4 Why Optical? Enormous bandwidth made available Low bit error rates DWDM makes ~160 channels/ possible in a fiberEach wavelength “potentially” carries about 40 GbpsHence Tbps speeds become a realityLow bit error rates10-9 as compared to 10-5 for copper wiresVery large distance transmissions with very little amplification.
5 Dense Wave Division Multiplexing (DWDM) 123Long-haul fiber4Output fibersMultiple wavelength bands on each fiberTransmit by combining multiple different frequencies
6 Anatomy of a DWDM System Terminal ATerminal BDEMUXTransponderInterfacesMUXTransponderInterfacesPost-AmpLine AmplifiersPre-AmpDirectConnectionsDirectConnectionsBasic building blocksOptical amplifiersOptical multiplexersStable optical sources
7 Core Transport Services ProvisionedSONET circuits.Aggregated intoLamdbas.CircuitOriginCarried overFiber optic cables.CircuitDestinationOC-3OC-3OC-12STS-1STS-1STS-1
9 Relationship of IP and Optical Optical bringsBandwidth multiplicationNetwork simplicity (removal of redundant layers)IP bringsScalable, mature control planeUniversal OS and application supportGlobal InternetCollectively IP and Optical (IP+Optical) introduces a set of service-enabling technologies
10 OXC Typical Super POP SONET Core Core ATM Large Voice IP Switch InterconnectionNetworkSONETCoreATMSwitchVoiceSwitchCoreIProuterLargeMulti-serviceAggregationSwitchCoupler&Opt.ampDWDM+ADMOXCDWDM Metro Ring
12 What are the Challenges with Optical Networks? Processing: Needs to be done with electronicsNetwork configuration and managementPacket processing and schedulingResource allocation, etc.Traffic BufferingOptics still not mature for this (use Delay Fiber Lines)1 pkt = Gbps requires 1.2 s of delay => 360 m of fiber)Switch configurationRelatively slow
13 Wavelength Converters Improve utilization of available wavelengths on linksAll-optical WCs being developedGreatly reduce blocking probabilitiesNo converters123New request1 3With convertersWC
14 Wavelength Cross-Connects (WXCs) A WDM network consists of wavelength cross-connects (WXCs) (OXC) interconnected by fiber links.2 Types of WXCsWavelength selective cross-connect (WSXC)Route a message arriving at an incoming fiber on some wavelength to an outgoing fiber on the same wavelength.Wavelength continuity constraintWavelength interchanging cross-connect (WIXC)Wavelength conversion employedYield better performanceExpensive
15 Wavelength Router Control Plane: Data Plane: Wavelength Router Wavelength Routing IntelligenceData Plane:Optical Cross Connect MatrixUnidirectional DWDM Links to other Wavelength RoutersUnidirectional DWDM Links to other Wavelength RoutersSingle Channel Links to IP Routers, SDH Muxes, ...
17 OXC Control Unit Each OXC has a control unit Responsible for switch configurationCommunicates with adjacent OXCs or the client network through single-hop light pathsThese are Control light pathsUse standard signaling protocol like GMPLS for control functionsData light paths carry the data flowOriginate and terminate at client networks/edge routers and transparently traverse the core
18 Optical Cross-connects (No wavelength conversion) All Optical Cross-connect (OXC) Also known as Photonic Cross-connect (PXC)l1l3OpticalSwitchFabricl3
19 Optical Cross-Connect with Full Wavelength Conversion Convertersl1l2l1,l2,... ,lnl1,l2,... ,lnl2l11l1nlnl1l1l1,l2,... ,lnl1,l2,... ,lnl2l22lnl2n......l1lnl1,l2,... ,lnl1,l2,... ,lnl2l1Mlnl2MWavelengthWavelengthOptical CrossBarDemuxMuxSwitchM demultiplexers at incoming sideM multiplexers at outgoing sideMn x Mn optical switch has wavelength converters at switch outputs
20 Wavelength Router with O/E and E/O Cross-ConnectIncoming InterfaceIncoming WavelengthOutgoing InterfaceOutgoing Wavelengthl1l3
21 Individual wavelengths O-E-O Crossconnect Switch (OXC)OutgoingfibersIncomingfibersIndividual wavelengthsOODemuxMuxO/E1EE/O1E/OE/O2O/EE/O2E/OWDM(many λs)E/ONO/EE/ONE/OE/OSwitches information signal on a particular wavelength on anincoming fiber to (another) wavelength on an outgoing fiber.
