William StallingsData and Computer Communications
اسلاید 1: William Stallings Data and Computer CommunicationsChapter 10Packet Switching
اسلاید 2: PrinciplesCircuit switching designed for voiceResources dedicated to a particular callMuch of the time a data connection is idleData rate is fixedBoth ends must operate at the same rate
اسلاید 3: Basic OperationData transmitted in small packetsTypically 1000 octetsLonger messages split into series of packetsEach packet contains a portion of user data plus some control infoControl infoRouting (addressing) infoPackets are received, stored briefly (buffered) and past on to the next nodeStore and forward
اسلاید 4: Use of Packets
اسلاید 5: AdvantagesLine efficiencySingle node to node link can be shared by many packets over timePackets queued and transmitted as fast as possibleData rate conversionEach station connects to the local node at its own speedNodes buffer data if required to equalize ratesPackets are accepted even when network is busyDelivery may slow downPriorities can be used
اسلاید 6: Switching TechniqueStation breaks long message into packetsPackets sent one at a time to the networkPackets handled in two waysDatagramVirtual circuit
اسلاید 7: DatagramEach packet treated independentlyPackets can take any practical routePackets may arrive out of orderPackets may go missingUp to receiver to re-order packets and recover from missing packets
اسلاید 8: Virtual CircuitPreplanned route established before any packets sentCall request and call accept packets establish connection (handshake)Each packet contains a virtual circuit identifier instead of destination addressNo routing decisions required for each packetClear request to drop circuitNot a dedicated path
اسلاید 9: Virtual Circuits v DatagramVirtual circuitsNetwork can provide sequencing and error controlPackets are forwarded more quicklyNo routing decisions to makeLess reliableLoss of a node looses all circuits through that nodeDatagramNo call setup phaseBetter if few packetsMore flexibleRouting can be used to avoid congested parts of the network
اسلاید 10: Packet Size
اسلاید 11: Circuit v Packet SwitchingPerformancePropagation delayTransmission timeNode delay
اسلاید 12: Event Timing
اسلاید 13: External and Internal OperationPacket switching - datagrams or virtual circuitsInterface between station and network nodeConnection orientedStation requests logical connection (virtual circuit)All packets identified as belonging to that connection & sequentially numberedNetwork delivers packets in sequenceExternal virtual circuit servicee.g. X.25Different from internal virtual circuit operationConnectionlessPackets handled independentlyExternal datagram serviceDifferent from internal datagram operation
اسلاید 14: Combinations (1)External virtual circuit, internal virtual circuitDedicated route through networkExternal virtual circuit, internal datagramNetwork handles each packet separatelyDifferent packets for the same external virtual circuit may take different internal routesNetwork buffers at destination node for re-ordering
اسلاید 15: Combinations (2)External datagram, internal datagramPackets treated independently by both network and userExternal datagram, internal virtual circuitExternal user does not see any connectionsExternal user sends one packet at a timeNetwork sets up logical connections
اسلاید 16: External Virtual Circuit and Datagram Operation
اسلاید 17: Internal Virtual Circuit and Datagram Operation
اسلاید 18: RoutingComplex, crucial aspect of packet switched networksCharacteristics requiredCorrectnessSimplicityRobustnessStabilityFairnessOptimalityEfficiency
اسلاید 19: Performance CriteriaUsed for selection of routeMinimum hopLeast costSee Stallings appendix 10A for routing algorithms
اسلاید 20: Costing of Routes
اسلاید 21: Decision Time and PlaceTimePacket or virtual circuit basisPlaceDistributedMade by each nodeCentralizedSource
اسلاید 22: Network Information Source and Update TimingRouting decisions usually based on knowledge of network (not always)Distributed routingNodes use local knowledgeMay collect info from adjacent nodesMay collect info from all nodes on a potential routeCentral routingCollect info from all nodesUpdate timingWhen is network info held by nodes updatedFixed - never updatedAdaptive - regular updates
اسلاید 23: Routing StrategiesFixedFloodingRandomAdaptive
