CS335 Networking

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CS335 Networking Network Administration

• Tuesday, April 6

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Local Asynchronous Communication (RS-332)

• Binary digits (bits) represent data
• Short distances ex. Keyboard to computer
• http//www2.rad.com/networks/1995/rs232/hist.htmh
  ist

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Electric current to send bits
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Parallel vs. Serial

• Parallel
• Multiple parallel data paths
• Transmit bits simultaneously
• Travel in 8,16, 32 or 64 bit paths
• Need physical circuit for each channel
• Serial
• Single lane
• Used in modems, older terminal connections, some
  serial printers

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Asynchronous

• Sender and receiver do not need to coordinate
  before each transmission.
• The electrical signal the transmitter sends does
  not contain info that the user can use to
  determine where individual bits begin and end.
• Receiving hardware must be built to accept and
  interpret the signal the sending hardware
  generates.

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Asynchronous

• Telegraph
• Morse code HE is 4 dots for H and 1 dot for E
• Numeral 5 is 5 dots
• Operators need to know there is a timed pause
• Digital transmissions need a timing mechanism or
  coding mechanism between bytes of data

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Start bit and Stop bit
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Synchronous transmissions

• Send multiple bytes of data as one transmission
  without a start and stop bit for each byte
• Instead sends preceding sync bits with info about
  transmission rate to alert receiving device that
  it is about to receive data
• Other types use a separate channel instead of
  sync bits

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Standards again!

• How long to hold voltage for a single bit?
• What is the maximum rate at which hardware can
  change the voltage?
• Will hardware be interchangeable with other
  vendors?

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EIA Standard RS-232

• Defines serial, asynchronous communication
• Serial bits travel on the wire one after
  another
• Parallel multiple wires allow one bit on each
  wire
• Connection less than 50 ft
• Voltages range between -15 and 15 volts
• Can send 8 bit characters but often configured to
  send 7 data bits.

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RS-232

• Can send a character any time
• Delay arbitrarily long to send another
• Asynchronous because sender and receiver do not
  coordinate before transmission
• Once starts sends all bits one after another with
  no delay between
• Never leaves 0 volts on the wire, when there is
  nothing to send it leaves the wire with a
  negative voltage that corresponds to bit value 1.

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Bits, Bytes, Data Encoding

• Each 1 or 0 is a bit
• 8 bits is a byte
• Bits and bytes are encoded to represent
  characters
• ASCII, EBCDIC, and Unicode

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ASCII standard

• 7 bit representation
• What character is represented here?

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EBCDIC

• Extended binary coded decimal interchange code
• IBM proprietary encoding scheme
• Used in legacy IBM mainframes
• Eight bits to represent letters, numerals, and
  special characters
• 256 characters can be represented

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Unicode

• http//www.unicode.org
• First 128 characters are same as ASCII
• Unicode uses 16 bits instead of the 7 bits of
  ascii
• Allows for 65,536 different characters to include
  Chinese, Greek, Hebrew, Russian, etc.
• Unicode supported by modern browsers as well as
  OSs like Windows, Netware, Linux, Unix

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Baud Rate

• Baud - Number of changes in signal per second
  that the hardware generates
• Sending and receiving hardware must agree on the
  length of time voltage will be held on the line
• Instead of time per bit, bits per second
• Baud rates are configured by hardware or software
• Early connections operated at 300 baud
• Currently 19,200 or 33,600 bits per second more
  common

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Baud

• At slow speeds, only one bit of information
  (signaling element) is encoded in each electrical
  change. The baud, therefore, indicates the number
  of bits per second that are transmitted. For
  example, 300 baud means that 300 bits are
  transmitted each second (abbreviated 300 bps ).
  Assuming asynchronous communication, which
  requires 10 bits per character, this translates
  to 30 characters per second (cps). For slow rates
  (below 1,200 baud), you can divide the baud by 10
  to see how many characters per second are sent.
• At higher speeds, it is possible to encode more
  than one bit in each electrical change. 4,800
  baud may allow 9,600 bits to be sent each second.
  At high data transfer speeds, therefore, data
  transmission rates are usually expressed in bits
  per second (bps) rather than baud. For example, a
  9,600 bps modem may operate at only 2,400 baud.

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Simplex, half and full duplex

• Simplex one way data flow
• Half-duplex one way at a time
• Full-duplex bidirectional on a single channel
  or one channel for each direction

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RS-232 full-duplex
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RS-232 resources

• http//www.camiresearch.com/Data_Com_Basics/RS232_
  standard.html
• http//www.arcelect.com/rs232.htm
• http//www2.rad.com/networks/1995/rs232/rs232.htm

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Hardware limitations

• Cant change voltage instantly
• Electric conductivity not perfect
• RS-232 standard allows for these imperfections

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Hardware limitations
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Hardware bandwidth

• Measured in cycles per second or Hertz (Hz)
• Nyquist theorem states maximum data rate
• Shannon theorem gives a limit to data rate
  because of noise (background interference)

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Long-distance communication

• Carrier wave
• Modify or modulate
• Amplitude and Frequency modulation

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Amplitude modulation
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Phase Shift modulation
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Phase shift modulation

• If the system can shift the phase by 8 possible
  amounts (23) the transmitter uses 3 bits of data
  to select the shift
• Receiver determines how much the carrier shifted
  and uses the shift to recreate the bits that
  caused it
• Thus bits per second is a multiple of the baud
  rate

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Modulator/Demodulator

• Modem

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Dialup modems

• Send bits a long distance

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Multiplexing

• Two or more signals that use different carrier
  frequencies can be transmitted over a single
  medium simultaneously without interference
• Cable television for instance

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Multiplexing

• Wavelength division (FDM) optical fiber
• Spread spectrum
• Time division (TDM) sources take turns