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

2. 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. zOnce all the packets forming a message arrive at the destination, they are recompiled into the original message.

3. 3 Packet Switching Application zMost modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. zCircuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. zPacket switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.

4. 4 Recall Circuit Switching zCall Set-up zData Transfer zCall disconnect

5. 5 Features of Circuit Switching zCircuit switching is connection oriented. zResources are allocated for the call throughout the network. zCalls may be blocked if the resources are not available. zCircuit Switching originated due to need for voice communications.

6. 6 Circuit Switching for Data zWhen Circuit Switching networks started to be used for data communications it became clear that: yIn circuit switching resources dedicated to a particular call whereas data traffic is bursty so most of the time allocated resources would be unutilized. yData rate is fixed in circuit switching so both ends must operate at the same rate - whereas there is asymmetry in the data rate required for each communicating party for data communication needs.

7. 7 Packet Switching zPacket Switching started in the 1970s. zARPANET that became Internet zIn the beginning most people did not believe it would work zThe basic technology of packet switching is fundamentally the same today as it was in the early 1970’s networks zPacket switching remains one of the few technologies for effective long-distance data communications.

8. 8 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

9. 9 Packet Switching Operation

10. 10 Use of Packets

11. 11 Packet Switching Networks are Switched Networks

12. 12 Advantages zLine efficiency ySingle node to node link can be dynamically shared by many packets over time yPackets queued and transmitted as fast as possible zData rate conversion y Two stations of different data rates can exchange packets because each connects to its node at its proper data rate

13. 13 Advantages zWhen traffic becomes heavy in a circuit switching network, some calls are blocked i.e the network refuses to accept additional connection requests until the load on the network decreases. zOn a packet switching network, packets are still accepted, but delivery delay increases zPriorities can be used. If a node has a number of packets queued for transmission, it can transmit the higher priority packets first.

14. 14 Switching Technique zIf a station sends a message through packet switching network that is of length greater than the maximum packet size, it breaks the message up into packets and sends these packets, one at a time, to the network zQuestion?? How the network will handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination

15. 15 Switching Technique - Virtual Circuits and Datagrams zPackets handled in two ways yDatagram approach yVirtual circuit approach

16. 16 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.

17. 17 Packet Switching Datagram Approach

18. 18 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

19. 19 Virtual Circuit Packet Switching 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

20. 20 Packet Switching Virtual Circuit Approach

21. 21 Virtual Circuit

22. 22 One Station Can Have Many Virtual Circuit Connections

23. 23 Virtual Circuits vs. Datagram zSo the main characteristic of the virtual circuit technique is that a route between stations is setup prior to data transfer, this does not mean that this is a dedicated path as in the circuit switching zA packet is still buffered at each node, and queued for output over a line, while other packets on other virtual circuits may share the use of the line

24. 24 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

25. 25 Packet Size

26. 26 Packet Size zIn this example it is assumed that there is a virtual circuit from station X through nodes a and b to station Y. zThe message to be sent comprises 40 octets and 3 octets of control information called header. zIf the entire message is sent the packet first transmitted from station X to node a, when the entire packet is received, it can be transmitted from a to b and then transmitted to Y. ignoring switching time, total transmission time is 129 octet- time(43octets x 3 packet transmission)

27. 27 Circuit vs. Packet Switching zPerformance yPropagation delay xThe time it takes a signal to propagate from one node to the next. This time generally negligible. Typically on a wire medium 2x10 8 yTransmission time xThe time it takes for a transmitter to send out a block of data, e.g it takes 1s to transmit 10,000 bit block of data onto a 10-kbps line. yNode delay xThe time it takes for a node to perform the necessary processing as it switches data

28. 28 Comparison with Circuit Switching - Event Timing

29. 29 Circuit SwitchingPacket Switching Datagram Packet switching Virtual-circuit Packet Switching Dedicated transmission pathNo dedicated path Continuous transmission of dataTransmission of packets Fast enough for interactive Messages are not storedPackets may be stored until delivered Packets stored until delivered The path is established for entire conversation Route established for each packet Route established for entire conversation Call setup delay; negligible transmission delay Packet transmission delayCall setup delay; packet transmission delay Busy signal if called party busySender may be notified if packet not delivered Sender notified of connection denial Overload may block call setup; no delay for established calls Overload increases packet delay Overload may block call setup; increase packet delay Electromechanical or computerized switching Small switching nodes User responsible for message loss protection Network may be responsible for individual packets Network may be responsible for packet sequences Usually no speed or code conversion Speed and code conversion Fixed bandwidthDynamic use of bandwidth No overhead bits after call setupOverhead bits in each packet

30. 30 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

31. 31 Combinations (1) zExternal virtual circuit, internal virtual circuit yWhen the user requests a virtual circuit, a dedicated route through the network is constructed. y All packets follow the same route zExternal virtual circuit, internal datagram yNetwork handles each packet separately yDifferent packets for the same external virtual circuit may take different internal routes yNetwork buffers at destination node for re-ordering

