Switching Architectures for Optical Networks CSIT 5600 by M. Hamdi 1
Internet Reality Data Center SONET DWD M SONET Access Metro Long Haul CSIT 5600 by M. Hamdi Metro Access 2
Hierarchies of Networks: IP / ATM / SONET / WDM CSIT 5600 by M. Hamdi 3
Why Optical? • Enormous bandwidth made available – DWDM makes ~160 channels/ possible in a fiber – Each wavelength “potentially” carries about 40 Gbps – Hence Tbps speeds become a reality • Low bit error rates – 10 -9 as compared to 10 -5 for copper wires • Very large distance transmissions with very little amplification. CSIT 5600 by M. Hamdi 4
Dense Wave Division Multiplexing (DWDM) 1 2 3 Long-haul fiber 4 Output fibers Multiple wavelength bands on each fiber Ø Transmit by combining multiple lasers @ different frequencies CSIT 5600 by M. Hamdi 5
Anatomy of a DWDM System Terminal B Terminal A Transponder Interfaces M U X Post. Amp Line Amplifiers Direct Connections Pre. Amp D E M U X Transponder Interfaces Direct Connections Basic building blocks • Optical amplifiers • Optical multiplexers • Stable optical sources CSIT 5600 by M. Hamdi
• Provisioned SONET circuits. • Aggregated into Lamdbas. Core Transport Services Circuit Origin • Carried over Fiber optic cables. Circuit Destination OC-3 OC-12 STS-1 CSIT 5600 by M. Hamdi 7
WDM Network: Wavelength View WDM link Edge Router Legacy Interfaces Legacy ( e. g. , Po. S, Gigabit Interfaces Ethernet, IP/ATM) Interfaces Legacy Interfaces Optical Switch CSIT 5600 by M. Hamdi 8
Relationship of IP and Optical • Optical brings –Bandwidth multiplication –Network simplicity (removal of redundant layers) • IP brings –Scalable, mature control plane –Universal OS and application support –Global Internet • Collectively IP and Optical (IP+Optical) introduces a set of service-enabling technologies CSIT 5600 by M. Hamdi 9
Typical Super POP Interconnectio n Network Core IP router DWDM + Metro Ring ADM Large Multi-service Aggregation Switch Voice Switch Core ATM Switch OXC CSIT 5600 by M. Hamdi SONET Coupler & Opt. amp 10
Typical POP Voice Switch D W D M OXC D W D M SONET-XC CSIT 5600 by M. Hamdi 11
What are the Challenges with Optical Networks? • Processing: Needs to be done with electronics – Network configuration and management – Packet processing and scheduling – Resource allocation, etc. • Traffic Buffering – Optics still not mature for this (use Delay Fiber Lines) – 1 pkt = 12 kbits @ 10 Gbps requires 1. 2 s of delay => 360 m of fiber) • Switch configuration – Relatively slow CSIT 5600 by M. Hamdi 12
Wavelength Converters • Improve utilization of available wavelengths on links • All-optical WCs being developed • Greatly reduce blocking probabilities 3 2 WC No converters 1 New request 1 3 With converters 1 New request 1 3 CSIT 5600 by M. Hamdi 13
Wavelength Cross-Connects (WXCs) • A WDM network consists of wavelength cross-connects (WXCs) (OXC) interconnected by fiber links. • 2 Types of WXCs – Wavelength 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 constraint – Wavelength interchanging cross-connect (WIXC) • Wavelength conversion employed • Yield better performance • Expensive CSIT 5600 by M. Hamdi 14
Wavelength Router Control Plane: Wavelength Routing Intelligence Data Plane: Optical Cross Connect Matrix Unidirectional DWDM Links to other Wavelength Routers Single Channel Links to IP Routers, SDH Muxes, . . . CSIT 5600 by M. Hamdi Unidirectional DWDM Links to other Wavelength Routers 15
Optical Network Architecture UNI Mesh Optical Network UNI IP Network IP Router OXC Control unit Optical Cross Connect (OXC) Control Path Data Path CSIT 5600 by M. Hamdi 16
