Local Asynchronous Communication

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Local Asynchronous Communication
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Asynchronous

• Means untimed, unexpected
• Sender and receiver dont coordinate before
  communication
• Sender sends whenever
• Receiver must be ready to accept communication at
  any point

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Asynchronous Continued

• Good for lots of devices where data are generated
  randomly
• Pretty much anything from humans
• One possible scenario negative voltage for 1 and
  positive for 0

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Voltage Diagram
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Need For Standards

• How long to keep voltage? How many times a
  second? How much voltage?
• Standards specify these in great detail
• Allows for interoperability
• Organizations
• ITU, EIA, IEEE, ISO, CCITT

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Standards

• Documents created
• EIA created on for serial communication RS-232
• Sends characters serially
• Serial communication is one bit at a time
• Parallel is more than one

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RS-232

• Sends data a character at a time
• Has start and stop bit to denote beginning and
  end of character
• -15 Volts
• Each bit same length

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RS232 Picture
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Baud

• Jean-Maurice-Emile Baudot (b. 1845, Magneux,
  France--d. March 28, 1903, Sceaux)
• Dont usually specify time per bit
• Instead bits per second
• Baud changes per second
• Not bits per second

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Baud Continued

• A baud is a signalling event in which bits can be
  encoded. Baud rate is the number of signalling
  events per second.
• A baud is a bucket that can hold a fixed number
  of bits.
• Receiver measures channel multiple times a bit
• Compares against valid bit time
• Errors are called framing errors

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Simplex Et Cetera

• Simplex one direction only
• Half duplex one direction at a time
• Full duplex both directions at the same time
• Two wires needed to communicate
• Signal and ground
• One signal/ground pair can send in only one
  direction
• RS-232 needs three wires for full duplex

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RS232 Full Duplex
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RS-232 Connectors

• Data Communication Equipment (DCE)
• at the computer end
• Data Termination Equipment (DTE)
• At the serial device end
• These two have different transmit and receive
  pins
• So connecting to DCEs requires crossover cable

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Limits

• Real hardware cannot perfectly transmit signals
  distortion
• Wires do not conduct perfectly
• Distance limits degradation of signal over space

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Limits Picture
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Bandwidth

• Non-perfect transmission limits overall rate
• Each medium has bandwidth
• Maximum rate at which hardware can change and
  detect
• Bandwidth measured in Hertz (Hz)
• 4000 Hz (4 kHz) 4000 changes per second maximum
• Bandwidth is a measure of the width of spectrum a
  transmission medium can carry and deliver
• The range of frequencies the hardware can carry

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Bandwidth Continued

• Nyquists sampling theorem gives theoretical
  upper limit to amount of information transmitted
  based on bandwidth
• Bandwidth B
• K possible values of voltage in each unit of
  transmission
• 2?B?log2K (note log2X (ln(X) / ln(2)) )

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Bandwidth Continued

• Nyquist doesnt address noise
• Shannon limit does
• B?log2(1 S/N)
• Signal to noise ratio
• Measured in decibels (dB 10log10(S/N))
• 20 dB x100
• 3000?log2(1 1000) 30,000 bps

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Frequency Spectrum
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Example Calculations

• Review logs
• Nyquist Max data rate D 2Blog2(K)
• K 8 23, B 3000 Hz, can transmit D
  230003 18000 bits per second (each
  half-cycle can encode 3 bits)
• Shannon Channel Capacity (bits/sec) C
  Blog2(1S/N)
• Signal level actually in S/N in a measure called
  decibels
• db 10log10(S/N)
• S/N 100 gives 20 db
• S/N 1000 gives 30 db, whats 40 db?
• Given BW 3000 Hz, 40 db signal (S/N 104
  10000)
• C (3000)log2(10001) ? 39800 bits/sec. Try 20
  db?

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Problems

• Given bandwidth of 4000 Hz, with K 8 levels of
  energy sent in each baud, what is the maximum
  theoretical bit transmission rate?
• Given bandwidth of 4000 Hz, we send a 50 db
  signal. What is the actual channel capacity?
• What if it was 60 db?