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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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Prof. Abdelshakour Abuzneid

growth of computer networking
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
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
organisation of the text
Organisation of the text
  • Introduction
  • Data transmission
  • Packet transmission
  • Internetworking
  • Networking applications
resource sharing
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
growth of the internet
Growth Of The Internet
  • Early research prototype to a global communication system

Millions of computers


probing the internet
Probing the Internet
  • Simplest probing tool :- ping
  • Ex: ping
    • 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
interpreting a ping response
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
tracing a route
Tracing a Route
  • Traceroute determines the intermediate computers along the path to a remote destination
  • Ex: traceroute
  • Provides more information than ping
transmission media
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
copper wires
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

copper wires cont
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
glass fibers
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
  • 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
  • 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
geosynchronous satellites
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
low earth orbit satellites
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
low earth orbits satellite arrays
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
  • 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
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

asynchronous communication
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
electronic communication
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
standards for communication
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 )
baud rate and errors
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
full duplex transmission
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


limitations of real hardware
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
  • 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
  • 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
long distance communication
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
  • 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
modulation techniques
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
modulation techniques cont
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
  • 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
leased analog data circuits
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
optical and radio frequency
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
dial up modems
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)
carrier frequencies and multiplexing
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
fdm technologies
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
spread spectrum and tdm
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