OXC Control Unit • Each OXC has a control unit • Responsible for switch configuration • Communicates with adjacent OXCs or the client network through single-hop light paths – These are Control light paths – Use standard signaling protocol like GMPLS for control functions • Data light paths carry the data flow – Originate and terminate at client networks/edge routers and transparently traverse the core CSIT 5600 by M. Hamdi 17
Optical Cross-connects (No wavelength conversion) l 2 l 4 All Optical Cross-connect (OXC) Also known as Photonic Cross-connect (PXC) l 1 l 3 Optical Switch Fabric l 3 l 4 l 1 l 2 CSIT 5600 by M. Hamdi 18
Optical Cross-Connect with Full Wavelength Conversion Wavelength Converters l 2 l 1 l 2 ln l 1, l 2, . . . , ln 2. . . l 1, l 2, . . . , ln M Wavelength Demux l 1, l 2, . . . , ln l 1 ln 1 l 2 ln ln l 1 l 2 Optical Cross. Bar Switch l 1, l 2, . . . , ln 2. . . l 1, l 2, . . . , ln M Wavelength Mux • M demultiplexers at incoming side • M multiplexers at outgoing side • Mn x Mn optical switch has wavelength converters at switch outputs CSIT 5600 by M. Hamdi 19
Wavelength Router with O/E and E/O Incoming Interface Incoming Wavelength Cross-Connect Outgoing Interface Outgoing Wavelength l 1 l 3 CSIT 5600 by M. Hamdi 20
O-E-O Crossconnect Switch (OXC) Incoming fibers Demux 1 2 N WDM (many λs) Individual wavelengths O O O/E E/O E O/E E/O O/E O/E E/O E/O O/E O/E Outgoing fibers Mux 1 E/O E/O 2 N Switches information signal on a particular wavelength on an incoming fiber to (another) wavelength on an outgoing fiber. CSIT 5600 by M. Hamdi 21
Optical core network Opaque (O-E-O) and transparent (O-O) sections Client signals E/O Transparent optical island O/E O O from other nodes O O O E E O O to other nodes E Opaque optical network by M. Hamdi CSIT 5600 O O 22
OEO vs. All-Optical Switches OEO • Capable of status monitoring • Optical signal regenerated – improve signal-to-noise ratio • Traffic grooming at various levels All-Optical • Unable to monitor the contents of the data stream • Only optical amplification – signal -to-noise ratio degraded with distance • No traffic grooming in subwavelength level • Less aggregated throughput • More expensive • Higher aggregated throughput • More power consumption • ~10 X cost saving • ~10 X power saving CSIT 5600 by M. Hamdi 23
Large customers buy “lightpaths” A lightpath is a series of wavelength links from end to end. optical fibers One fiber Repeater cross-connect CSIT 5600 by M. Hamdi 24
Hierarchical switching: Node with switches of different granularities A. Entire fibers Fibers O O O Fibers B. Wavelength subsets O O O C. Individual wavelengths O E “Express trains” O CSIT 5600 by M. Hamdi “Local trains” 25
Wide Area Network (WAN) GAN links WAN : Up to 200 -500 wavelengths 40 -160 Gbit/s/l wavebands (> 10 l) OXC: Optical Wavelength/Waveband Cross Connect CSIT 5600 by M. Hamdi 26
Packet (a) vs. Burst (b) Switching CSIT 5600 by M. Hamdi 27
MAN (Country / Region) IP packets optical burst formation CSIT 5600 by M. Hamdi 28
Optical Switching Technologies • • MEMs – Micro. Electro. Mechanical Liquid Crystal Opto-Mechanical Bubble Technology Thermo-optic (Silica, Polymer) Electro-optic (Li. Nb 03, SOA, In. P) Acousto-optic Others… Maturity of technology, Switching speed, Scalability, Cost, Relaiability (moving components or not), etc. CSIT 5600 by M. Hamdi 29
MEMS Switches for Optical Cross. Connect Proven technology, switching time (10 to 25 msec), moving mirrors is a reliability problem. CSIT 5600 by M. Hamdi 30
WDM “transparent” transmission system (O-O nodes) Wavelengths disaggregator O Fibers Wavelengths aggregator O O O multiple λs O O Optical switching fabric (MEMS devices, etc. ) Incoming fiber Outgoing fibers Tiny mirrors CSIT 5600 by M. Hamdi 31
