Local Asynchronous Communication

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Local Asynchronous Communication

• Chaminade University
• Department of Computer Science and Computer
  Information Systems

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Asynchronous Communication

• A communication is called asynchronous if a
  sender and a receiver do not need to coordinate
  before data can be transmitted.
• Transmitter and receiver do not synchronize
  before each transmission.
• Example Keyboards use asynchronous communication.

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Using voltage to send info
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The RS-232 Standard

• RS-232 is a popular standard used for
  asynchronous, serial communication over short
  distances between a computer and a modem or ASCII
  terminal.

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RS-232 Voltages
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RS-232 Voltages

• Note that the sender must leave the line idle for
  a minimum time after the transmission of the last
  bit. This idle time is called the stop bit.
• Because a receiver cannot distinguish between an
  idle line and an initial 1 bit, the RS-232
  standard requires a sender to transmit an extra 0
  bit before transmitting the bits of a character.
  The extra bit is known as a start bit.

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Exercise

• Create the RS232 waveform diagram for the
  transmission of the ASCII character a.

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Baud Rate

• Transmission hardware is rated in baud, the
  number of changes in the signal per second that
  the hardware generates.
• For the simple RS-232 protocol, the baud rate is
  exactly equal to the number of bits per second.
• Thus, 9600 baud means 9600 bits per second.

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Wiring for Full-Duplex RS-232
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Limitations of Real Hardware

• How fast can hardware transmit bits across a
  wire?
• No electronic device can produce an exact voltage
  or change from one voltage to another instantly.
• No wire conducts electricity perfectly
• Signals lose energy, signals are not perfect.

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Example
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Bandwidth
Sampling interval
An electric signal that is transmitted beyond the
bandwidth of the channel. Can you tell where is
the 1 and here is the 0 ? Can a receiver decode
the 1 and the 0 ?
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Bandwidth

• Each transmission system has a limited bandwidth
• Bandwidth is the maximum rate that the hardware
  can change the signal.
• If the sender attempts to transmit changes faster
  than the bandwidth, the hardware will not be able
  to keep up because it will not have enough time
  to complete one change before the sender
  attempts to make another.

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Bandwidth

• Bandwidth is measured in cycles per second, or
  Hertz (Hz).
• Bandwidth is the fastest continuously oscillating
  signal that can be sent across the hardware.
• Example A transmission system has BW of 4000Hz
  the underlying HW can transmit any signal that
  oscillates at a rate less than or equal to 4000
  cycles per second.

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Bandwidth

• Bandwidth limitations arise from the physical
  properties of matter and energy.
• Any transmission system that uses radio, waves,
  sound, light or electric current will have a
  limited bandwidth.

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The Nyquist Theorem

• Relationship between the bandwidth (B) of a
  transmission channel and the maximum number of
  bits per second (D) that can be transferred over
  that channel. Known as the Nyquist theorem.
• It provides a theoretical bound on the maximum
  rate at which data can be sent.
• D 2Blog2 K
• K is the possible values of voltage used.

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Exercise

• Working with a neighbor, find out the maximum
  data rate that can be sent for a transmission
  scheme like RS-232.
• Determine the maximum rate in bits per second at
  which data can be sent across a transmission
  system that has a bandwidth of 4000 Hz and uses
  four values of voltage to encode information.

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The Shannons Theorem

• Nyquists theorem provides an absolute maximum
  that cannot be achieved in practice.
• Real communication systems are subject to small
  amounts of background interference called noise.
• Noise makes it impossible to achieve the
  theoretical maximum transmission rate.

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The Shannons Theorem

• The theorem can be stated as
• C Blog2 (1 S/N) where
• C is the effective limit
• S/N is the signal to noise ratio.
• The signal to noise ratio is given in decibels by
  the expression 10log10S/N.
• Example a ratio of S/N equal to 100 is 20 db.

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Exercise

• The telephone system has a signal-to-noise ratio
  of approximately 30db and a bandwidth of
  approximately 3000 Hz. What is the data rate of
  such a system?
• Answer about 30,000 bps.
• How dialup modems achieve higher throughput than
  the Shannon Theorem allows?
• Answer Data Compression (only 6 bits are used to
  send English letters)

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