Transcript Switching Architectures for Optical Networks
Switching Architectures for Optical Networks
CSIT5600 by M. Hamdi 1
Internet Reality
Data Center SONET SONET SONET
Access Metro
DWD M DWD M SONET
Long Haul
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Metro Access
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Hierarchies of Networks: IP / ATM / SONET / WDM
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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.
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Dense Wave Division Multiplexing (DWDM)
Output fibers 1 2 3 4 Long-haul fiber
Multiple wavelength bands on each fiber
Transmit by combining multiple lasers @ different frequencies
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Anatomy of a DWDM System
Terminal A Terminal B Transponder Interfaces M U X Direct Connections Post Amp Line Amplifiers
Basic building blocks • Optical amplifiers • Optical multiplexers • Stable optical sources
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Provisioned SONET circuits.
Core Transport Services
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Aggregated into Lamdbas.
Circuit Origin •
Carried over Fiber optic cables.
OC-3 OC-3 OC-12 STS-1 STS-1 STS-1
Circuit Destination
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WDM Network: Wavelength View
WDM link Legacy Interfaces ( e.g., PoS, Gigabit Ethernet, IP/ATM) Interfaces Edge Router Legacy Interfaces Legacy Interfaces Optical Switch
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Relationship of IP and Optical
• • •
Optical brings
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Bandwidth multiplication
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Network simplicity (removal of redundant layers) IP brings
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Scalable, mature control plane
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Universal OS and application support
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Global Internet Collectively IP and Optical (IP+Optical) introduces a set of service-enabling technologies CSIT5600 by M. Hamdi 9
Core IP router
Typical Super POP
Interconnectio n Network Large Multi-service Aggregation Switch Voice Switch Core ATM Switch SONET DWDM DWDM + Metro Ring ADM OXC Coupler & Opt.amp
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D W D M
Typical POP
Voice Switch OXC D W D M
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SONET-XC
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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
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Wavelength Converters
• Improve utilization of available wavelengths on links • All-optical WCs being developed • Greatly reduce blocking probabilities 3 2 No converters 1 New request 1 3 2 3 WC With converters 1 New request 1 3
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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
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Wavelength Router
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, ...
CSIT5600 by M. Hamdi Unidirectional DWDM Links to other Wavelength Routers 15
Optical Network Architecture
Mesh Optical Network IP Network UNI UNI IP Network IP Router Optical Cross Connect (OXC) OXC Control unit
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Control Path Data Path
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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
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Optical Cross-connects (No wavelength conversion)
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All Optical Cross-connect (OXC) Also known as Photonic Cross-connect (PXC)
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Optical Switch Fabric
3 4 1 2
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Optical Cross-Connect with Full Wavelength Conversion 1
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n M Wavelength Optical CrossBar Demux Switch
• M demultiplexers at incoming side
Wavelength Mux
• M multiplexers at outgoing side • Mn x Mn optical switch has wavelength converters at switch outputs
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Wavelength Router with O/E and E/O Incoming Interface Incoming Wavelength Cross-Connect Outgoing Interface Outgoing Wavelength
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O-E-O Crossconnect Switch (OXC)
Incoming 1 N fibers Demux 2 WDM (many λs) Individual wavelengths O O/E O/E O/E O/E O/E O/E O/E O/E O/E E O E/O E/O E/O E/O E/O E/O E/O E/O E/O Mux Outgoing Switches information signal on a particular wavelength on an incoming fiber to (another) wavelength on an outgoing fiber.
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fibers 1 2 N
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Optical core network
Opaque (O-E-O) and transparent (O-O) sections
Transparent optical island Client signals E/O O/E O O O O E E O to other nodes E O from other nodes O E O O O O Opaque optical network
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OEO vs. All-Optical Switches
OEO All-Optical • Capable of status monitoring • Optical signal regenerated – improve signal-to-noise ratio • Traffic grooming at various levels • Unable to monitor the contents of the data stream • Only optical amplification – signal-to-noise ratio degraded with distance • No traffic grooming in sub wavelength level • Less aggregated throughput • More expensive • More power consumption • Higher aggregated throughput • ~10X cost saving • ~10X power saving
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Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.
optical fibers One fiber cross-connect Repeater
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Hierarchical switching: Node with switches of different granularities
A. Entire fibers Fibers O O O Fibers B. Wavelength subsets O O O “Express trains” C. Individual wavelengths O E
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O “Local trains”
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Wide Area Network (WAN)
WAN : Up to 200-500 wavelengths 40-160 Gbit/s/
wavebands (> 10
) OXC: Optical Wavelength/Waveband Cross Connect CSIT5600 by M. Hamdi 26
Packet (a) vs. Burst (b) Switching
Payload A Header
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Incoming fibers
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Fixed-length (but unaligned) B A Control wavelengths Data wavelengths B Control packets
2 1
Synchronizer
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Header recognition, processing, and generation Setup Switch
1 2 2 2
FDL’s Offset time
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(a) O/E/O Switch Control packet processing (setup/bandwidth reservation)
2 1
New headers
2 1
Data bursts (b)
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C C D D
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IP packets optical burst formation
MAN (Country / Region)
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Optical Switching Technologies
• • • • • • • • MEMs – MicroElectroMechanical Liquid Crystal Opto-Mechanical Bubble Technology Thermo-optic (Silica, Polymer) Electro-optic (LiNb03, SOA, InP) Acousto-optic Others…
Maturity of technology, Switching speed, Scalability, Cost, Relaiability (moving components or not), etc.
