Transcript Chapter 4: Electronic and Signals

University of the Western Cape
Chapter 4: Electronic and Signals
Aleksandar Radovanovic
University of the Western Cape
Parts of an Atom
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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
Basics of Electricity
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Voltage (V): Electrical pressure due
to separation of electrical charge.
Unit of measurement: Volt
Current (I): Flow of electrons
Unit of measurement: Ampere
Resistance (R): Property of material
that opposes electrical flow.
Unit of measurement: Ohm
Electrons flow only in
circuits that are closed, or complete,
loops.
Measuring electricity
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A millimeter is a electrical testing
tool that can perform measurement
of:
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voltage
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resistance,
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continuity (the level of
resistance of a path)
Some of the networking related
measurements:
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CAT 5 cable
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terminated coaxial cable
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telephone wire
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CAT 5 jacks
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wall outlets
Analog and Digital
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Analog signals:
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Continuous, “wave” voltageversus-time graphs
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used in telecommunications for
over 100 years
Digital signals:
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has discrete, or jumpy, voltageversus-time graphs
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Usually has two voltage levels
Bits on Media
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One bit, on an electrical medium, is
the electrical signal corresponding
to binary 0 or binary 1.
The bit takes a small amount of
time to travel (propagate) along the
medium.
The time it takes the bit to travel
from one end of the medium and
back again is referred to as the
round trip time, (RTT).
Attenuation
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A 1 bit voltage signal loses
amplitude as energy passes from
the signal to the cable.
Attenuation is the loss of signal
strength
Attenuation also happens to optical
signals; the optical fiber absorbs
and scatters some of the light
energy.
One way to fix the problem is to
change the medium. A second way
is to use a repeater after a certain
distance.
Reflection
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When voltage pulses, or bits, hit a
discontinuity some energy can be
reflected. If not carefully controlled,
this energy can interfere with later
bits.
Can be resolved by ensuring that all
networking components are
impedance matched.
Reflection occurs with optical
signals. Optical signals reflect
whenever they hit a discontinuity in
the glass fiber, such as when a
connector is plugged into a device.
Dispersion, jitter, and latency
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Dispersion is when the signal
broadens in time.
Timing jitter happens when
source and destination clock loose
synchronization.
Latency, is a signal delay. To travel
a distance, a bit takes at least a
small amount of time to get to
where it's going. If the bit goes
through any devices, the transistors
and electronics introduce more
latency.
Noise
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Noise is unwanted additions to voltage,
optical, or electromagnetic signals.
1. NEXT-A and NEXT-B (near-end
crosstalk): when electrical noise on the
cable originates from signals on other
wires in the cable, this is known as
crosstalk.
Solution: quality cables and termination.
2. Thermal noise, due to the random
motion of electrons, is unavoidable but
usually relatively small compared to our
signals.
3. AC Power/Reference Ground Noise:
bad grounding, long wires, faulty
electrical devices.
Solution: separate LAN power supply.
4. Electromagnetic interference (EMI),
and radio frequency interference (RFI).
Solution: shielding and cancellation
Collision
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A collision occurs when two bits
from two different communicating
computers are on a shared-medium
at the same time.
In the case of copper media, the
voltages of the two binary signals
are added, and cause a third
voltage level. This voltage variation
is not allowed in a binary system,
which only understands two voltage
levels. The bits are corrupted
"destroyed".
Summary
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Electricity is based on the ability of electrons of certain types of atoms to
separate, or flow. Electricity flows from negative to positive within electrical
circuits
The concepts of voltage, current, and resistance, provide a means of
measuring electricity which is required to be able to design and manufacture
electronic devices.
One bit, on an electrical medium, is the electrical signal corresponding to
binary 0 or binary 1.
Six things that can happen to 1 bit:
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propagation
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attenuation
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reflection
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noise
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timing problems (dispersion, jitter, and latency)
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collisions