WELCOME TO THE COURSE

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WELCOME TO THE COURSE
LOCAL AREA NETWORK
Prof. Abdelshakour Abuzneid
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Growth Of Computer Networking

• Internet started as a research project
• Today, internet reaches millions
• Data networks made telecommuting available to
  individuals

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Complexity In Network Systems

• Networking is complex because multiple
  technologies exist
• No simple and uniform technology for networking
  concepts
• Look beyond the details and concentrate on
  understanding concepts

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Organisation of the text

• Introduction
• Data transmission
• Packet transmission
• Internetworking
• Networking applications

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Resource Sharing

• The first networks were designed to share large
  scale computational powers
• Advanced Research Project Agency(ARPA) started
  investigating data networks
• Early internet emerged in 1970s at ARPA
• Became commercial success in the 1990s

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Growth Of The Internet

• Early research prototype to a global
  communication system

Millions of computers
year
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Probing the Internet

• Simplest probing tool - ping
• Ex ping www.netbook.cs.purdue.edu
• Send a message and then waits for a response
• Reports computer is alive/not responding
• Options
• Specify packet size
• Compute round trip time
• Repeatedly send one message/sec until program
  stops

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Interpreting a Ping Response

• Ping outputs
• Number of packets sent and receives
• Packets lost, and
• Min/avg/max round trip times
• Disadvantages
• Round trip times provides little information
• Cannot explain why time required is higher
• Output occurs only when computer responds
• Sometimes fails because of network congestion
• Uses
• A diagnostic tool for network administrators
• Pinpoints failures quickly

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Tracing a Route

• Traceroute determines the intermediate computers
  along the path to a remote destination
• Ex traceroute www.netbook.cs.purdue.edu
• Provides more information than ping

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Transmission Media

• Computer communication involves encoding data in
  form of energy
• Hardware devices attached to the computer
  performs the the encoding and decoding of data

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Copper Wires

• Primary medium to connect computers because
• Inexpensive easy to install
• Low resistance to electric current
• When wires placed close together in parallel,
  interference takes place
• To minimize interference, networks use
• Twisted pair
• Advantages
• Limits electromagnetic energy emission
• Prevents signals from other wires from
  interfering

Plastic coated wires
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Copper Wires(cont.)

• Coaxial cable(coax)
• Single wire surrounded by a heavier metal shield
• Provides barrier to electromagnetic radiation
• More protection than twisted pair
• Shielded twisted pair
• A pair of wires surrounded by a metal shield

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Glass Fibers

• Optical fibers uses light to transport data
• Advantages
• Use of light eliminates interference
• Carries of pulse of light much farther
• Carries more information than wires
• Requires only a single fiber
• Disadvantages
• Installation requires special equipment
• Difficult to locate a break in fiber
• Difficult to repair a broken fiber

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Radio

• Uses electromagnetic radiation to transmit data
• Operates at radio frequency
• Transmissions referred to as RF transmissions
• Does not require a direct physical connection
  between computers

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Satellites

• Combined with RF technology to provide
  communication across longer distances
• Satellite contains a transponder which
• Accepts incoming radio transmission
• Amplifies it, and
• Transmits the amplified signal
• Contains multiple transponders operating
  independently at different frequency

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Geosynchronous Satellites

• Placed in an orbit exactly synchronized with the
  rotation of the earth
• Appears at exactly the same spot at all times
• Ex A satellite above equator over Atlantic ocean

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Low Earth Orbit Satellites

• They orbit a few hundred miles above the
  earth(typically 200-400 miles)
• Disadvantages
• Rate at which satellite must travel
• Can only be used during the time its orbit passes
  between two ground stations
• Maximal utilization requires complex control
  systems

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Low Earth Orbits Satellite Arrays

• Satellite arrays
• Launching a set of satellites into low earth
  orbits
• Each point in ground has at least one satellite
  overhead
• Satellites in an array communicate with one
  another

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Microwave

• A higher frequency version of radiowaves
• Can be aimed in a single direction
• Can carry more information than lower frequency
  RF transmissions
• Cannot penetrate metal structures

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Infrared

• Infrared technology can be used for data
  communication
• Limited to a small area
• Especially convenient for small, portable
  computers
• Advantages of wireless communication
• Light from a laser can be used to carry data

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Comparision of Cable Media
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Asynchronous Communication

• Sender and receiver do not need to coordinate
  before data can be transmitted
• Receiver must be ready to accept data whenever it
  arrives
• Ex Keyboard
• Transmitter signal does not contain any
  information to determine where individual bits
  begin and end

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Electronic Communication

• A small electric current used to encode data
• Negative voltage represents 1
• Positive voltage represents 0
• Waveform diagram
• Represents the variation of electric signal over
  time

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Standards for Communication

• RS232
• Accepted to transfer characters over short
  distances
• Defines serial,asynchronous communication
• Sends each bit in exactly the same length of time
• Precedes each character with a start bit ( 0 )
• Follows each character with an idle period at
  least one bit long ( stop bit 1 )

