Circuit Switching

Switches: hardware and/or software devices allowingtemporary connections between two or more devices...
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Circuit Switching CircuitSwitching by Dr.NguyenMinhHoang hoang.dhbk@gmail.com March2007 References Referencebooks:  DataCommunicationsandNetworking,4/e,byBehrouz A.Forouzan,DeAnzaCollege (http://highered.mcgraw hill.com/sites/0072967757/information_center_view0/)  Data&ComputerCommunicationsbyWilliamStallings (http://williamstallings.com/DCC/DCC7e.html) Introduction Howcanweconnectmultipledevices?  Pointtopointvs.  Multipoint Bettersolutionswitching Switches:hardwareand/orsoftwaredevicesallowing temporaryconnectionsbetweentwoormoredevices Threemethods:circuit,packet,andmessageExampleofaSwitchedNetwork CircuitSwitching Createsadirectphysicalconnectionbetweentwodevices Originallydevelopedforvoicetraffic Doesnotusepointtopointconnectionsbetweenevery devicetoeveryotherdevice Instead,deviceslinktoaswitchwhichmayinturnlink tootherswitches Allowsforadedicatedconnectionfordurationof transmission CircuitSwitching Spacedivisionswitching Circuitswitching:createadirect Timedivisionswitching physicalconnectionbetween2 TDMBus devicessuchasphonesorcomputers Combinations CircuitSwitchvs.FoldedSwitch ninputs,m outputs nlinesinfullduplexFoldedSwitch:Annbynfoldedswitchallowseverydevicetoconnecttoeveryotherdeviceinfullduplexmode SpaceDivisionSwitch Thepathsinthecircuitare separatedfromeachother spatially Originallyforanalog networks,butnowforboth analoganddigital Crossbarswitch MultistageswitchCrossbarSwitch Inputsandoutputsconnectedina gridusingelectronicmicroswitches (transistors)atcrosspoints Disadvantage??? Toomanycrosspoints,onlyafewused atanygiventime Inefficient;fewerthan25%of switchesareinusedatagiventime MultistageSwitches Combinecrossbarswitchesinseveralstages Devicesarelinkedtoswitchesthatarelinkedtoa hierarchyofotherswitches Middlestagesusuallyhavefewerswitchesthanfirstand laststages Fewercrosspointsbutstillallowingmultiplepaths throughthenetworkincreasedreliabilityMultistageSwitch(1) Asetofcrossbarswitchesinseveralstages Usea15x15crossbar=>225crosspoints Usetheabovemultistage=>78crosspoints MultistageSwitch Multiplepathsavailableforeachpairofdevices=>reliableDisadvantage??? Blockingproblemwhenheavytraffic,experienced inpublicphonesystems Blocking Reducednumberofcrosspointsmaymeanthatat timesofheavytraffic,aninputmaynotbeableto connecttoanoutputifthereisnopathavailable(i.e. allswitchesareoccupied) Doesnotoccurinsinglestageswitch;anonblocking pathisalwaysavailable TimeDivisionSwitches Usestimedivisionmultiplexingtoachieveswitching Usedbymostmodernswitches TDswitchinginvolvespartitioninglowspeedbitstreamintopieces thatsharehigherspeedstream Twomethods:  Timeslotinterchange(TSI)  TDMbus  Basedonsynchronoustimedivisionmultiplexing(TDM)  Eachstationconnectsthroughcontrolledgatestohighspeedbus  Timeslotallowssmallamountofdataontobus  Anotherline’sgateisenabledforoutputatthesametimeTDMwithaTimeSlotInterchange WithoutaTSI,orderisunchanged 1->3, 2->4, 3->1, 4->2 TimeSlotInterchange(TSI)  ConsistsofRAMwith severalmemorylocations  Fillsupwithincomingdata fromtimeslotsinorder received  Slotsaresentoutinorder basedoncontrolunitlogic#memorylocations=#inputs Cannothandleburstysizeofmem.loc.=sizeofatimeslot trafficTDMBus  Inputandoutputlinesare connectedtoahighspeed busthroughinputand outputgates  Controlunitopensand closesgatesasneeded Timevs.SpaceDivisionSwitch Timedivision  Nocrosspoints  Delay Spacedivision  Manycrosspoints  Zerodelay SpaceandTimeDivisionSwitching Combinations Advantageofspacedivisionswitchingisitisinstantaneous; however,requirestoomanycrosspoints Advantageoftimedivisionswitchingisitneedsno crosspoints;however,processingconnectionscreatesdelays Combiningthemoptimizesthenumberofcrosspointsand reducestheamountofdelay  TSS:TimeSpaceSpace  TSST:TimeSpaceSpaceTime  STTS:SpaceTimeTimeSpaceTSTSwitch ...