Download Electronics & Signals

Electronics &
Signals
Andres, Wen-Yuan Liao
Department of Computer Science and Engineering
De Lin Institute of Technology
[email protected]
http://www.cse.dlit.edu.tw/~andres
Overview
The basic theory of electricity
Learn how data is transmitted
through physical media
Learn about the different factors
that affect data transmission
 Alternating
line noise
current (AC) power
Basics of Electricity
A helium atom
Creating stable atoms
Static electricity
Electrical current
Electrical measurement terms
Analogy for voltage, resistance, and current
Graphing AC and DC voltage
Constructing a simple series electrical current
Purpose of grounding networking equipment
A helium atom
nucleus - the center part of the atom,
formed by protons and neutrons
protons - particles have a positive charge ,
and along with neutrons, form the nucleus
neutrons - particles have no charge
(neutral), and along with protons, form the
nucleus
electrons - particles have a negative charge,
and orbit the nucleus
Helium Atom
Atomic number of 2
 Has
2 protons and 2 electrons.
It has an atomic weight of 4
 Subtract
the atomic number (2)
from the atomic weight (4) you
learn that helium also has 2
neutrons
Danish physicist, Niels Bohr
If the protons and neutrons
of this atom were the size of
a soccer ball, in the middle of
a soccer field, the electrons
would be the size of cherries
Creating stable atoms
Coulomb's Electric Force Law
 Opposite
charges react to each
other with a force that causes
them to be attracted to each
other
 Like charges react to each other
with a force that causes them to
repel each other
Free flow of electrons
The protons and neutrons are bound
together by a very powerful force
The electrons are bound to their orbit
around the nucleus by a weaker force
Electrons in certain atoms can be
pulled free from the atom, and made to
flow. This is electricity - a "free flow of
electrons"
Static electricity
Loosened electrons that stay in one
place, without moving and with a
negative charge
Electrostatic discharge (ESD)
 These
static electrons have an
opportunity to jump to a conductor
 harmless to people
 problems for sensitive electronic
equipment
Electrical current
Insulators
Conductors
Semiconductors
Electrical insulators
Materials that allow electrons
to flow through them with
great difficulty, or not at all.
Electrical conductors
Materials that allow electrons to
flow through them with great ease
The outermost electrons are bound
very loosely to the nucleus, and are
easily freed.
The introduction of voltage causes
the free electrons to move, causing
a current to flow
Semiconductors
Materials where the amount of
electricity they conduct can be
precisely controlled
Analogy for voltage,
resistance, and current
A simple series electrical
current
Electrons flow only in circuits that are
closed, or complete, loops
The chemical processes in the battery
cause charges to be separated, which
provides a voltage, or electrical
pressure, enabling electrons to flow
through various devices.
Oscilloscope
x-axis that represents time
y-axis that represents voltage
There are usually two y-axis
voltage inputs so that two
waves can be observed and
measured at the same time.
Basics of Digital
Multimeters
Perform voltage, resistance, and
continuity measurements
Resistance: Short circuit
DC/AC Voltage
Ground
The place on the earth that touches
your house
 The
third prong is the ground
 Gives the electrons an extra
conducting path to flow to the earth,
rather than through your body.
The reference point, or the 0 volts
level
Purpose of grounding
Electrical current follows the path of
least resistance
Pole-mounted transformer

Reduces the high voltages, used in the
transmission, to the 120 or 240 volts used
by typical consumer electrical appliances
Safety ground connection
In electrical equipment, the safety
ground wire is connected to any
exposed metal part of the equipment
The motherboards and computing
circuits in computing equipment are
electrically connected to the chassis
This also connects them to the
safety grounding wire, which is
used to dissipate static electricity
Prevent such metal parts from
becoming energized with a
hazardous voltage resulting from a
wiring fault inside the device
Basics of Signals and Noise
Comparing analog and digital signals
Using analog signals to build digital signals
Representing one bit on a physical medium
Network signal propagation
Network attenuation
Network reflection
Noise
Dispersion, jitter, and latency
Collision
Messages in terms of bits
Signal
Refers to a desired electrical
voltage, light pattern, or modulated
electromagnetic wave
Analog signal
is wavy
has a continuously varying voltageversus-time graph
is typical of things in nature
has been widely used in
telecommunications for over 100 years
Important characteristics
Amplitude (A) - its height and depth
Period (T) - length of time to
complete 1 cycle
Frequency (f) - f = 1/T
Digital signal
Has discrete, or jumpy, voltage-versustime graphs
Is typical of technology, rather than
nature
Have a fixed amplitude but their pulse
width and frequency can be changed
Representing one bit
0 volts for binary 0, and +5 volts for binary 1
Signal reference ground is an important
concept
Optical signals


binary 0: low-light, or no-light intensity
binary 1: higher-light intensity
Wireless signals


