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  • William StallingsData and Computer Communications7th Edition

    Chapter 10

    Circuit Switching and Packet Switching

  • Switching Networks

    Long distance transmission is typically done over a network of switched nodesNodes not concerned with content of dataEnd devices are stationsComputer, terminal, phone, etc.A collection of nodes and connections is a communications networkData routed by being switched from node to node

  • Nodes

    Nodes may connect to other nodes onlystations and other nodesNode to node links usually multiplexedNetwork is usually partially (not fully) connectedSome redundant connections are desirable for reliabilityTwo different switching technologiesCircuit switchingPacket switching

  • Simple Switched Network

  • Circuit Switching

    Dedicated communication path between two stationsThree phasesEstablishTransferDisconnectMust have switching capability and channel capacity to establish connectionMust have intelligence to work out routing

  • Circuit Switching - Applications

    InefficientChannel capacity dedicated for duration of connectionIf no data, capacity wastedSet up (connection) takes timeOnce connected, transfer is transparentDeveloped for voice traffic (phone)

  • Public Circuit Switched Network

  • Telecomms Components

    SubscriberDevices attached to networkSubscriber lineLocal LoopSubscriber loopConnection to networkFew km up to few tens of kmExchangeSwitching centersEnd office - supports subscribersTrunksBranches between exchangesMultiplexed

  • Circuit Establishment

  • Circuit Switch Elements

  • Circuit Switching Concepts

    Digital SwitchProvide transparent signal path between devicesNetwork InterfaceControl UnitEstablish connectionsGenerally on demandHandle and acknowledge requestsDetermine if destination is freeconstruct pathMaintain connectionDisconnect

  • Blocking or Non-blocking

    BlockingA network is unable to connect stations because all paths are in useA blocking network allows thisUsed on voice systemsShort duration callsNon-blockingPermits all stations to connect (in pairs) at onceUsed for some data connections

  • Space Division Switching

    Developed for analog environmentSeparate physical pathsCrossbar switchNumber of crosspoints grows as square of number of stationsLoss of crosspoint prevents connectionThere is only one way to connect two stationsInefficient use of crosspointsAll stations connected, only a few crosspoints in useNon-blocking

  • Space Division Switch

  • Multistage Switch

    Reduced number of crosspointsMore than one path through networkIncreased reliabilityMore complex controlMay be blocking

  • Three Stage Space Division Switch

  • Time Division Switching

    Modern digital systems rely on intelligent control of space and time division elementsUse digital time division techniques to set up and maintain virtual circuitsPartition low speed bit stream into pieces that share higher speed stream

  • Control Signaling Functions

    Audible communication with subscriberTransmission of dialed numberCall can not be completed indicationCall ended indicationSignal to ring phoneBilling infoEquipment and trunk status infoDiagnostic infoControl of special equipment

    * DTMF

  • Control Signal Sequence

    Both phones on hookSubscriber lifts receiver (off hook)End office switch signaledSwitch responds with dial toneCaller dials numberIf target not busy, send ringer signal to target subscriberFeedback to callerRinging tone, engaged tone, unobtainableTarget accepts call by lifting receiverSwitch terminates ringing signal and ringing toneSwitch establishes connectionConnection release when source or target subscriber hangs up

  • Switch to Switch Signaling

    Subscribers connected to different switchesOriginating switch seizes interswitch trunkSend off-hook signal on trunk, requesting digit register at target switch (for address)Terminating switch sends off-hook followed by on-hook (wink) to show register readyOriginating switch sends address

  • Location of Signaling

    Subscriber to networkDepends on subscriber device and switchWithin networkManagement of subscriber calls and networkmore complex

  • In Channel Signaling

    Use same channel for signaling and callRequires no additional transmission facilitiesInbandUses same frequencies as voice signalCan go anywhere a voice signal canImpossible to set up a call on a faulty speech pathOut of bandVoice signals do not use full 4kHz bandwidthNarrow signal band within 4kHz used for controlCan be sent whether or not voice signals are presentNeed extra electronicsSlower signal rate (narrow bandwidth)

  • Drawbacks of In Channel Signaling

    Limited transfer rateDelay between entering address (dialing) and connectionOvercome by use of common channel signaling

  • Common Channel Signaling

    Control signals carried over paths independent of voice channelOne control signal channel can carry signals for a number of subscriber channelsCommon control channel for these subscriber linesAssociated ModeCommon channel closely tracks interswitch trunks Disassociated ModeAdditional nodes (signal transfer points)Effectively two separate networks

  • Common vs. In Channel Signaling

    Fig. 10.7

  • CommonChannelSignaling Modes

    Fig. 10.8

  • Signaling System Number 7

    SS7Common channel signaling schemeISDNOptimized for 64k digital channel networkCall control, remote control, management and maintenanceReliable means of transfer of info in sequenceWill operate over analog and below 64kbpsPoint to point terrestrial and satellite links

  • SS7 Signaling Network Elements

    Signaling point (SP)Any point in the network capable of handling SS7 control messageSignal transfer point (STP)A signaling point capable of routing control messagesControl planeResponsible for establishing and managing connectionsInformation planeOnce a connection is set up, info is transferred in the information plane

  • Transfer Points

    Fig. 10.9

  • Signaling Network Structures

    STP capacitiesNumber of signaling links that can be handledMessage transfer timeThroughput capacityNetwork performanceNumber of SPsSignaling delaysAvailability and reliabilityAbility of network to provide services in the face of STP failures

  • Softswitch Architecture

    General purpose computer running software to make it a smart phone switchLower costsGreater functionalityPacketizing of digitized voice dataAllowing voice over IPMost complex part of telephone network switch is software controlling call processCall routingCall processing logicTypically running on proprietary processorSeparate call processing from hardware function of switchMedia gateway control protocolPhysical switching done by media gatewayCall processing done by media gateway controller

  • Traditional Circuit Switching

  • Softswitch

  • Packet Switching Principles

    Circuit switching designed for voiceResources dedicated to a particular callData rate is fixed (and symmetric)Both ends must operate at the same rateFor data connections, the circuit switching technique is inefficient.Much of the time a data connection is idleData rate is varying (and asymmetric)Best-effort

  • Basic Operation

    Data 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

  • Use of Packets

  • Advantages

    Line efficiencySingle node to node link can be shared by many packets over timePackets queued and transmitted as fast as possibleSimilar to Asynchronous (statistical) TDMData 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

  • Switching Technique

    Station breaks long message into packetsPackets sent one at a time to the networkPackets handled in two waysDatagramVirtual circuit

  • Datagram

    Each 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

  • DatagramDiagram

  • Virtual Circuit

    Preplanned 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 bandwidth

  • VirtualCircuitDiagram

  • Virtual Circuits vs Datagram

    Virtual circuitsNetwork can provide sequencing and error controlPackets are forwarded more quicklyNo routing decisions to makeLess reliablefailure of a node loses all circuits through that nodeDatagramNo call setup phaseBetter if few packetsMore flexibleRouting can be used to avoid congested/failed parts of the network

  • Packet Size

    Fig. 10.14

  • Circuit vs Packet Switching

    PerformancePropagation delayTransmission timeNode (processing) delay

  • Event Timing

  • X.25

    1976Interface between host and packet switched networkAlmost universal on packet switched networks and packet switching in ISDNDefines three layersPhysicalLinkPacket

  • X.25 - Physical

    Interface between attached station and link to nodeData terminal


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