1 Packet Switching Around 1970, research began on a new form of architecture for long distance communications: Packet Switching.

2 Introduction zPacket 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.

3 Packet Switching Operation zData are transmitted in short packets. Typically an upper bound on packet size is 1000 octets. zIf 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 user's data and some control information. zThe control information should at least contain: yDestination Address ySource Address zStore and forward - Packets are received, stored briefly (buffered) and past on to the next node

4 Advantages zLine efficiency ySingle node to node link can be shared by many packets over time yPackets queued and transmitted as fast as possible zData rate conversion yEach station connects to the local node at its own speed yNodes buffer data if required to equalize rates zPackets are accepted even when network is busy yDelivery may slow down zPriorities can be used

5 Switching Technique - Virtual Circuits and Datagrams zStation breaks long message into packets zPackets sent one at a time to the network zPackets handled in two ways yDatagram yVirtual circuit

6 Datagram Packet Switching zIn datagram approach each packet is treated independently with no reference to packets that have gone before. No connection is set up. zPackets can take any practical route zPackets may arrive out of order zPackets may go missing zUp to receiver to re-order packets and recover from missing packets zMore processing time per packet per node zRobust in the face of link or node failures.

7 Packet Switching Datagram Approach

8 Virtual Circuit Packet Switching zIn the Virtual Circuit approach a pre-planned route is established before any packets are sent. zThere is a call set up before the exchange of data (handshake). zAll packets follow the same route and therefore arrive in sequence. zEach packet contains a virtual circuit identifier instead of destination address zMore set up time zNo routing decisions required for each packet - Less routing or processing time zSusceptible to data loss in the face of link or node failure zClear request to drop circuit zNot a dedicated path

9 Packet Switching Virtual Circuit Approach

10 Virtual Circuits vs. Datagram zVirtual circuits yNetwork can provide sequencing and error control yPackets are forwarded more quickly xNo routing decisions to make yLess reliable xLoss of a node looses all circuits through that node zDatagram yNo call setup phase xBetter if few packets yMore flexible xRouting can be used to avoid congested parts of the network

11 Packet switching - datagrams or virtual circuits zInterface between station and network node yConnection oriented xStation requests logical connection (virtual circuit) xAll packets identified as belonging to that connection & sequentially numbered xNetwork delivers packets in sequence xExternal virtual circuit service xe.g. X.25 xDifferent from internal virtual circuit operation yConnectionless xPackets handled independently xExternal datagram service xDifferent from internal datagram operation

12 External Virtual Circuit and Datagram Operation

13 Internal Virtual Circuit and Datagram Operation

14 Circuit vs. Packet Switching zPerformance yPropagation delay yTransmission time yNode delay

15 Comparison with Circuit Switching - Event Timing

16 Comparison with Circuit Switching

17 Routing zComplex, crucial aspect of packet switched networks zCharacteristics required yCorrectness ySimplicity yStability yFairness yEfficiency

18 Routing Performance Criteria zUsed for selection of route zMinimum hop zLeast cost yUsing some algorithm zDelay zThroughput

19 Routing Decision Time and Place zTime yPacket basis yvirtual circuit basis zPlace yDistributed xMade by each node yCentralized ySource (originating node)

20 Network Information Source zRouting decisions usually based on knowledge of network (not always) zDistributed routing yNodes use local knowledge yMay collect info from adjacent nodes yMay collect info from all nodes on a potential route zCentral routing yCollect info from all nodes