo - Dept of Computing
Noise
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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:
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NEXT (Near End Cross Talk) whether at
the source (A) or the destination (B)
EMI/RFI
Institute of Technology Sligo - Dept of Computing
NEXT Noise
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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:
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Strict adherence to RJ-45 termination
procedures (Chapter 5);
Using high quality twisted pair cabling
Institute of Technology Sligo - Dept of Computing
EMI/RFI Noise
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EMI (Electromagnetic Interference) and
RFI (Radio Frequency Interference)
attack the quality of electrical signals on
the cable.
Sources of EMI/RFI include:
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Fluorescent lighting (EMI)
Electrical motors (EMI)
Radio systems (RFI)
Institute of Technology Sligo - Dept of Computing
EMI/RFI Noise Example
Digital Signal
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Source computer sends
out a digital signal.
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Along the path, the signal
encounters EMI noise.
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The digital signal and EMI
combine to distort the signal.
EMI
Distorted Signal
Institute of Technology Sligo - Dept of Computing
EMI/RFI Noise
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Two ways to prevent EMI/RFI Noise:
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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.
Institute of Technology Sligo - Dept of Computing
Canceling EMI/RFI Noise
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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.
Institute of Technology Sligo - Dept of Computing
Canceling EMI/RFI Noise
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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
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing
Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Timing Problems
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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:
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Time it takes a bit to travel to its destination
Devices the bit travels through
Institute of Technology Sligo - Dept of Computing
Bits Are Distorted By...
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Propagation
Attenuation
Reflection
Noise
Timing Problems
Collisions
Institute of Technology Sligo - Dept of Computing
Collisions
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Collisions occur in broadcast topologies
where devices share access to the
network media.
A collision happens when two devices
attempt to communicate on the sharedmedium 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.
Institute of Technology Sligo - Dept of Computing
Final Topic: Encoding
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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:
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Manchester
NRZ (non-return to zero)
Institute of Technology Sligo - Dept of Computing
Good Luck
on the
Test!!