22 Optical core network Opaque (O-E-O) and transparent (O-O) sections optical islandE/OO/EClientsignalsOOOOEEOto other nodesfrom other nodesOOOOOEEOOpaque optical network
23 OEO vs. All-Optical Switches Capable of status monitoringOptical signal regenerated – improve signal-to-noise ratioTraffic grooming at various levelsLess aggregated throughputMore expensiveMore power consumptionUnable to monitor the contents of the data streamOnly optical amplification – signal-to-noise ratio degraded with distanceNo traffic grooming in sub-wavelength levelHigher aggregated throughput~10X cost saving~10X power saving
24 Large customers buy “lightpaths” A lightpath is a series of wavelength links from end to end.opticalfibersOne fiberRepeatercross-connect
25 Hierarchical switching: Node with switches of different granularities A. Entire fibersOFibersFibers“Expresstrains”OOB. WavelengthsubsetsOEOC. IndividualwavelengthsO“Localtrains”
26 Wide Area Network (WAN) GAN linksWAN :Up to wavelengthsGbit/s/lwavebands (> 10 l)OXC: Optical Wavelength/Waveband Cross Connect
32 Upcoming Optical Technologies WDM routing is circuit switchedResources are wasted if enough data is not sentWastage more prominent in optical networksTechniques for eliminating resource wastageBurst SwitchingPacket SwitchingOptical burst switching (OBS) is a new method to transmit dataA burst has an intermediate characteristics compared to the basic switching units in circuit and packet switching, which are a session and a packet, respectively
33 Optical Burst Switching (OBS) Group of packets a grouped in to ‘bursts’, which is the transmission unitBefore the transmission, a control packet is sent outThe control packet contains the information of burst arrival time, burst duration, and destination addressResources are reserved for this burst along the switches along the wayThe burst is then transmittedReservations are torn down after the burst
35 Optical Packet Switching Fully utilizes the advantages of statistical multiplexingOptical switching and bufferingPacket has Header + PayloadSeparated at an optical switchHeader sent to the electronic control unit, which configures the switch for packet forwardingPayload remains in optical domain, and is re-combined with the header at output interface
36 Optical Packet Switch Has Input interface separates payload and header Input interface, Switching fabric, Output interface and control unitInput interface separates payload and headerControl unit operates in electronic domain and configures the switch fabricOutput interface regenerates optical signals and inserts packet headersIssues in optical packet switchesSynchronizationContention resolution
37 Main operation in a switch: The header and the payload are separated.Header is processed electronically.Payload remains as an optical signal throughout the switch.Payload and header are re-combined at the output interface.hdrCPUpayloadhdrpayloadhdrpayloadRe-combinedWavelength ioutput port jOpticalpacketWavelength iinput port jOptical switch
38 Output port contention Assuming a non-blocking switching matrix, more than one packet may arrive at the same output port at the same time.Input portsOptical SwitchOutput portspayloadhdr. . .. . .payload. . .hdr. . .payloadhdr
39 OPS Architecture: Synchronization Occurs in electronic switches – solved by input bufferingSlotted networksFixed packet sizeSynchronization stages requiredSync.
45 OPS: Contention Resolution More than one packet trying to go out of the same output port at the same timeOccurs in electronic switches too and is resolved by buffering the packets at the outputOptical buffering ?Solutions for contentionOptical BufferingWavelength multiplexingDeflection routing
47 OPS: Contention Resolution Optical BufferingShould hold an optical signalHow? By delaying it using Optical Delay Lines (ODL)ODLs are acceptable in prototypes, but not commercially viableCan convert the signal to electronic domain, store, and re-convert the signal back to optical domainElectronic memories too slow for optical networks
57 Deflection routingWhen there is a conflict between two optical packets, one will be routed to the correct output port, and the other will be routed to any other available output port.A deflected optical packet may follow a longer path to its destination. In view of this:The end-to-end delay for an optical packet may be unacceptably high.Optical packets may have to be re-ordered at the destination
59 Scalable Multi-Rack Switch Architecture Optical linksLine card rackSwitch CoreNumber of linecards is limited in a single rackLimited power supplement, i.e. 10KWPhysical consideration, i.e. temperature, humidityScaling to multiple racksFiber links and central fabrics
60 Logical Architecture of Multi-rack Switches SchedulerLine CardLine CardLocalBuffersCrossbarLocalBuffersFiber I/OFramerLaserLaserLaserLaserFramerFiber I/OLine CardLine CardLocalBuffersLocalBuffersFiber I/OFramerLaserLaserLaserLaserFramerFiber I/OSwitch Fabric SystemOptical I/O interfaces connected to WDM fibersElectronic packet processing and bufferingOptical buffering, i.e. fiber delay lines, is costly and not matureOptical interconnectHigher bandwidth, lower latency and extended link length than copper twisted linesSwitch fabric: electronic? Optical?