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Baud rate and errors

• Baud is the number of changes in the signal per
  second
• Framing errors occurs when receiving and sending
  hardware not configured to use the same baud rate
• Receiver measures voltage multiple times per bit
  and compares
• RS-232 hardware makes use of framing errors
• Ex BREAK key

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Full duplex transmission

• Simultaneous transfers in two directions
• Transfers in one direction known as simplex or
  half duplex
• RS-232 requires
• A wire for data traveling in either directions
• A single ground wire
• RS-232 defines a 25 pin connector

Ground
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Limitations of real hardware

• No electronic device can produce an exact voltage
  or change from one voltage to another instantly
• No wire conducts electricity perfectly
• RS-232 recognizes that real hardware is imperfect
• Recommends taking samples during the middle of
  the time allocated to the bit

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Bandwidth

• The maximum rate at which hardware can change a
  signal
• Measured in cycles per second or Hertz
• Every physical transmission system has a finite
  bandwidth
• Nyquist sampling theorem for RS-232
• D 2B log2 K
• D Max. data rate in bps
• B Bandwidth
• K Possible values of voltages

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Noise

• Noise is interference (static) that destroys the
  integrity of signals
• Shannons theorem
• C B log2 (1 S/N)
• C Effective limit on channel capacity in bps
• B Bandwidth
• S average signal power
• N average noise power
• S/N signal to noise ratio

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Long distance communication

• Electric current becomes weaker as it travels
• RS-232 will not suffice for long distance
  communication
• A continuous , oscillating signal will propagate
  farther than other signals
• An electric current changes when a bit changes
• Long distance communication usually use a sine
  wave , called a carrier

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Modulation

• To send data , transmitter modifies the carrier
  slightly
• Used with telephone , radio and television
• Long distance computer networks use the same
  scheme as radio station
• Transmitter generates a continuously oscillating
  carrier signal
• Modulates according to the data being sent
• The receiver
• Monitors the incoming carriers
• Detects modulation
• Reconstructs the original data
• Discard the carrier

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Modulation techniques

• Amplitude modulation
• Varies the strength of the outgoing signal in
  proportion to the signal been sent
• Frequency modulation
• Varies the frequency of the underlying carrier in
  proportion to the information being sent
• Both require at least one cycle of a carrier to
  send a bit

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Modulation techniques (cont)

• Phase shift modulation
• Changes the timing of the carrier wave to encode
  data
• Each change is called phase shift
• Each phase shift can encode more than one bit of
  data
• Phase shifts chosen to represent a power of two
  possible values
• Data rate is twice the baud rate

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Modem

• Modulator
• Accepts a sequence of data bits and applies
  modulation
• Demodulator
• Accepts a modulated carrier wave and recreate the
  sequence of data bits
• Most network systems are full duplex
• Each location needs both a modulator and a
  demodulator
• Both circuits combined into a single device
  called modem

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Leased analog data circuits

• Companies use 4-wire circuits as part of their
  network
• Private companies cannot install circuits along
  long distances
• Telephone company provides the necessary wiring
  because
• Each cable includes extra wires for future
  expansion
• It agrees to lease the wires for a monthly fee
• Can be used only with special modems
• Also called serial data circuit, serial line or
  leased line
• Constant availability
• Limited connectivity and cost

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Optical and radio frequency

• Modems can be used with
• RF transmission (RF modem)
• Glass fibers (optical modems)
• Conventional telephone connections
• EX A pair of radio frequency modems can be used
  to send data using radio frequency
• Operating principle remains the same
• RF modems promotes wireless networking

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Dial-up modems

• Connects to an ordinary telephone line
• Differs from 4 wire modems in three ways
• Contains circuitry that mimics a telephone
• Uses a carrier that is an audible tone
• Offers full duplex communication
• Modems must use different carrier tones or
  coordinate
• Data exchanged between the waiting modem(answer
  mode) and other modem(calling mode)

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Carrier frequencies and multiplexing

• Separate channels permit multiple communications
  to share a single, physical connection
• Sender transmits a signal using particular
  carrier frequency
• Receiver accepts at given frequency, unaffected
  by signals at other frequencies
• Technically termed as Frequency division
  multiplexing(FDM)
• Used on high bandwidth transmission systems over
  wire, RF or optical fiber

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FDM technologies

• Broadband technology
• Uses a larger part of electromagnetic
  spectrum(larger bandwidth)
• Base band technology
• Uses a small part of electromagnetic spectrum and
  sends only one bit at a time
• Optical FDM called Wave division multiplexing
• Operates by sending multiple light waves across
  a single optical fiber

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Spread spectrum and TDM

• Spread spectrum involves use of multiple carriers
• Optimum carrier frequency may vary over time
• Improves reliability by arranging for a
  transmitter to send the same signal on a set of
  carrier frequencies
• Dialup modems use a form of spread spectrum
  transmission
• TDM- Time division multiplexing
• Sources sharing a medium takes turns