اسلاید 24: Fixed RoutingSingle permanent route for each source to destination pairDetermine routes using a least cost algorithm (appendix 10A)Route fixed, at least until a change in network topology
اسلاید 25: Fixed Routing Tables
اسلاید 26: FloodingNo network info requiredPacket sent by node to every neighborIncoming packets retransmitted on every link except incoming linkEventually a number of copies will arrive at destinationEach packet is uniquely numbered so duplicates can be discardedNodes can remember packets already forwarded to keep network load in boundsCan include a hop count in packets
اسلاید 27: Flooding Example
اسلاید 28: Properties of FloodingAll possible routes are triedVery robustAt least one packet will have taken minimum hop count routeCan be used to set up virtual circuitAll nodes are visitedUseful to distribute information (e.g. routing)
اسلاید 29: Random RoutingNode selects one outgoing path for retransmission of incoming packetSelection can be random or round robinCan select outgoing path based on probability calculationNo network info neededRoute is typically not least cost nor minimum hop
اسلاید 30: Adaptive RoutingUsed by almost all packet switching networksRouting decisions change as conditions on the network changeFailureCongestionRequires info about networkDecisions more complexTradeoff between quality of network info and overheadReacting too quickly can cause oscillationToo slowly to be relevant
اسلاید 31: Adaptive Routing - AdvantagesImproved performanceAid congestion control (See chapter 12)Complex systemMay not realize theoretical benefits
اسلاید 32: ClassificationBased on information sourcesLocal (isolated)Route to outgoing link with shortest queueCan include bias for each destinationRarely used - do not make use of easily available infoAdjacent nodesAll nodes
اسلاید 33: Isolated Adaptive Routing
اسلاید 34: ARPANET Routing Strategies(1)First Generation1969Distributed adaptiveEstimated delay as performance criterionBellman-Ford algorithm (appendix 10a)Node exchanges delay vector with neighborsUpdate routing table based on incoming infoDoesnt consider line speed, just queue lengthQueue length not a good measurement of delayResponds slowly to congestion
اسلاید 35: ARPANET Routing Strategies(2)Second Generation1979Uses delay as performance criterionDelay measured directlyUses Dijkstra’s algorithm (appendix 10a)Good under light and medium loadsUnder heavy loads, little correlation between reported delays and those experienced
اسلاید 36: ARPANET Routing Strategies(3)Third Generation1987Link cost calculations changedMeasure average delay over last 10 secondsNormalize based on current value and previous results
اسلاید 37: X.251976Interface between host and packet switched networkAlmost universal on packet switched networks and packet switching in ISDNDefines three layersPhysicalLinkPacket
اسلاید 38: X.25 - PhysicalInterface between attached station and link to nodeData terminal equipment DTE (user equipment)Data circuit terminating equipment DCE (node)Uses physical layer specification X.21Reliable transfer across physical linkSequence of frames
اسلاید 39: X.25 - LinkLink Access Protocol Balanced (LAPB)Subset of HDLCsee chapter 7
اسلاید 40: X.25 - PacketExternal virtual circuitsLogical connections (virtual circuits) between subscribers
اسلاید 41: X.25 Use of Virtual Circuits
اسلاید 42: Virtual Circuit ServiceVirtual CallDynamically establishedPermanent virtual circuitFixed network assigned virtual circuit
اسلاید 43: Virtual Call
اسلاید 44: Packet Format
اسلاید 45: MultiplexingDTE can establish 4095 simultaneous virtual circuits with other DTEs over a single DTC-DCE linkPackets contain 12 bit virtual circuit number
اسلاید 46: Virtual Circuit Numbering
اسلاید 47: Flow and Error ControlHDLS (Chapter 7)
اسلاید 48: Packet SequencesComplete packet sequencesAllows longer blocks of data across network with smaller packet size without loss of block integrityA packetsM bit 1, D bit 0B packetsThe restZero or more A followed by B
اسلاید 49: Reset and RestartResetReinitialize virtual circuitSequence numbers set to zeroPackets in transit lostUp to higher level protocol to recover lost packetsTriggered by loss of packet, sequence number error, congestion, loss of network internal virtual circuitRestartEquivalent to a clear request on all virtual circuitsE.g. temporary loss of network access
اسلاید 50: Required ReadingStalling Chapter 10X.25 info from ITU-T web siteRouting information from Comer
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