Lecture 3

Lecture 3

 

• MAC
  
  • Acronym: Medium Access Control
    
• To prevent collisions the host computer would poll each terminal based on its MAC address and at that point they would communicate
  
• Frame
  
  • Kept information on where it is going
    
  • And where its source is
    
• Modification 2
  
  • Use multiplexing to
    
    • Transmit multiple messages simultaneously and
      
    • To detect communication errors
      
  • Multiplexers provide a second approach for sharing the communication line
    
  • CRC was used to detect errors
    
  • At this point every frame had a header, CRC and data
    
  • CRC
    
    • Acronym: Cyclic Redundancy Check
      

Second Generation of Networks

 

• the second generation of networks were Computer to computer networks
  
  • as cost of computers dropped, dumb terminals were replaced by PC's
    
  • interconnecting computers were required to support
    
    • file transferring
      
    • remote telnet to allow remote application
      
    • parallel processing to execute a single program over multiple computers
      
• ARPANET was the first WAN connecting universities
  
  • Operated using packet switching
    
  • Each message is converted into several smaller packets
    
  • At the destination computer the packets are combined into the original message from the host
    
  • Acronym:
    Advanced Research Projects Agency Net
    
  • Missing packets / corrupt packets became a concern
    
• Internet is the interconnection of many networks
  
  • Resulted in compatibility issues with speeds of networks and bandwidth
    
  • Standards needed to be created to connect the networks seamlessly
    

 

Comparison of Switching Techniques

 

• Circuit switching (designed for phone networks)
  
  • End to end path is established between transmitter and receiver
    
  • Complete blocks transmitted and once complete, circuit is terminated
    
  • Transmitter and Receiver were inaccessible for the duration of the connection
    
  • Definition Trunk: a major line connection in a telephone network
    
• Message switching (designed for telegraphic networks)
  
  • No physical path is established between Transmitter and Receiver
    
  • Connection is established between the Transmitter and first switching office (router)
    
  • Entire block of data is transmitted to the switching office
    
  • Block is forwarded one hop at a time
    
  • No limit on block size, switching stations inaccessible for duration of transfer
    
• Packet switching (used in internet)
  
  • A tight limit is placed on maximum block size
    
  • Data is broken in different sub-blocks and each sub-block is transmitted one hop at a time, on after the other
    
  • Message switching and packet switching are very alike
    
  • Packet switching is quicker because the original data is broken into packets and the length of time the message takes to send everything at once is broken into fragments