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14 congestionin datanetworks
 

14 congestionin datanetworks

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    14 congestionin datanetworks 14 congestionin datanetworks Presentation Transcript

    • Data and Computer Communications (DEE 3213) - Congestion in Data Networks
    • 4.3 Introduction -What Is Congestion?
      • Congestion occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network
      • Congestion control aims to keep number of packets below level at which performance falls off dramatically
      • Data network is a network of queues
      • Generally 80% utilization is critical
      • Finite queues mean data may be lost
    • Queues at a Node
    • Effects of Congestion
      • Packets arriving are stored at input buffers
      • Routing decision made
      • Packet moves to output buffer
      • Packets queued for output transmitted as fast as possible
        • Statistical time division multiplexing
      • If packets arrive to fast to be routed, or to be output, buffers will be filled up
      • Can discard packets
      • Can use flow control
        • Can propagate congestion through network
    • Practical Performance
      • Ideal assumes infinite buffers and no overhead
      • Buffers are finite
      • Overheads occur in exchanging congestion control messages
    • Effects of Congestion - No Control
    • Traffic Management
      • Fairness
      • Quality of service
        • May want different treatment for different connections
      • Reservations
        • e.g. ATM
        • Traffic contract between user and network