32. 32 Combinations (2) zExternal datagram, internal datagram yEach packets is treated independently from both the user’s and the network’s point of view zExternal datagram, internal virtual circuit yExternal user does not see any connections simply sending packets one at a time yNetwork sets up logical connection between stations for packet delivery

33. 33 External Virtual Circuit and Datagram Operation

34. 34 Internal Virtual Circuit and Datagram Operation

35. 35 External and Internal Operation - ED/IVC

36. 36 External and Internal Operation - ED/ID

37. 37 External and Internal Operation - EVC/IVC

38. 38 External and Internal Operation - EVC/ID

39. 39 Packet switching - datagram v/s virtual circuits zThe datagram service, coupled with internal datagram operation, allows for efficient use of the network zThere is no call setup and no need to hold up packets while a packet in error is retransmitted zThe virtual circuit service can provide end-to- end sequencing and error control

40. 40 Packet switching - datagram v/s virtual circuits z virtual circuit is attractive for supporting connection-oriented applications such as file transfer and remote terminal access zIn practice, the virtual circuit service is much more common than the datagram service zThe reliability and convenience of a connection-oriented service is seen as more attractive than the benefits of the datagram service

41. 41 Routing zComplex, crucial aspect of packet switched networks zCharacteristics required yCorrectness ySimplicity yRobustness yStability yFairness yOptimality yEfficiency

42. 42 Routing Performance Criteria zThe selection of a route is generally based on some performance criterion. zMinimum hop zLeast cost Routing yCost is associated with each link yCost could be inversely related to the data rate yUsing some algorithm zDelay zThroughput

43. 43 Routing Decision Time and Place zRouting decisions are made on the basis of some performance criterion. Two key characteristics of the decision are the time and place that the decision is made. zDecision time is determined by whether the routing decision is made on a packet or virtual circuit basis. zWhen the internal operation of the network is datagram, a routing decision is made individually for each packet. zFor the internal virtual circuit operation, a routing decision is made at the time the virtual circuit is established

44. 44 Routing Decision Time and Place zThe term decision place refers to which node or nodes in the network are responsible for the routing decision. yDistributed xMade by each node (more complex but more robust) yCentralized (network control centre) xLoss of the control centre may block operation of the network ySource routing x(decision is actually made by the source station rather than by a network node)

45. 45 Network Information Source zRouting decisions usually based on knowledge of network (not always) zSome strategies use no such information and yet manage to get packets through; flooding and some random strategies zDistributed routing yNodes use local knowledge yMay collect info from adjacent nodes yMay collect info from all nodes on a potential route zCentral routing yCentral node collect info from all nodes

46. 46 Network Information Update Timing zA related concept is that of information update timing yWhich is a function of both the information source and the routing strategy yIf no information is used (as in flooding), there is no information to update yFixed strategy – information is never updated yAdaptive strategy – information is updated from time to time to enable the routing decision to adapt to changing decision (regular updates)

47. 47 Elements of Routing Techniques for Packet-Switching Networks zPerformance Criteria yNumber of Hops yCost yDelay yThroughput zDecision time yPacket (Datagram) ySession (Virtual Circuit) zDecision Place yEach node (distributed) yCentral node (Centralized) yOriginating node (Source) z Network Information Source yNone yLocal yAdjacent node yNode along route yAll nodes z Network information update yContinuous yPeriodic yMajor load change yTopology change

48. 48 Routing Strategies zFixed zFlooding zRandom zAdaptive

49. 49 Fixed Routing zSingle permanent route for each source to destination pair zDetermine routes using a least cost algorithm (appendix 10A) zRoute fixed, at least until a change in network topology

50. 50 Fixed Routing Tables

51. 51 Fixed Routing zWith fixed routing, there is no difference between routing for datagrams and virtual circuits zAll packets from a given source to a given destination follow the same route zThe advantage of fixed routing is simplicity and it should work well in a reliable network with a stable load zIts disadvantage is its lack of flexibility. It does not react to network congestion or failures

52. 52 Flooding zNo network info required zPacket sent by node to every neighbor zIncoming packets retransmitted on every link except incoming link zEventually a number of copies will arrive at destination zEach packet is uniquely numbered so duplicates can be discarded zNodes can remember packets already forwarded to keep network load in bounds zCan include a hop count in packets

53. 53 Flooding Example

54. 54 Properties of Flooding zAll possible routes between source and destination are tried yVery robust (used to send emergency messages) zBecause all routes are tried at least one copy of the packet to arrive at the destination will have used a minimum hop route zAll nodes are visited yUseful to distribute information (e.g. routing) zThe principal disadvantage of flooding is the high traffic load

55. 55 Random Routing zRandom routing has the simplicity and robustness of flooding with far less traffic load zNode selects one outgoing path for retransmission of incoming packet zSelection can be random or round robin zCan select outgoing path based on probability calculation zNo network info needed zRoute is typically not least cost nor minimum hop

