Lecture 13 Virtual Circuits, ATMPacket SwitchingCircuit SwitchingCircuit Switching DiscussionVirtual CircuitsPacket Switching and Virtual Circuits: SimilaritiesVirtual Circuits Versus Packet SwitchingSlide 8Packet switched vs. VCVirtual CircuitConnections and SignalingVirtual Circuit Switching: Label (“tag”) SwappingLabel (“tag”) SwappingPVC connection setupSVC Connection SetupVirtual Circuits In PracticeAsynchronous Transfer Mode: ATMCell SwitchingATM FeaturesWhy 53 Bytes?ATM Adaptation LayersAAL5 Adaptation LayerATM Packet Shredder EffectIP over ATMIP over ATM Static VCsATM DiscussionATM + IPIP SwitchingIP Switching ExampleSlide 30Slide 31Another ViewIP Switching DiscussionAn Alternative Tag SwitchingIP Switching versus Tag Switching1Lecture 13Virtual Circuits, ATM15-441 Networking, Fall 2007These slides swiped, with pride, from Dave Anderson, 15-441 Spring 20062Packet SwitchingSource sends information as self-contained packets that have an address.»Source may have to break up single message in multipleEach packet travels independently to the destination host.»Routers and switches use the address in the packet to determine how to forward the packetsDestination recreates the message.Analogy: a letter in surface mail.3Circuit SwitchingSource first establishes a connection (circuit) to the destination.»Each router or switch along the way may reserve some bandwidth for the data flowSource sends the data over the circuit.»No need to include the destination address with the data since the routers know the pathThe connection is torn down.Example: telephone network.4Circuit Switching DiscussionTraditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth.»Physical connection - clearly no need to include addresses with the dataAdvantages, relative to packet switching:»Implies guaranteed bandwidth, predictable performance »Simple switch design: only remembers connection information, no longest-prefix destination address look upDisadvantages:»Inefficient for bursty traffic (wastes bandwidth)»Delay associated with establishing a circuitCan we get the advantages without (all) the disadvantages?5Virtual CircuitsEach wire carries many “virtual” circuits. »Forwarding based on virtual circuit (VC) identifier– IP header: src, dst, etc.–Virtual circuit header: just “VC”»A path through the network is determined for each VC when the VC is established»Use statistical multiplexing for efficiencyCan support wide range of quality of service.»No guarantees: best effort service»Weak guarantees: delay < 300 msec, …»Strong guarantees: e.g. equivalent of physical circuit6Packet Switching andVirtual Circuits: Similarities“Store and forward” communication based on an address.»Address is either the destination address or a VC identifierMust have buffer space to temporarily store packets.»E.g. multiple packets for some destination arrive simultaneouslyMultiplexing on a link is similar to time sharing.»No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern»Reservations: some flows are guaranteed to get a certain number of “slots”AB ACBD7Virtual Circuits Versus Packet Switching Circuit switching:»Uses short connection identifiers to forward packets»Switches know about the connections so they can more easily implement features such as quality of service»Virtual circuits form basis for traffic engineering: VC identifies long-lived stream of data that can be scheduledPacket switching:»Use full destination addresses for forwarding packets»Can send data right away: no need to establish a connection first»Switches are stateless: easier to recover from failures»Adding QoS is hard»Traffic engineering is hard: too many packets!8Circuit SwitchingInput PortsOutput PortsSwitchConnects (electrons or bits) ports to ports9Packet switched vs. VCABR2R1R3R4R1 packet forwarding table:Dst R2R1 VC table:VC 1 R2VC 2 R3Different paths to same destination!(useful for traffic engineering!)VCIVCIPayloadDstDstPayloadDst123433311122444210Virtual CircuitABR2R1R3R4R1 VC table:VC 5 R2VCIVCIPayloadPayloadDst1234333111224442R2 VC table:VC 5 R4Challenges: - How to set up path? - How to assign IDs??11Connections and SignalingPermanent vs. switched virtual connections (PVCs, SVCs)»static vs. dynamic. PVCs last “a long time”–E.g., connect two bank locations with a PVC that looks like a circuit–SVCs are more like a phone call»PVCs administratively configured (but not “manually”)»SVCs dynamically set up on a “per-call” basisTopology»point to point»point to multipoint»multipoint to multipointChallenges:»How to configure these things?–What VCI to use?–Setting up the path12Virtual Circuit Switching:Label (“tag”) SwappingGlobal VC ID allocation -- ICK! Solution: Per-link uniqueness. Change VCI each hop. Input Port Input VCI Output Port Output VCI R1: 1 5 3 9 R2: 2 9 4 2R4: 1 2 3 5ABR2R1R3R4 Dst123433311122444213Label (“tag”) SwappingResult: Signalling protocol must only find per-link unused VCIs.»“Link-local scope”»Connection setup can proceed hop-by-hop.–Good news for our setup protocols!14PVC connection setupManual?»Configure each switch by hand. Ugh.Dedicated signalling protocol»E.g., what ATM usesPiggyback on routing protocols»Used in MPLS. E.g., use BGP to set up15SVC Connection Setupcallingpartynetwork calledpartySETUPSETUPCONNECTACKCONNECTACKCONNECTCONNECT16Virtual Circuits In PracticeATM: Teleco approach»Kitchen sink. Based on voice, support file transfer, video, etc., etc.»Intended as IP replacement. That didn’t happen. :)»Today: Underlying network protocol in many teleco networks. E.g., DSL speaks ATM. IP over ATM in some cases.MPLS: The “IP Heads” answer to ATM»Stole good ideas from ATM»Integrates well with IP»Today: Used inside some networks to provide VPN support, traffic engineering, simplify core.Other nets just run IP.Older tech: Frame Relay»Only provided PVCs. Used for quasi-dedicated 56k/T1 links between offices, etc. Slower, less flexible than ATM.17Asynchronous Transfer Mode: ATMConnection-oriented, packet-switched»(e.g., virtual circuits).Teleco-driven. Goals:»Handle voice, data, multimedia»Support both PVCs and
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