binary 0: short burst of waves
binary 1: longer burst of waves
Six things that can
happen to 1 bit
Propagation
Attenuation
Reflection
Noise
Timing problem
Collisions
Noise
Unwanted additions to voltage,
optical, or electromagnetic
signals
 Keep
the signal-to-noise (S/N)
ratio as high as possible
 Corrupt a bit turning a binary 1
into a binary 0, or a 0 into a 1
NEXT-A and NEXT-B
Near-end crosstalk (NEXT)
Electrical noise on the cable originates from
signals on other wires in the cable
When two wires are near each other and
untwisted, energy from one wire can wind
up in an adjacent wire
Cause noise at both ends of a terminated
cable
Thermal Noise
Thermal noise, due to the
random motion of electrons, is
unavoidable but usually
relatively small compared to our
signals.
AC Power/Reference
Ground Noise
Ideally the signal reference ground
should be completely isolated from the
electrical ground
Isolation would keep AC power leakage
and voltage spikes off the signal
reference ground
AC Power/Reference
Ground Noise
But the chassis of a computing
device serves as the signal
reference ground, and as the AC
power line ground
 Problems
with the power ground can
lead to interference with the data
system
EMI/RFI
Electromagnetic interference (EMI)
Radio frequency interference (RFI)
Lighting, electrical motors, and radio
systems
Each wire in a cable can act like an
antenna
Noices to other media
Optical fiber is immune to NEXT
and AC power/reference ground
noise
Wireless systems are particularly
prone to EMI/RFI
Solution to NEXT
The problem of NEXT can be
addressed by termination
technology, strict adherence to
standard termination
procedures, and the use of
quality twisted pair cables
Solution to Thermal
noice
There is nothing that can be done
about thermal noise, other than to
give the signals a large enough
amplitude so that it doesn't matter
Solution to AC/reference
Work closely with your electrical contractor
and power company
 Ask that separate power distribution
panels, known as breaker boxes, be
installed for each office area
Get the best and shortest electrical ground
Solution to EMI/RFI - 1
Shielding
A
metal braid or foil surrounds each
wire pair or group of wire pairs.
 A barrier to any interfering signals
 Increases the diameter of the cable
and the cost
Solution to EMI/RFI - 2
Cancellation:more commonly used
technique
The magnetic fields of two wires are the
exact opposite of each other
The two magnetic fields will cancel each
other out
Cancel out any outside magnetic fields
Twisting the wires can enhance this
cancellation effect
Dispersion, jitter, and
latency
Affect the same thing - the timing of a bit
Dispersion
The signal broadens in time

1 bit can start to interfere with the next bit and
confuse it with the bits before and after it.
Solution
Be fixed by proper cable design, limiting cable
lengths, and finding the proper impedance.
 In optical fibers: Laser light
 Wireless communications
 Be minimized by the frequencies used to
transmit

Jitter
The clock on the source host is not
synchronized with the destination
Bits will arrive a little earlier and later
than expected
Solution
 Be fixed by a series of complicated
clock synchronizations, including
hardware and software, or protocol
synchronizations
Latency (delay)
Einstein’s theory of relativity states,
"nothing can travel faster than the speed of
light in a vacuum (3.0 x 108
meters/second)."
If the bit goes through any devices, the
transistors and electronics introduce more
latency
Solution
 The careful use of internetworking
devices, different encoding strategies,
and various layer protocols.
The Speed
Light in vacuum: 3.0 x 108 m/s
Networking signals on copper media
they travel in the range of 1.9x108 m/s
to 2.4x108 m/s
Networking signals on optical fiber
travel at approximately 2.0x108 m/s
Collision
Two bits from two different
communicating computers are on a
shared-medium at the same time
The voltages of the two binary signals
are added, and cause a third voltage
level
Excessive collisions can slow the
network down or bring it to a halt.
Solution to Collision
A set of rules for dealing with them
when they occur (Ethernet)
Prevent collisions (FDDI)
Basics of Encoding
Networking Signals
Historical examples of encoding
Modulation and encoding
Analog and Digital
Communication
Analog
Transmission
Analog Data Analog Signal
(Analog modulation)
Digital
Transmission
Digital Signal
(Pulse modulation,
quantization)
Digital Data
Digital Signal
(Coding)
Analog Signal
(Digital modulation)
Amplifier, Repeater,
Modem, Codec
Analog signal
Amplifier
Analog signal
Digital signal
Repeater
Digital signal
Digital data
Modem
Analog signal
Analog data
Digital data
Codec
Digital signal
Encoding & Modulation
How to express the message
The method to use to transport the
message (carrier)
Encoding

Convert binary data into a form that can
travel on a physical communications link
Modulation

Use the binary data to manipulate a wave
Modulation and encoding
Encoding means converting 1s and 0s
into something real and physical, such
as:
an electrical pulse on a wire
 a light pulse on an optical fiber
 a pulse of electromagnetic waves into
space

TTL/Manchester
Encoding
Manchester encoding is more
complex, but is more immune to
noise and is better at remaining
synchronized
Modulation
Taking a wave and changing, or
modulating it so that it carries
information.
AM (amplitude modulation)
FM (frequency modulation)
PM (phase modulation)
Summary
Electricity
Signals and noise
Encoding and modulation