Chapter 4 Layer 1 Electronics and Signals

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Chapter 4Layer 1 - Electronics and Signals
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Lecture Objective (Week 4)

• After finishing this chapter, students should be
  able to
• Understand the electricity terminology
• Differentiate an analog signal against a digital
  signal
• Explain factors affecting signal transmission
  attenuation, propagation, reflection, noise,
  dispersion, jitter, latency, and collision
• Describe modulation and encoding

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Basics of Electricity

• Voltage
• Current
• Resistance, Impedance
• Current
• Ground

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Electrical Measurement Terms

• Voltage (electromotive force EMF)
• force that is created pushes toward the opposite
  charge and away from the like charge.
• Voltage can also be created by friction (static
  electricity), by magnetism (electric generator),
  or by light (solar cell).
• Voltage is represented by the letter "V", and
  sometimes by the letter "E", for electromotive
  force. The unit of measurement for voltage is
  volt (V).

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Electrical Measurement Terms

• Current
• the flow of charges that is created when
  electrons move.
• When voltage (electrical pressure) is applied,
  and there is a path for the current, electrons
  move from the negative terminal (which repels
  them), along the path, to the positive terminal
  (which attracts them).
• Current is represented by the letter "I". The
  unit of measurement for current is Ampere (Amp),
  and is defined as the number of charges per
  second that pass by a point along a path.

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Electrical Measurement Terms

• Resistance (R)
• Materials through which current flows, offer
  varying amounts of opposition, or resistance, to
  the movement of the electrons.
• conductors.
• insulators.
• semiconductors.
• unit of measurement for resistance is the ohm
  (?).
• Alternating Current (AC)
• AC flows in one direction, then reverses its
  direction, and repeats the process.
• Direct Current (DC)
• always flows in the same direction, and DC
  voltages always have the same polarity.

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Electrical Measurement Terms

• Impedance (Z)
• Its unit of measurement, like that for
  resistance, is the ohm (?).
• impedance is the total opposition to current flow
  (due to AC and DC voltages).
• resistance is generally used when referring to DC
  voltages.
• Voltage, Current, Resistance Relationship
• Currents only flow in closed loops called
  circuits. These circuits must be composed of
  conducting materials, and must have sources of
  voltage. Voltage causes current to flow, while
  resistance and impedance oppose it V IR

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Electrical Measurement Terms

• Ground
• the place on the earth that touches your house
  (probably via the buried water pipes), eventually
  making an indirect connection to your electric
  outlets.
• When you use an electric appliance that has a
  plug with three prongs, the third prong is the
  ground. It 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, when
  making electrical measurements. Voltage is
  created by the separation of charges, which means
  that voltage measurements must be made between
  two points. A negative terminal on a battery is
  also referred to as 0 volts, or reference ground.

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Purpose of Grounding Networking Equipment

• electrical current follows the path of least
  resistance
• AC power supply has 3 outlets
• Live/Hot power in
• Neutral power out
• Ground connected to any exposed metal part of
  the equipment to prevent electrical shock
• An accidental connection between the hot wire and
  the chassis is an example of a wiring fault that
  could occur in a network device the safety
  ground wire connected to the device would serve
  as a low resistance path to the earth ground.

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Comparing Analog Digital Signals

• An analog signal has the following
  characteristics
• is wavy
• has a continuously varying voltage-versus-time
  graph
• is typical of things in nature
• has been widely used in telecommunications for
  over 100 years
• e.g. a Sine wave

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Comparing Analog Digital Signals

• A digital signal has the following
  characteristics
• has discrete, or jumpy, voltage-versus-time
  graphs
• is typical of technology, rather than nature
• have a fixed amplitude but their pulse width and
  frequency can be changed

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Using Analog Signals to Build Digital Signals
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Network Signal Propagation

• 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).
• Assuming no other delays, the time it takes the
  bit to travel down the medium to the far end is
  RTT/2.

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Network Attenuation

• Attenuation also happens to optical signals,
  radio waves and microwaves
• There are repeaters for electrical, optical, and
  wireless bits.

