Transcript CS335 Networking & Network Administration

CS335
Networking &
Network Administration
Tuesday, April 6
Local Asynchronous
Communication (RS-332)
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Binary digits (bits) represent data
Short distances ex. Keyboard to computer
http://www2.rad.com/networks/1995/rs232/his
t.htm#hist
Electric current to send bits
Parallel vs. Serial
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Parallel
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Multiple parallel data paths
Transmit bits simultaneously
Travel in 8,16, 32 or 64 bit paths
Need physical circuit for each channel
Serial
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Single lane
Used in modems, older terminal connections,
some serial printers
Asynchronous
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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.
Asynchronous
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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
Start bit and Stop bit
Synchronous transmissions
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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
Standards again!
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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?
EIA Standard RS-232
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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.
RS-232
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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.
Bits, Bytes, Data Encoding
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Each 1 or 0 is a bit
8 bits is a byte
Bits and bytes are encoded to represent
characters
ASCII, EBCDIC, and Unicode
ASCII standard
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7 bit representation
What character is represented here?
EBCDIC
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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
Unicode
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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
OS’s like Windows, Netware, Linux, Unix
Baud Rate
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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
Baud
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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.
Simplex, half and full duplex
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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
RS-232 full-duplex
RS-232 resources
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http://www.camiresearch.com/Data_Com_Ba
sics/RS232_standard.html
http://www.arcelect.com/rs232.htm
http://www2.rad.com/networks/1995/rs232/rs
232.htm
Hardware limitations
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Can’t change voltage instantly
Electric conductivity not perfect
RS-232 standard allows for these
imperfections
Hardware limitations
Hardware bandwidth
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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)
Long-distance communication
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Carrier wave
Modify or modulate
Amplitude and Frequency modulation
Amplitude modulation
Phase Shift modulation
Phase shift modulation
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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
Modulator/Demodulator
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Modem
Dialup modems
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Send bits a long distance
Multiplexing
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Two or more signals that use different carrier
frequencies can be transmitted over a single
medium simultaneously without interference
Cable television for instance
Multiplexing
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Wavelength division (FDM) – optical fiber
Spread spectrum
Time division (TDM) – sources “take turns”