61 Optical Switch FabricSchedulerLine CardLine CardLocalBuffersCrossbarLocalBuffersFiber I/OFramerLaserLaserLaserLaserFramerFiber I/OLine CardLine CardLocalBuffersLocalBuffersFiber I/OFramerLaserLaserLaserLaserFramerFiber I/OSwitch Fabric SystemLess optical-to-electrical conversion inside switchCheaper, physically smallerCompare to electronic fabric, optical fabric brings advantages inLow power requirement, Scalability, Port density, High capacityTechnologies that can be used2D/3D MEMS, liquid crystal, bubbles, thermo-optic, etc.Hybrid architecture takes advantage of the strengths of both electronics and optics
62 Electronic Vs. Optical Fabric Trans. LineBufferInter- connectionInter- connectionBufferTrans. LineSwitching FabricOpticalElectronicE/O or O/E ConversionOpticalfavorredTrans. LineBufferInter- connectionInter- connectionBufferTrans. LineSwitching Fabric
64 Features of Optical Fabric Less E/O or O/E conversionHigh capacityLow power consumptionLess costHowever,Reconfiguration overhead (50-100ns)Tuning of lasers (20-30ns)System clock synchronization (10-20ns or higher)
65 Scheduling Under Reconfiguration Overhead Traditional slot-by-slot approachSchedulerTransferScheduleReconfigureTime LineLow bandwidth usage
66 Reduced Rate Scheduling Fabric setup (reconfigure)Traffic transferTime slotSlot-by-slot Scheduling, zero fabric setup timeSlot-by-slot Scheduling with reconfigure delayReduced rate Scheduling, each schedule is held for some timeChallenge: fabric reconfiguration delayTraditional slot-by-slot scheduling brings lots of overheadSolution: slow down the scheduling frequency to compensateEach schedule will be held for some timeScheduling taskFind out the matchingDetermine the holding time
67 Scheduling Under Reconfiguration Overhead Reduce the scheduling rateBandwidth Usage = Transfer/(Reconfigure+Transfer)ConstantApproachesBatch scheduling: TSA-basedSingle scheduling: Schedule + Hold
68 Single Scheduling Schedule + Hold One schedule is generated each time Each schedule is held for some time (holding time)Holding time can be fixed or variableExample: LQF+Hold
70 Optical Circuit Switching An optical path established between two nodesCreated by allocation of a wavelength throughout the path.Provides a ‘circuit switched’ interconnection between two nodes.Path setup takes at least one RTTNo optical buffers since path is pre-setDesirable to establish light paths between every pair of nodes.Limitations in WDM routing networks,Number of wavelengths is limited.Physical constraints:limited number of optical transceivers limit the number of channels.
71 Routing and Wavelength Assignment (RWA) Light path establishment involvesSelecting a physical path between source and destination edge nodesAssigning a wavelength for the light pathRWA is more complex than normal routing becauseWavelength continuity constraintA light path must have same wavelength along all the links in the pathDistinct Wavelength ConstraintLight paths using the same link must have different wavelengths
72 No Wavelength Converters WSXCAccess FiberWavelength 1POPPOPWavelength 2Wavelength 3
73 With Wavelength Converters WIXCWavelength 1Access FiberPOPPOPWavelength 2Wavelength 3
74 Routing and Wavelength Assignment (RWA) RWA algorithms based on traffic assumptions:Static TrafficSet of connections for source and destination pairs are givenDynamic TrafficConnection requests arrive to and depart from network one by one in a random manner.Performance metrics used fall under one of the following three categories:Number of wavelengths requiredConnection blocking probability: Ratio between number of blocked connections and total number of connections arrived
75 Static and Dynamic RWA Static RWA Dynamic RWA Light path assignment when traffic is known well in advanceArises in capacity planning and design of optical networksDynamic RWALight path assignment to be done when requests arrive in random fashionEncountered during real-time network operation
76 Static RWARWA is usually solved as an optimization problem with Integer Programming (IP) formulationsObjective functionsMinimize average weighted number of hopsMinimize average packet delayMinimize the maximum congestion levelMinimize number of Wavelenghts
77 Static RWAMethodologies for solving Static RWAHeuristics for solving the overall ILP sub-optimallyAlgorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-setMethodologies for solving Static RWAHeuristics for solving the overall ILP sub-optimallyAlgorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-setMethodologies for solving Static RWAHeuristics for solving the overall ILP sub-optimallyAlgorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set
78 Solving Dynamic RWADuring network operation, requests for new light-paths come randomlyThese requests will have to be serviced based on the network state at that instantAs the problem is in real-time, dynamic RWA algorithms should be simpleThe problem is broken down into two sub-problemsRouting problemWavelength assignment problem
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