Transmission Media

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Transmission Media
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Transmission of Information

• From physics
• Energy
• Electromagnetic wave propagation
• From mathematics
• Coding theory

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Transmission Media

• Copper wire
• Need two wires
• Possibilities
• Twisted pair
• Coaxial Cable
• Optical Fiber
• Flexible
• Light stay in
• Air/space
• Used for electromagnetic transmission

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Twisted pair wiring
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Coaxial Cable
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Type of Satellite

• Geosynchronous Satellite
• Low Earth Orbit
• Low Earth Orbit Satellite Arrays

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Satellite
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Two important Physical Limits of a Transmission
System

• Propagation Delay
• Time required for signal to travel across media
• Example electromagnetic radiation travels
  through space at the speed of light (3 108
  meters per second)
• Bandwidth
• Maximum times per second the signal can change

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Transmission of Data

• Network hardware encodes information for
  transmission
• Two types of encoding
• Analog (amount of energy proportional to the
  value of item sent)
• Digital (two forms of energy to encode 0 and 1)
• Computer networks use the latter

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Example Digital Encoding

• Medium
• Copper wire
• Energy form
• Electric current
• Encoding
• Negative voltage encodes 1
• Positive voltage encodes 0

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Illustration of Digital Encoding

• Known as waveform diagram
• X-axis corresponds to time
• Y-axis corresponds to voltage

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Encoding Details

• All details specified by a standard
• ASCII standard
• Several organizations produce networking
  standards
• IEEE
• Institute for Electrical and Electronic Engineers
• ITU
• International Telecommunications Union
• EIA
• Electronic Industries Association
• Standard RS-232-C
• Hardware that adheres to standard interoperable

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Standard RS-232-C

• Example use
• Connection to keyboard/mouse
• Serial port on PC
• Voltage 15 or -15
• Cable limit to lt 50 feet
• Uses asynchronous communication
• When the transmitter has nothing to send, it
  leaves the wire with a negative voltage that
  corresponds to bit value 1

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Asynchronous Communication

• Sender and receiver must agree on
• Number of bits per character
• Duration of each bit
• Receiver
• Does not know when a character will arrive
• May wait forever
• To ensure meaningful exchange
• Start bit before character
• One or more stop bits after character

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Illustration of RS-232

• Start bit
• Same as 0
• Not part of data
• Stop bit
• Same as 1
• Follows data

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Duration of a bit in RS-232C

• Determined by baud rate
• Example baud rate 9.6Kbaud, 28.8Kbaud, 33.6Kbaud
• Duration of bit is 1/baud_rate
• Sender and receiver must agree a priori
• Received samples signal
• Disagreement results in framing error

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Two Way Communication

• Desirable in practice
• Requires each side to have transmitter and
  receiver
• Called full duplex
• Distinguished from half duplex transmission
  (simplex transmission)

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Illustration of Full-Duplex Communication

• Transmitter on one side connected to receiver on
  other side
• Separate wires needed to carry current in each
  direction
• Common ground wire
• DB-9, DB-15, or DB-25 connector used
• Pin 2 is transmit
• Pin 3 is receive
• A computer transmits on pin 2 and receives on pin
  3
• A modem transmits on pin 3 and receives on pin 2

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Electrical Transmission

• Its is an ugly world
• Electrical energy dissipates as it travels along
• Wires have resistance, capacitance, and
  inductance which distort signals
• Magnetic or electrical interference distort
  signals
• Distortion can result in loss or
  misinterpretation

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Illustration of Distorted Signal For a Single Bit

• In practice
• Distortion can be much worse than illustrated

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Consequences

• RS-232 hardware must handle minor distortions
• Take multiple samples per bit
• Tolerate less than full voltage
• Cannot use electrical current for long-distance
  transmission

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Nyquist Intersymbol Interference Theorem

• Nyquists theorem states that the maximum data
  rate in bits per second that can be achieved over
  a transmission system of bandwidth B is 2B
• If the transmission system uses K possible values
  of voltage instead of two, D denotes the maximum
  data rate in bits per second, then D 2Blog2K

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Shannons Theorem

• Noise Background interference
• C Blog2(1S/N)
• C effective limit on the channel capacity in
  bits per second
• B hardware bandwidth
• S average signal power
• N average noise power
• Example Voice telephone system
• S/N 1000, B 3000Hz
• C 30,000bps

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The Bottom Line

• Nyquists theorem means finding a way to encode
  more bits per cycle improves the data rate
• Shannons theorem means that no amount of clever
  engineering can overcome the fundamental physical
  limits of a real transmission system