Packet switching

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Packet switching

  1. 1. Packet SwitchingAround 1970, research began on a new form of architecture for long distance communications: Packet Switching. 1
  2. 2. Introduction Packet Switching refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message. 2
  3. 3. Packet Switching Operation Data are transmitted in short packets. Typically an upper bound on packet size is 1000 octets. If a station has a longer message to send it breaks it up into a series of small packets. Each packet now contains part of the users data and some control information. The control information should at least contain: Destination Address Source Address Store and forward - Packets are received, stored briefly (buffered) and past on to the next node 3
  4. 4. AdvantagesLine efficiency Single node to node link can be shared by many packets over time Packets queued and transmitted as fast as possibleData rate conversion Each station connects to the local node at its own speed Nodes buffer data if required to equalize ratesPackets are accepted even when network is busy Delivery may slow downPriorities can be used 4
  5. 5. Switching Technique - VirtualCircuits and Datagrams Station breaks long message into packets Packets sent one at a time to the network Packets handled in two ways Datagram Virtual circuit 5
  6. 6. Datagram Packet Switching In datagram approach each packet is treated independently with no reference to packets that have gone before. No connection is set up. Packets can take any practical route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing packets More processing time per packet per node Robust in the face of link or node failures. 6
  7. 7. PacketSwitchingDatagramApproach 7
  8. 8. Virtual Circuit PacketSwitchingIn the Virtual Circuit approach a pre-planned route isestablished before any packets are sent.There is a call set up before the exchange of data(handshake).All packets follow the same route and therefore arrive insequence.Each packet contains a virtual circuit identifier instead ofdestination addressMore set up timeNo routing decisions required for each packet - Less routingor processing timeSusceptible to data loss in the face of link or node failureClear request to drop circuitNot a dedicated path 8
  9. 9. PacketSwitchingVirtualCircuitApproach 9
  10. 10. Virtual Circuits vs. DatagramVirtual circuits Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node looses all circuits through that nodeDatagram No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network 10
  11. 11. Packet switching -datagrams or virtual circuitsInterface between station and network node Connection oriented Station requests logical connection (virtual circuit) All packets identified as belonging to that connection & sequentially numbered Network delivers packets in sequence External virtual circuit service e.g. X.25 Different from internal virtual circuit operation Connectionless Packets handled independently External datagram service Different from internal datagram operation 11
  12. 12. ExternalVirtualCircuit andDatagramOperation 12
  13. 13. InternalVirtualCircuit andDatagramOperation 13
  14. 14. Circuit vs. Packet Switching Performance Propagation delay Transmission time Node delay 14
  15. 15. Comparison with CircuitSwitching - Event Timing 15
  16. 16. Comparison with CircuitSwitching 16
  17. 17. Packet Switching Evolution X.25 packet-switched network Router-based networking Switching vs. routing Frame relay network ATM network 17
  18. 18. Switching vs RoutingSwitching Routingpath set up at connection can work as connectionlesstime complex routing algorithmsimple table look up table maintainance viatable maintainance via protocolsignaling out of sequence deliveryno out of sequence delivery likelylost path may lose robust: no connections lostconnection significant processing delaymuch faster than pure output link decision based onrouting packet header contents - atlink decision made ahead of every nodetime, and resourcesallocated then 18

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