56. 56 Adaptive Routing zUsed by almost all packet switching networks zRouting decisions change as conditions on the network change yFailure xWhen a node or trunk fails, it can no longer be used as part of a route yCongestion xPortion of a network is heavily congested, packets route around rather than the area of congestion zRequires info about network zDecisions more complex (processing burden on network nodes increases)

57. 57 Adaptive Routing - Advantages zReacting too quickly, causing congestion- producing oscillation, or too slowly, being irrelevant zImproved performance, as seen by the network user zAn adaptive routing strategy can aid in congestion control

58. 58 ARPANET Routing Strategies(1) zFirst Generation y1969 yDistributed adaptive yEstimated delay as performance criterion yBellman-Ford algorithm yNode exchanges delay vector with neighbors yUpdate routing table based on incoming info yDoesn't consider line speed, just queue length yQueue length not a good measurement of delay yResponds slowly to congestion

59. 59 ARPANET Routing Strategies(2) zSecond Generation y1979 yUses delay as performance criterion yDelay measured directly yUses Dijkstra’s algorithm yGood under light and medium loads yUnder heavy loads, little correlation between reported delays and those experienced

60. 60 ARPANET Routing Strategies(3) zThird Generation y1987 yLink cost calculations changed yMeasure average delay over last 10 seconds yNormalize based on current value and previous results

61. 61 Costing of Routes

62. 62 Dijkstra’s Algorithm zDefine: zN = set of all nodes in the network zs = source node zM = set of nodes so far incorporated by the algorithm zd ij = link cost from node i to node j; d ii = 0 and d ij =  if the two nodes are not directly connected; d ij  0 if the two nodes are directly connected zD n = cost of the least cost path from node s to node n that is currently known to the algorithm

63. 63

64. 64 Example Algorithm Dijkstra

65. 65 Results for Dijkstra Algorithm It er ati on TL(2)PathL(3)PathL(4)PathL(5)PathL(6 ) Path 1{1}21–251-311–4  -  - 2{1,4}21–241-4-311–421-4–5  - 3{1, 2, 4}21–241-4-311–421-4–5  - 4{1, 2, 4, 5} 21–231-4-5– 3 11–421-4–541-4-5–6 5{1, 2, 3, 4, 5} 21–231-4-5– 3 11–421-4–541-4-5–6 6{1, 2, 3, 4, 5, 6} 21-231-4-5- 3 11-421-4–541-4-5-6

66. 66 Bellman-Ford Algorithm Definitions zFind shortest paths from given node subject to constraint that paths contain at most one link zFind the shortest paths with a constraint of paths of at most two links zAnd so on zs =source node zw(i, j) = link cost from node i to node j yw(i, i) = 0 yw(i, j) =  if the two nodes are not directly connected yw(i, j)  0 if the two nodes are directly connected zh =maximum number of links in path at current stage of the algorithm zL h (n) = cost of least-cost path from s to n under constraint of no more than h links

67. 67 Example of Bellman-Ford Algorithm

68. 68 Results of Bellman-Ford Example hL h (2 ) Pat h L h (3 ) PathL h (4 ) Pat h L h (5 ) PathL h (6 ) Path 0  -  -  -  -  - 121-251-311-4  -  - 221-241-4-311-421-4- 5 101-3-6 321-231-4-5- 3 11-421-4- 5 41-4-5-6 421-231-4-5- 3 11-421-4- 5 41-4-5-6

69. 69 Comparison zResults from two algorithms agree zInformation gathered yBellman-Ford xCalculation for node n involves knowledge of link cost to all neighboring nodes plus total cost to each neighbor from s xEach node can maintain set of costs and paths for every other node xCan exchange information with direct neighbors xCan update costs and paths based on information from neighbors and knowledge of link costs yDijkstra xEach node needs complete topology xMust know link costs of all links in network xMust exchange information with all other nodes

70. 70 Evaluation zDependent on processing time of the algorithms and the amount of information that must be collected from other nodes in the network zThe evaluation will be depend on the implementation approach and the specific implementation zBoth algorithms are known to converge under static conditions of topology and link costs and will converge to the same solution zIf the link costs change over time, the algorithms will attempt to catch up with these changes zHowever, if the link costs depend on traffic, which in turn depends on routes chosen, then a feedback conditions exists, and instabilities may result

71. 71 Packet Switching Evolution zX.25 packet-switched network zRouter-based networking zSwitching vs. routing zFrame relay network zATM network

72. 72 Switching v/s Routing zSwitching zpath set up at connection time zsimple table look up ztable maintainance via signaling zno out of sequence delivery zlost path may lose connection zmuch faster than pure routing zlink decision made ahead of time, and resources allocated then z Routing z can work as connectionless z complex routing algorithm z table maintainance via protocol z out of sequence delivery likely z robust: no connections lost z significant processing delay z output link decision based on packet header contents - at every node