Transcript Electronics and Signals

High Voltage!!
Parts of An Atom
electron
Proton
Neutron
neutron
Electron
proton
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:

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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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High Voltage and Low Current
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High Voltage and High Current
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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
Three Required Parts
of an Electrical Circuit
Source or Battery
Complete Path
Resistance
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
Attenuation
Reflection
Noise
Timing Problems
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)
EMI/RFI Noise Example
Digital Signal

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
Distorted 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 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.

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!!