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MEMS Switches for Optical Cross Connect
Moveable Micromirror Proven technology, switching time (10 to 25 msec), moving mirrors is a reliability problem.
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WDM “transparent” transmission system
(O-O nodes) Wavelengths disaggregator O Fibers O Wavelengths aggregator O multiple λs O O O Optical switching fabric (MEMS devices, etc.) Tiny mirrors Incoming fiber Outgoing fibers
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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
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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
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Optical Burst Switching (OBS)
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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 re combined with the header at output interface
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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
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• 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.
payload hdr Wavelength i input port j Optical packet hdr payload CPU Optical switch payload hdr Re-combined Wavelength i output port j CSIT5600 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 payloadhdr payloadhdr . . .
payloadhdr Output ports .
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OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering
Slotted networks
• Fixed packet size • Synchronization stages required
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OPS Architecture: Synchronization
Slotted networks
• Fixed packet size • Synchronization stages required
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OPS Architecture: Synchronization
Slotted networks
• Fixed packet size • Synchronization stages required
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OPS Architecture: Synchronization
Slotted networks
• Fixed packet size • Synchronization stages required
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OPS Architecture: Synchronization
Slotted networks
• Fixed packet size • Synchronization stages required
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OPS Architecture: Synchronization
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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
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OPS Architecture
Contention Resolutions
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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 re convert the signal back to optical domain • Electronic memories too slow for optical networks
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OPS Architecture
Contention Resolutions
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Optical buffering
1 2 3 4 1 1 1 2 3 4
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OPS Architecture
Contention Resolutions
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Optical buffering
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OPS Architecture
Contention Resolutions
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Optical buffering
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OPS: Contention Resolution
• Wavelength multiplexing – Resolve contention by transmitting on different wavelengths – Requires wavelength converters - $$$
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OPS Architecture
Contention Resolutions
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Wavelength conversion
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OPS Architecture
Contention Resolutions
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Wavelength conversion
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OPS Architecture
Contention Resolutions
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Wavelength conversion
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OPS Architecture
Contention Resolutions
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Wavelength conversion
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OPS Architecture
Contention Resolutions
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Wavelength conversion
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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: – T he end-to-end delay for an optical packet may be unacceptably high.
– Optical p ackets may have to be re-ordered at the destination
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Electronic Switches Using Optical Crossbars
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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. 10KW – Physical consideration, i.e. temperature, humidity • Scaling to multiple racks – Fiber links and central fabrics
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Logical Architecture of Multi-rack Switches Fiber I/O Line Card Framer Local Buffers Laser Laser Scheduler Laser Laser Line Card Local Buffers Framer Fiber I/O Crossbar Fiber I/O Line Card Framer Local Buffers Laser Laser Laser Laser Line Card Local Buffers Framer 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?
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Optical Switch Fabric
Fiber I/O Line Card Framer Local Buffers Laser Laser Scheduler Laser Laser Line Card Local Buffers Framer Fiber I/O Crossbar Fiber I/O Line Card Framer Local Buffers Laser Laser Laser Laser Line Card 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 – 2D/3D MEMS, liquid crystal, bubbles, thermo-optic, etc.
Hybrid architecture takes advantage of the strengths of both electronics and optics CSIT5600 by M. Hamdi 61
Electronic Vs. Optical Fabric
Electronic Trans.
Line Buffer Inter connection Switching Fabric Inter connection Buffer Trans.
Line Optical Trans.
Line Buffer Inter connection Switching Fabric Inter connection Buffer Trans.
Line
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Optical Electronic E/O or O/E Conversion
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Multi-rack Hybrid Packet Switch
Rack Buf f er E/O O/E Buf f er Buf f er E/O Optical Fiber Optical Crossbar Buf f er E/O Linecard Buf f er E/O O/E Buf f er Optical Fiber O/E Buf f er O/E Buf f er Switch Core
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Features of Optical Fabric
• Less E/O or O/E conversion • High capacity • Low power consumption • Less cost However, • Reconfiguration overhead (50-100ns) – Tuning of lasers (20-30ns) – System clock synchronization (10-20ns or higher)
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Scheduling Under Reconfiguration Overhead
• Traditional slot-by-slot approach Scheduler Schedule Reconfigure Transfer Time Line • Low bandwidth usage
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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 1.
2.
Scheduling task Find out the matching Determine the holding time
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Scheduling Under Reconfiguration Overhead
• Reduce the scheduling rate – Bandwidth Usage = Transfer/(Reconfigure+Transfer) Constant • Approaches – Batch scheduling: TSA-based – Single scheduling: Schedule + Hold
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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
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Routing and Wavelength Assignment
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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
– Physical constraints: is limited.
• limited number of optical transceivers limit the number of channels.
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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
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Access Fiber
No Wavelength Converters
WSXC Wavelength 1 POP POP Wavelength 2 Wavelength 3
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With Wavelength Converters
WIXC Wavelength 1 Access Fiber POP POP Wavelength 2 Wavelength 3
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Routing and Wavelength Assignment (RWA)
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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
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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
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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
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Static RWA
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http://www.tct.hut.fi/~esa/java/wdm/
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Solving Dynamic RWA
• During network operation, requests for new light paths 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
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