Circuit Switching Reading: 3.1.2, 3.3, 4.5, and 6.5Goals of Today’s LectureCircuit Switching (e.g., Phone Network)Timing in Circuit SwitchingCircuit Switching With Human OperatorCircuit Switching: Multiplexing a LinkAdvantages of Circuit SwitchingDisadvantages of Circuit SwitchingVirtual Circuit (VC)Establishing the CircuitVirtual Circuit Identifier (VC ID)Swapping the Label at Each HopVirtual Circuits Similar to IP DatagramsVirtual Circuits Differ from IP DatagramsQuality of ServiceAdmission ControlAdmission Control: FlowspecSpecifying Bursty TrafficLeaky Bucket Traffic ModelService Requested From the NetworkEnsuring the Source BehavesEnforcing BehaviorLink Scheduling: FIFOLink Scheduling: Strict PriorityLink Scheduling: Weighted FairnessLink Schedulers: Trade-OffsRouting in Virtual Circuit NetworksQuality-of-Service RoutingSource-Directed QoS RoutingQoS Routing: Link-State AdvertisementsQoS Routing: Example Path SelectionAsynchronous Transfer Mode (ATM)Picking the ATM Cell SizeInterfacing to End HostsInferring the Need for a Virtual CircuitGrouping IP Packets Into FlowsChallenges for IP Over ATMATM TodayDifferentiated Services in IPExample Packet ClassifierReal Guarantees?Example Uses of QoS TodayConclusions1Circuit SwitchingReading: 3.1.2, 3.3, 4.5, and 6.5 COS 461: Computer NetworksSpring 2006 (MW 1:30-2:50 in Friend 109)Jennifer RexfordTeaching Assistant: Mike Wawrzoniak http://www.cs.princeton.edu/courses/archive/spring06/cos461/2Goals of Today’s Lecture•Circuit switching–Establish, transfer, and teardown–Comparison with packet switching–Virtual circuits as a hybrid scheme•Quality of service in virtual-circuit networks–Traffic specification and enforcement–Admission control and resource reservation–Link scheduling (FIFO, priority, and weighted fairness)–Path selection (quality-of-service routing)•Quality of service for IP traffic–IP over virtual circuits–Differentiated services3Circuit Switching (e.g., Phone Network)•Establish: source creates circuit to destination–Node along the path store connection info–Nodes may reserve resources for the connection•Transfer: source sends data over the circuit–No destination address, since nodes know path•Teardown: source tears down circuit when done4Timing in Circuit SwitchingInformationCircuit Establishment Transfer Circuit Teardown Host 1 Host 2Switch 1 Switch 2propagation delay between Host 1 and Switch1propagation delay between Host 1 and Host 2Transmission delaytime5Circuit Switching With Human Operator6Circuit Switching: Multiplexing a Link•Time-division–Each circuit allocated certain time slots•Frequency-division–Each circuit allocated certain frequenciestimefrequencytime7Advantages of Circuit Switching•Guaranteed bandwidth –Predictable communication performance–Not “best-effort” delivery with no real guarantees•Simple abstraction–Reliable communication channel between hosts–No worries about lost or out-of-order packets•Simple forwarding –Forwarding based on time slot or frequency–No need to inspect a packet header•Low per-packet overhead–Forwarding based on time slot or frequency–No IP (and TCP/UDP) header on each packet8Disadvantages of Circuit Switching•Wasted bandwidth–Bursty traffic leads to idle connection during silent period–Unable to achieve gains from statistical multiplexing•Blocked connections–Connection refused when resources are not sufficient–Unable to offer “okay” service to everybody•Connection set-up delay –No communication until the connection is set up–Unable to avoid extra latency for small data transfers•Network state–Network nodes must store per-connection information–Unable to avoid per-connection storage and state9Virtual Circuit (VC)•Hybrid of packets and circuits–Circuits: establish and teardown along end-to-end path–Packets: divide the data into packets with identifiers•Packets carry a virtual-circuit identifier–Associates each packet with the virtual circuit–Determines the next link along the path•Intermediate nodes maintain state VC–Forwarding table entry–Allocated resources10Establishing the Circuit•Signaling–Creating the entries in the forwarding tables–Reserving resources for the virtual circuit, if needed•Two main approaches to signaling–Network administrator configures each node–Source sends set-up message along the path•Set-up latency–Time for the set-up message to traverse the path–… and return back to the source•Routing–End-to-end path is selected during circuit set-up11Virtual Circuit Identifier (VC ID)•Virtual Circuit Identifier (VC ID)–Source set-up: establish path for the VC–Switch: mapping VC ID to an outgoing link–Packet: fixed length label in the header 121: 72: 7link 71: 142: 8link 14link 812Swapping the Label at Each Hop•Problem: using VC ID along the whole path–Each virtual circuit consumes a unique ID–Starts to use up all of the ID space in the network•Label swapping–Map the VC ID to a new value at each hop–Table has old ID, and next link and new ID121: 7, 202: 7, 53link 720: 14, 7853: 8, 42link 14link 813Virtual Circuits Similar to IP Datagrams•Data divided in to packets–Sender divides the data into packets–Packet has address (e.g., IP address or VC ID)•Store-and-forward transmission–Multiple packets may arrive at once–Need buffer space for temporary storage •Multiplexing on a link –No reservations: statistical multiplexing Packets are interleaved without a fixed pattern–Reservations: resources for group of packetsGuarantees to get a certain number of “slots”14Virtual Circuits Differ from IP Datagrams •Forwarding look-up–Virtual circuits: fixed-length connection id–IP datagrams: destination IP address•Initiating data transmission–Virtual circuits: must signal along the path–IP datagrams: just start sending packets•Router state–Virtual circuits: routers know about connections–IP datagrams: no state, easier failure recovery•Quality of service–Virtual circuits: resources and scheduling per VC–IP datagrams: difficult to provide QoS15Quality of Service•Allocating resources to the virtual circuit–E.g., guaranteed bandwidth on each link in the path–E.g., guaranteeing a maximum delay along the path•Admission control–Check during signaling that the resources are available–Saying “no” if they are not, and reserving them if they
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