Upcoming Optical Technologies • WDM routing is circuit switched – Resources are wasted if enough data is not sent – Wastage more prominent in optical networks • Techniques for eliminating resource wastage – Burst Switching – Packet Switching • Optical burst switching (OBS) is a new method to transmit data • A burst has an intermediate characteristics compared to the basic switching units in circuit and packet switching, which are a session and a packet, respectively CSIT 5600 by M. Hamdi 32
Optical Burst Switching (OBS) • Group of packets a grouped in to ‘bursts’, which is the transmission unit • Before the transmission, a control packet is sent out – The control packet contains the information of burst arrival time, burst duration, and destination address • Resources are reserved for this burst along the switches along the way • The burst is then transmitted • Reservations are torn down after the burst CSIT 5600 by M. Hamdi 33
Optical Burst Switching (OBS) CSIT 5600 by M. Hamdi 34
Optical Packet Switching • Fully utilizes the advantages of statistical multiplexing • Optical switching and buffering • Packet has Header + Payload – Separated at an optical switch • Header sent to the electronic control unit, which configures the switch for packet forwarding • Payload remains in optical domain, and is recombined with the header at output interface CSIT 5600 by M. Hamdi 35
Optical Packet Switch • Has – Input interface, Switching fabric, Output interface and control unit • Input interface separates payload and header • Control unit operates in electronic domain and configures the switch fabric • Output interface regenerates optical signals and inserts packet headers • Issues in optical packet switches – Synchronization – Contention resolution CSIT 5600 by M. Hamdi 36
• 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. hdr payload CPU hdr payload Wavelength i input port j Re-combined Wavelength i output port j Optical packet Optical switch CSIT 5600 by M. Hamdi 37
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 ports Optical Switch Output ports payloadhdr . . . payloadhdr CSIT 5600 by M. Hamdi . . . 38
OPS Architecture: Synchronization Occurs in electronic switches – solved by input buffering Slotted networks • Fixed packet size • Synchronization stages required Sync. CSIT 5600 by M. Hamdi 39
OPS Architecture: Synchronization Slotted networks • Fixed packet size • Synchronization stages required Sync. CSIT 5600 by M. Hamdi 40
OPS Architecture: Synchronization Slotted networks • Fixed packet size • Synchronization stages required Sync. CSIT 5600 by M. Hamdi 41
OPS Architecture: Synchronization Slotted networks • Fixed packet size • Synchronization stages required Sync. CSIT 5600 by M. Hamdi 42
OPS Architecture: Synchronization Slotted networks • Fixed packet size • Synchronization stages required Sync. CSIT 5600 by M. Hamdi 43
OPS Architecture: Synchronization Sync. CSIT 5600 by M. Hamdi 44
OPS: Contention Resolution • More than one packet trying to go out of the same output port at the same time – Occurs in electronic switches too and is resolved by buffering the packets at the output – Optical buffering ? • Solutions for contention – Optical Buffering – Wavelength multiplexing – Deflection routing CSIT 5600 by M. Hamdi 45
OPS Architecture Contention Resolutions 1 2 3 1 1 2 1 3 4 4 CSIT 5600 by M. Hamdi 46
OPS: Contention Resolution • Optical Buffering – Should hold an optical signal • How? By delaying it using Optical Delay Lines (ODL) – ODLs are acceptable in prototypes, but not commercially viable – Can convert the signal to electronic domain, store, and reconvert the signal back to optical domain • Electronic memories too slow for optical networks CSIT 5600 by M. Hamdi 47
