Page 1Peter A. Steenkiste, SCS, CMU1Lecture 13ATM, SONET, and MPLSPeter SteenkisteSchool of Computer ScienceCarnegie Mellon University15-441 Networking, Spring 2006http://www.cs.cmu.edu/~prs/15-441Peter A. Steenkiste, SCS, CMU2Outline● Circuit switching.● ATM overview.● SONET.● MPLS.Peter A. Steenkiste, SCS, CMU3Packet Switching● Source sends information as self-contained packets that have a destination address.» Source may have to break up single message in multiple● Each packet travels independently to the destination host.» Routers and switches use the destination address in the packet to determine how to forward the packets● Destination recreates the message.● Analogy: a letter in surface mail.Peter A. Steenkiste, SCS, CMU4Circuit Switching● Source first establishes a connection (circuit) to the destination.» Each router or switch along the way may reserve some bandwidth for the data flow● Source sends the data over the circuit.» No need to include the destination address with the data since the routers know the path● The connection is torn down.● Example: telephone network.Peter A. Steenkiste, SCS, CMU5Connection SetupSETUPSETUPCONNECTACKCONNECTACKCONNECTCONNECTSETUPCONNECTACKCONNECTPeter A. Steenkiste, SCS, CMU6Circuit Switching Discussion● Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth.» Physical connection - clearly no need to include addresses with the data● Advantages, relative to packet switching:» Implies guaranteed bandwidth, predictable performance » Simple switch design: only remembers connection information, no destination address look up● Disadvantages:» Inefficient for bursty traffic (wastes bandwidth)» Delay associated with establishing a circuit● Can we get the advantages without (all) the disadvantages?Page 2Peter A. Steenkiste, SCS, CMU7Virtual Circuits● Each wire carries many “virtual” circuits. » Forwarding based on virtual circuit (VC) identifier» A path through the network is determined for each VC when the VC is established» Use statistical multiplexing for efficiency● Can support wide range of quality of service.» No guarantees: best effort service» Weak guarantees: delay < 300 msec, …» Strong guarantees: e.g. equivalent of physical circuitPeter A. Steenkiste, SCS, CMU8Packet Switching andVirtual Circuits: Similarities● “Store and forward” communication based on an address.» Address is either the destination address or a VC identifier● Must have buffer space to temporarily store packets.» E.g. multiple packets for some destination arrive simultaneously● Multiplexing 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 ACBDPeter A. Steenkiste, SCS, CMU9Virtual 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 scheduled» Requires “hard” state in the network● Packet 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– Only have “soft” state» Adding QoS is hard» Traffic engineering is hard: too many packets!Peter A. Steenkiste, SCS, CMU10How To Assign VC Ids?● Easy solution: globally unique VC identifiers.» But very large number of VCs makes this inefficient– Requires large ids that have to be stored in header– Centralized assignment of ids to avoid conflicts● Better solution: use ids that have only local significance for a link.» Smaller number of VCs on a link reduces the id space» Allows local selection of VC identifiers» But requires “VC id remapping”149202435906153136Peter A. Steenkiste, SCS, CMU11VC Packet Forwarding● Lookup based on VC identifier.» Short, fixed-sized ids» Easier than MAC addresses (48 bits) or hierarchical IP addresses● Next hop: output port for packet.● Info: priority, VC id, ..● Table is filled in by routing protocol.» Similar to IP, but applies to setup request only3 3Switch8 3A21023C905900128.2.15.31VC ID Next Hop132-(2,34)InfoPeter A. Steenkiste, SCS, CMU12Outline● Circuit switching.● ATM overview.● SONET.● MPLS.Page 3Peter A. Steenkiste, SCS, CMU13ATM History● Telephone companies supported voice telephony: 4 kHz analog, 64 kbs digital.● They provided lines for data networking.» ISDN: 64 kbps and faster channels» T1 (1.544 Mbps)» T3 (44.736 Mbps)● They wanted to become the primary service provider for data networking services.» file transfer: bursty, many Mbps peak» database access: bursty, low latency» Multimedia: synchronized » Video: 6 MHz analog, 1.2-200 Mbps digital● How?Peter A. Steenkiste, SCS, CMU14ATM● Asynchronous Transfer Mode» Replace Synchronous Transfer Mode, which used slots in fixed frame structure (circuits)● Instead of predefined TDM slots, tag each slot with a virtual connection ID.» Bandwidth can change dynamically● Small packets allow good real time behavior.● Fixed sized packets (cells) support fast switchingVCIVCIdatadataPeter A. Steenkiste, SCS, CMU15ATM Features● Fixed size cells (53 bytes).● Virtual circuit technology using hierarchical virtual circuits (VP,VC).● Physical layer processing delineates cells by frame structure, cell header error check.● Support for multiple traffic classes by adaptation layer.» E.g. voice channels, data traffic● Elaborate signaling stack.» Backwards compatible with respect to the telephone standards● Standards defined by ATM Forum.» Organization of manufacturers, providers, usersPeter A. Steenkiste, SCS, CMU16ATM Standard Protocol LayersUpper Layer ProtocolsUpper Layer ProtocolsATM adaptation layerPhysical medium dependentCSCSSARSARAALAALATMATMPMDPMDTCTCPHYPHYTransmission convergenceSegmentation and reassemblyConvergence sublayerPeter A. Steenkiste, SCS, CMU17GFCGFCVPIVPIVPIVPIVCIVCIVCIVCIVCIVCIPTPTCLPCLPHECHECpayloadpayloadThe ATM Cell (UNI)5 bytes48 byteshdrhdrpldpld(proportional)Peter A. Steenkiste, SCS, CMU18Why 53 Bytes?● Small cells favored by voice applications» delays of more than
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