交換技術TheNetworkCoremeshofinterconnectedroutersthefundamentalquestion:howisdatatransferredthroughnet?circuitswitching:dedicatedcircuitpercall:telephonenetpacket-switching:datasentthrunetindiscrete“chunks”(message-switching)CircuitSwitchingEnd-endresourcesreservedfor“call”linkbandwidth,switchcapacitydedicatedresources:nosharingcircuit-like(guaranteed)performancecallsetuprequiredCircuitSwitchingnetworkresources(e.g.,bandwidth)dividedinto“pieces”piecesallocatedtocallsresourcepieceidleifnotusedbyowningcall(nosharing)dividinglinkbandwidthinto“pieces”frequencydivisiontimedivisionCircuitSwitching:FDMandTDMFDMfrequencytimeTDMfrequencytime4usersExample:PacketSwitchingeachend-enddatastreamdividedintopacketsuserA,Bpacketssharenetworkresourceseachpacketusesfulllinkbandwidthresourcesusedasneeded

Bandwidthdivisioninto“pieces”DedicatedallocationResourcereservationresourcecontention:

aggregateresourcedemandcanexceedamountavailablecongestion:packetsqueue,waitforlinkusestoreandforward:packetsmoveonehopatatimeNodereceivescompletepacketbeforeforwardingPacket-switching:store-and-forwardtakesL/Rsecondstotransmit(pushout)packetofLbitsontolinkatRbpsstoreandforward:entirepacketmustarriveatrouterbeforeitcanbetransmittedonnextlinkdelay=3L/R(assumingzeropropagationdelay)Example:L=7.5MbitsR=1.5Mbpstransmissiondelay=15secRRRLmoreondelayshortly…Packetswitchingversuscircuitswitching1Mb/slinkeachuser:100kb/swhen“active”active10%oftimecircuit-switching:

10userspacketswitching:

with35users,probability>10activeatsametimeislessthan.0004Packetswitchingallowsmoreuserstousenetwork!Nusers1MbpslinkQ:howdidwegetvalue0.0004?Packetswitchingversuscircuitswitchinggreatforburstydataresourcesharingsimpler,nocallsetupexcessivecongestion:packetdelayandlossprotocolsneededforreliabledatatransfer,congestioncontrolQ:Howtoprovidecircuit-likebehavior?bandwidthguaranteesneededforaudio/videoappsstillanunsolvedproblemIspacketswitchinga“slamdunkwinner?”Q:humananalogiesofreservedresources(circuit