Packet Switching

• This is a better situation for data
• Particularly when most of the time the line is not fully utilized.
• Data is broken into a series of packets.
• Each packet is transmitted over the network.
• This is good because
  • The network can be shared to a much larger extent.
  • Data can be converted, format, rate, ...
  • It can handle overloads up to a point, but this will increase delays
• Networks can deal with data using a datagram or virtual circuit approach.
• Datagram
  • Each packet is independently routed through the network.
  • End systems are usually responsible for reassembly, packet loss detection, ...
  • At each node in the network
    • The packet's destination field is examined
    • It is sent to the next best step based on this.
  • This is more flexible: it can respond in the case of loss or congestion.
  • It suffers less in the face of failure.
• virtual circuit
  • A path through the network is precomputed.
  • All data follows this path.
  • Note, no allocation of resources.
  • This requires less overhead for each packet.
  • It also reduces error detection problems (packets should be in order)
• He discusses speed and packet size issues next.
  • You should read this section.
  • It discusses the trade off between packet size and performance.
  • In general data transmitted encounters three types of delay:
    • Propagation delay: move a signal from one machine to another.
      • 2x10⁸ m/sec
      • This is usually small and can be disregarded.
    • Transmission time
      • Data rate of the line, as we have been discussing.
    • Node Delay
      • The time it takes a node to perform the various functions. as it switches data.
  • The diagram at the bottom of 315 is most illustrative.