Tuesday, September 9, 2008

Lecture 2


Lecture 2


  • Distributed processing
    • Google uses it
      • To benefit search speeds for any particular query
  • Communication networking
    • Area is fragmented and hard to understand
    • Hardware progressed faster than logic for using it
    • Because the communication process is so complex, network architecture partitions the overall communication process into separate functional areas called layers. TCP/IP consists of five layers, while OSI has seven layers

Services vs. Applications

  • Definition of service: the basic information transfer capability of a communication network
    • Internet transfer of individual block of information
    • Internet reliable transfer of a stream of bytes
    • Real-time transfer of a voice signal
  • Definition of application: use communication services to build a complete feature, hiding most of the technical details from the users
    • Email and web built on reliable stream service
    • Fax and modems built on basic telephone service
  • Newer applications use multiple (heterogeneous) networks
    • SMS builds on internet reliable stream service and cellular telephone text messaging

Evolution of Network Architecture


  • Communication networks can basically be classified in the following categories
    • Telegraph networks
      • A message transferring or switching system
      • We forward a message to some intermediate point
      • The message must be received
      • Then the forwarding point with now forward message onto another intermediate point or to the destination
      • Switching involved deciding the path of intermediaries to the destination
    • Telephone networks
      • Originally used 'circuit switching'
      • Definition: A means of connecting two communicating parties that relies on the creation of a physical link between the two. Circuit switching is characteristic of telephone connections. Unlike other methods of transmission, such as packet switching, it requires the link to be established before any communication can take place.
      • In a switching circuit technology the circuits were physical switches that defined a path between two points to create a communication signal
    • Internet
      • Packet switching: breaks the data into small packets and transmits them over a communication network from source to destination
      • Sometimes mimic circuit switching
      • Often times send packets down different routes so that if prevents network congestion
      • Packets contain    
        • Header information
          • The address
    • Next generation internet
      • Planned to provide 'prioritized communication' instead of 'best effort' service
      • Will be based on multiservice packet switching network technology

Circuit Switching


  • Pair wise interconnections
    • Every node is connected to every other node
    • N(n-1) / 2 connections
    • Not used for large networks because connection points are overwhelming and unrealistic
  • Access network
    • All nodes are connected to an intermediary such as a switch
    • N connections

Computer Networks and Packet Switching


  • The initial computer networks were terminal-oriented networks with single central computer shared by multiple users
    • Each user had access to a dummy terminal connected with a dedicated line to the central computer
    • A user would type instructions using the dummy terminal
    • The instructions would be passed on to the central computer, where they were executed, and results, returned to the dummy terminal, were displayed on the terminal
    • To allow fair access to the central computer, time-sharing techniques were employed
  • Modification 1
    • Use a multidrop line to connect terminals with the central computer
    • Time sharing was provided by polling frames that polled each terminal in a sequential order
    • Address in the header of the polling frame was used to identify the terminal
    • If a terminal wanted to communicate to the central computer, it would do so when polled
    • The central computer would execute the instruction and pass the result back to the terminal

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