OPS Architecture Contention Resolutions • Optical buffering 1 1 2 3 1 2 1 3 4 4 CSIT 5600 by M. Hamdi 48
OPS Architecture Contention Resolutions • Optical buffering 1 1 2 2 3 3 4 4 CSIT 5600 by M. Hamdi 49
OPS Architecture Contention Resolutions • Optical buffering 1 1 1 2 2 3 3 4 4 1 CSIT 5600 by M. Hamdi 50
OPS: Contention Resolution • Wavelength multiplexing – Resolve contention by transmitting on different wavelengths – Requires wavelength converters - $$$ CSIT 5600 by M. Hamdi 51
OPS Architecture Contention Resolutions • Wavelength conversion 1 1 2 2 CSIT 5600 by M. Hamdi 52
OPS Architecture Contention Resolutions • Wavelength conversion 1 1 2 2 CSIT 5600 by M. Hamdi 53
OPS Architecture Contention Resolutions • Wavelength conversion 1 1 2 2 CSIT 5600 by M. Hamdi 54
OPS Architecture Contention Resolutions • Wavelength conversion 1 1 2 2 CSIT 5600 by M. Hamdi 55
OPS Architecture Contention Resolutions • Wavelength conversion 1 1 2 2 CSIT 5600 by M. Hamdi 56
Deflection routing • When 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 CSIT 5600 by M. Hamdi 57
Electronic Switches Using Optical Crossbars CSIT 5600 by M. Hamdi 58
Scalable Multi-Rack Switch Architecture Optical links Line card rack Switch Core • Number of linecards is limited in a single rack – Limited power supplement, i. e. 10 KW – Physical consideration, i. e. temperature, humidity • Scaling to multiple racks – Fiber links and central fabrics CSIT 5600 by M. Hamdi 59
Logical Architecture of Multi-rack Switches Scheduler Line Card Fiber I/O Local Framer Buffers Laser Line Card Laser Local Buffers Framer Fiber I/O Crossbar Line Card Fiber I/O Local Framer Buffers Line Card Laser Local Buffers Framer Fiber I/O Switch Fabric System • Optical I/O interfaces connected to WDM fibers • Electronic packet processing and buffering – Optical buffering, i. e. fiber delay lines, is costly and not mature • Optical interconnect – Higher bandwidth, lower latency and extended link length than copper twisted lines • Switch fabric: electronic? Optical? CSIT 5600 by M. Hamdi 60
Optical Switch Fabric Scheduler Line Card Fiber I/O Local Framer Buffers Laser Line Card Laser Local Buffers Framer Fiber I/O Crossbar Line Card Fiber I/O Local Framer Buffers Line Card Laser Local Buffers Framer Fiber I/O Switch Fabric System • Less optical-to-electrical conversion inside switch – Cheaper, physically smaller • Compare to electronic fabric, optical fabric brings advantages in – Low power requirement, Scalability, Port density, High capacity • Technologies that can be used – 2 D/3 D MEMS, liquid crystal, bubbles, thermo-optic, etc. CSIT 5600 by M. • Hybrid architecture takes advantage Hamdi strengths of both of the electronics and optics 61
Electronic Vs. Optical Fabric Electronic Trans. Buffer Inter. Line connection Inter- Buffer Trans. connection Line Switching Fabric Optical Electronic E/O or O/E Conversion fa v Optical orr ed Trans. Buffer Inter. Line connection Inter- Buffer Trans. connection Line Switching Fabric CSIT 5600 by M. Hamdi 62
Multi-rack Hybrid Packet Switch CSIT 5600 by M. Hamdi 63
Features of Optical Fabric • Less E/O or O/E conversion • High capacity • Low power consumption • Less cost However, • Reconfiguration overhead (50 -100 ns) – Tuning of lasers (20 -30 ns) – System clock synchronization (10 -20 ns or higher) CSIT 5600 by M. Hamdi 64
Scheduling Under Reconfiguration Overhead • Traditional slot-by-slot approach Scheduler Schedule Reconfigure Transfer Time Line • Low bandwidth usage CSIT 5600 by M. Hamdi 65