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Network Reflection

• When voltage pulses, or bits, hit a discontinuity
  some energy can be reflected. If not carefully
  controlled, this energy can interfere with later
  bits.
• Also, optical signals reflect whenever they hit a
  discontinuity in the glass fiber, such as when a
  connector is plugged into a device.
• Whether the system is electrical, optical, or
  wireless, impedance mismatches cause reflections.
  
• resolve this by ensuring that all networking
  components are carefully impedance matched.

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Network Reflection
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Noise

• Noise is unwanted additions to voltage, optical,
  or electromagnetic signals
• it is important to keep the signal-to-noise (S/N)
  ratio as high as possible

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Noise

• NEXT (near-end crosstalk)
• originates from signals on other wires in the
  cable
• Thermal Noise
• AC Power/Reference Ground Noise
• inside buildings, AC power line noise is all
  around us
• difficult to detect and trace.
• 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.
• when the ground wires are long, they can act as
  an antenna for electrical noise.

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Noise

• Electromagnetic interference (EMI), and radio
  frequency interference (RFI). 
• a cable can act like an antenna
• particularly a problem because most LANs use
  frequencies in the 1-100 megahertz (MHz)
  frequency region, which happens to be where FM
  Radio signals, TV signals, and lots of appliances
  have their operating frequencies as well.
• optical fiber is immune to NEXT and AC
  power/reference ground noise, and wireless
  systems are particularly prone to EMI/RFI.
• NEXT can be addressed by termination technology,
  strict adherence to standard termination
  procedures, and the use of quality twisted pair
  cables.

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Noise

• nothing that can be done about thermal noise,
  other than to give the signals a large enough
  amplitude so that it doesn't matter
• to avoid the problem of AC/reference ground,
• work closely with your electrical contractor and
  power company. This will enable you to get the
  best and shortest electrical ground.
• or installing a single power transformer,
  dedicated to your LAN installation area
• separate power distribution panels, known as
  breaker boxes, be installed for each office area
  shortening the length of the ground wires,
  i.e. shortening the length of the signal ground

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Noise

• to avoid the problem of EMI/RFI
• increase the size of the conductor wires
  increase cost
• improve the type of insulating material used
  increase cost
• by shielding
• by cancellation.

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Dispersion, Jitter, and Latency

• affect the timing of a bit.

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Dispersion, Jitter, and Latency

• Dispersion
• signal broadens in time caused by the type of
  media involved. 
• If serious enough, 1 bit can start to interfere
  with the next bit and confuse it with the bits
  before and after it.
• copper cables fixed by proper cable design,
  limiting cable lengths, and finding the proper
  impedance.
• optical fibers controlled by using specific
  wavelength laser light
• wireless communications minimized by the carrier
  frequencies

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Dispersion, Jitter, and Latency

• Timing Jitter
• clock on the source host is not synchronized with
  the destination
• bits will arrive a little earlier and later than
  expected.
• can be fixed by a series of complicated clock
  synchronizations, including hardware and
  software, or protocol synchronizations.

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Dispersion, Jitter, and Latency

• Latency (Delay)
• a bit takes at least a small amount of time to to
  travel a distance
• Also, if the bit goes through any devices, the
  transistors and electronics introduce more
  latency.
• Minimized by the use of internetworking devices,
  different encoding strategies, and various layer
  protocols

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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.

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Collision

• excessive collisions can slow the network down or
  bring it to a halt.
• 2 ways to deal with collisions.
• to detect them, e.g. Ethernet
• try to prevent collisions e.g. in token -ring
  and FDDI.

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Messages in Terms of Bits
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Modulation and Encoding

• Messages can be encoded
• As voltages on copper Manchester and NRZI
  encoding are popular on copper-based networks.
• As guided light Manchester and 4B/5B encoding
  are popular on fiber based networks.
• As radiated EM waves a wide variety of encoding
  schemes (variations on AM, FM, and PM) are used
  on wireless networks