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WELCOME TO THE COURSE

WELCOME TO THE COURSE LOCAL AREA NETWORK Prof. Abdelshakour Abuzneid Growth Of Computer Networking Internet started as a research project Today, internet reaches millions Data networks made telecommuting available to individuals Complexity In Network Systems

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WELCOME TO THE COURSE

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  1. WELCOME TO THE COURSE LOCAL AREA NETWORK Prof. Abdelshakour Abuzneid

  2. Growth Of Computer Networking • Internet started as a research project • Today, internet reaches millions • Data networks made telecommuting available to individuals

  3. 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

  4. Organisation of the text • Introduction • Data transmission • Packet transmission • Internetworking • Networking applications

  5. 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

  6. Growth Of The Internet • Early research prototype to a global communication system Millions of computers year

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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 Infrared

  21. Comparision of Cable Media

  22. 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

  23. 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

  24. 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 )

  25. 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

  26. 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

  27. 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

  28. 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

  29. Noise • Noise is interference (static) that destroys the integrity of signals • Shannon’s 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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)

  38. 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

  39. 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

  40. 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

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