Reduced Rate Scheduling Fabric setup (reconfigure) Traffic transfer Time slot Slot-by-slot Scheduling, zero fabric setup time Slot-by-slot Scheduling with reconfigure delay Reduced rate Scheduling, each schedule is held for some time • Challenge: fabric reconfiguration delay – • Traditional slot-by-slot scheduling brings lots of overhead Solution: slow down the scheduling frequency to compensate – • Each schedule will be held for some time Scheduling task 1. 2. Find out the matching Determine the holding time CSIT 5600 by M. Hamdi 66
Scheduling Under Reconfiguration Overhead • Reduce the scheduling rate – Bandwidth Usage = Transfer/(Reconfigure+Transfer) Constant • Approaches – Batch scheduling: TSA-based – Single scheduling: Schedule + Hold CSIT 5600 by M. Hamdi 67
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 variable – Example: LQF+Hold CSIT 5600 by M. Hamdi 68
Routing and Wavelength Assignment CSIT 5600 by M. Hamdi 69
Optical Circuit Switching • An optical path established between two nodes • Created by allocation of a wavelength throughout the path. • Provides a ‘circuit switched’ interconnection between two nodes. – Path setup takes at least one RTT – No optical buffers since path is pre-set Desirable 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. CSIT 5600 by M. Hamdi 70
Routing and Wavelength Assignment (RWA) • Light path establishment involves – Selecting a physical path between source and destination edge nodes – Assigning a wavelength for the light path • RWA is more complex than normal routing because – Wavelength continuity constraint • A light path must have same wavelength along all the links in the path – Distinct Wavelength Constraint • Light paths using the same link must have different wavelengths CSIT 5600 by M. Hamdi 71
No Wavelength Converters WSXC Access Fiber Wavelength 1 POP Wavelength 2 Wavelength 3 CSIT 5600 by M. Hamdi 72
With Wavelength Converters WIXC Access Fiber POP Wavelength 1 POP Wavelength 2 Wavelength 3 CSIT 5600 by M. Hamdi 73
Routing and Wavelength Assignment (RWA) • RWA algorithms based on traffic assumptions: • Static Traffic – Set of connections for source and destination pairs are given • Dynamic Traffic – Connection 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 required • Connection blocking probability: Ratio between number of blocked connections and total number of connections arrived CSIT 5600 by M. Hamdi 74
Static and Dynamic RWA • Static RWA – Light path assignment when traffic is known well in advance – Arises in capacity planning and design of optical networks • Dynamic RWA – Light path assignment to be done when requests arrive in random fashion – Encountered during real-time network operation CSIT 5600 by M. Hamdi 75
Static RWA • RWA is usually solved as an optimization problem with Integer Programming (IP) formulations • Objective functions – Minimize average weighted number of hops – Minimize average packet delay – Minimize the maximum congestion level – Minimize number of Wavelenghts CSIT 5600 by M. Hamdi 76
Static RWA • Methodologies for solving Static RWA – Heuristics for solving the overall ILP sub-optimally – Algorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set – http: //www. tct. hut. fi/~esa/java/wdm/ CSIT 5600 by M. Hamdi 77
Solving Dynamic RWA • During network operation, requests for new lightpaths come randomly • These requests will have to be serviced based on the network state at that instant • As the problem is in real-time, dynamic RWA algorithms should be simple • The problem is broken down into two sub-problems – Routing problem – Wavelength assignment problem CSIT 5600 by M. Hamdi 78
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