Asynchronous Transfer Modes By: Megan Cwiklinski Adam Nasset Brad Samples Will VanlueOutlineIntroductionWhy Cells?Slide 5Slide 6Cells in PracticeWhy virtual circuits?Slide 9Types of virtual circuits and pathsStaticDynamicSwitchedATM Cell StructuresGeneric Flow ControlVirtual Path/Circuit IdentifierPayload TypeCell Loss PriorityHeader Error CorrectionClasses of ServiceTechnical ParametersSlide 22BenefitsBenefits contd.Questions?Asynchronous Transfer Asynchronous Transfer ModesModesBy: Megan CwiklinskiBy: Megan CwiklinskiAdam NassetAdam NassetBrad SamplesBrad SamplesWill VanlueWill VanlueOutlineOutline•IntroductionIntroduction•Why Cells and Cells in PracticeWhy Cells and Cells in Practice•Why Virtual CircuitsWhy Virtual Circuits•Types of Virtual Circuits and PathsTypes of Virtual Circuits and Paths•Cell StructureCell Structure•Quality and Classes of ServiceQuality and Classes of Service•BenefitsBenefitsIntroductionIntroduction•ATM is a data link layer protocolATM is a data link layer protocol•Uses fixed sized cellsUses fixed sized cells•Utilizes Synchronous Optical NetworkUtilizes Synchronous Optical Network•Widely used by multiplex services and DSLWidely used by multiplex services and DSLWhy Cells?Why Cells?•Use small data cells to reduce jitterUse small data cells to reduce jitter•Important when carrying voice trafficImportant when carrying voice traffic•Good job includesGood job includes–Evenly spaced stream of data itemsEvenly spaced stream of data items–Measure in time variablesMeasure in time variablesWhy Cells?Why Cells?•Designed to run at 155 Mbit/sDesigned to run at 155 Mbit/s–Typical 1500-byte data packet sent in Typical 1500-byte data packet sent in 77.42 microseconds77.42 microseconds•DisadvantageDisadvantage–Queuing delays can be larger than Queuing delays can be larger than transmittal timetransmittal time–Packets expensive to implement at high Packets expensive to implement at high speedsspeedsWhy Cells?Why Cells?•Cells developed to carry large Cells developed to carry large datagrams and provide short queuing datagrams and provide short queuing delaysdelays•Breaks up packets into 48-byte chunk Breaks up packets into 48-byte chunk with 5-byte header to be with 5-byte header to be reassembled laterreassembled laterCells in PracticeCells in Practice•Reduced need for small packet cellsReduced need for small packet cells•However, Asymmetric Digital However, Asymmetric Digital Subscribers Lines use ATMSubscribers Lines use ATM•On slow links, makes sense to use On slow links, makes sense to use ATMATM•Provides compelling business Provides compelling business advantage for small companiesadvantage for small companiesWhy virtual circuits?Why virtual circuits?•Data is passed from source to destination Data is passed from source to destination over multiple real circuits over multiple real circuits •ATM cell header contains:ATM cell header contains:–8- or 12-bit Virtual Path Identifier (VPI)8- or 12-bit Virtual Path Identifier (VPI)–16-bit Virtual Channel Identifier (VCI)16-bit Virtual Channel Identifier (VCI)Why virtual circuits?Why virtual circuits?•VPI & VCI make up a unique path for a virtual circuitVPI & VCI make up a unique path for a virtual circuit•Switching is done by altering VPI & VCI Switching is done by altering VPI & VCI –VPI & VCI may not stay constant throughout a circuitVPI & VCI may not stay constant throughout a circuit–The circuit is consistent for the entire messageThe circuit is consistent for the entire message•Can be used as multiplexing layerCan be used as multiplexing layer•Hierarchically identified first by VPI, then by VCI Hierarchically identified first by VPI, then by VCI –Simpler switching by only looking at one identifierSimpler switching by only looking at one identifierTypes of virtual circuits and pathsTypes of virtual circuits and paths•StaticStatic•DynamicDynamic•SwitchedSwitchedStaticStatic•Permanent Virtual Circuits (PVCs) and Permanent Virtual Circuits (PVCs) and Permanent Virtual Paths (PVPs) Permanent Virtual Paths (PVPs) –Circuits are made of explicitly defined segments Circuits are made of explicitly defined segments between endpointsbetween endpoints–Simple to understand; not very scalableSimple to understand; not very scalable–No dynamic rerouting in case of failureNo dynamic rerouting in case of failureDynamicDynamic•Soft Permanent Virtual Circuits (SPVCs) and Soft Soft Permanent Virtual Circuits (SPVCs) and Soft Permanent Virtual Paths (SPVPs)Permanent Virtual Paths (SPVPs)–Dynamically built from specified service contract and Dynamically built from specified service contract and endpoints endpoints –Types of service contractsTypes of service contracts•ABR (Available Bit Rate) ABR (Available Bit Rate) •CBR (Constant Bit Rate)CBR (Constant Bit Rate)•UBR (Unspecified Bit Rate)UBR (Unspecified Bit Rate)•VBR (Variable Bit Rate)VBR (Variable Bit Rate)SwitchedSwitched•Switched Virtual Circuits (SVCs)Switched Virtual Circuits (SVCs)–Created on demand; one time useCreated on demand; one time use–Defined by service contract and endpointsDefined by service contract and endpoints–Example: Individual telephone callsExample: Individual telephone callsATM Cell StructuresATM Cell Structures77443300VPIVPIVPIVPIVCIVCIVCIVCIVCIVCIPTPTCLPCLPHECHECPayload (48 bytes)Payload (48 bytes)77443300GFCGFCVPIVPIVPIVPIVCIVCIVCIVCIVCIVCIPTPTCLPCLPHECHECPayload (48 bytes)Payload (48 bytes)UNI ATM CellNNI ATM CellGeneric Flow ControlGeneric Flow Control•Permits transmissions of several Permits transmissions of several terminals to be multiplexed on the terminals to be multiplexed on the same user interfacesame user interface•Contains 4 bitsContains 4 bitsVirtual Path/Circuit IdentifierVirtual Path/Circuit Identifier•Virtual paths are semi-permanent Virtual paths are semi-permanent connections between endpointsconnections between endpoints•Connections are identified by the Connections are identified by the virtual circuitvirtual circuitPayload TypePayload Type•The first bit indicates user data or The first bit indicates user data or control datacontrol data•If the cell contains user dataIf the cell contains user data–Second bit indicates congestionSecond bit indicates congestion–Third bit indicates whether the cell is the Third bit indicates whether the cell is the last in a series of cells that represent a last in a series of cells