Download - Switching Architectures for Optical Networks
CSIT5600 by M. Hamdi1
Switching Architectures for Optical Networks
CSIT5600 by M. Hamdi2
SONET
DataCenter SONET
SONET
SONET
DWDM DWD
M
AccessLong HaulAccess MetroMetro
Internet Reality
CSIT5600 by M. Hamdi3
Hierarchies of Networks: IP / ATM / SONET / WDM
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.
CSIT5600 by M. Hamdi5
Dense Wave Division Multiplexing (DWDM)
Multiple wavelength bands on each fiberTransmit by combining multiple lasers @ different
frequencies
Output fibers
Long-haul fiber
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2
3
4
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Anatomy of a DWDM System
Terminal A Terminal B
Post-Amp
Pre-Amp
Line Amplifiers
MUX
DEMUX
TransponderInterfaces
TransponderInterfaces
DirectConnections
DirectConnections
Basic building blocks• Optical amplifiers• Optical multiplexers• Stable optical sources
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Core Transport Services
OC-3
OC-3
OC-12
STS-1STS-1STS-1
• ProvisionedSONET circuits.
• Aggregated intoLamdbas.
• Carried overFiber optic cables.
CircuitOrigin
Circuit Destination
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WDM Network: Wavelength View
WDM link
Optical Switch
Edge Router
Legacy
InterfacesLegacy
Interfaces
Legacy
Interfaces
( e.g., PoS, Gigabit
Ethernet, IP/ATM)
Interfaces
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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
Swit
chin
g
Transm
ission
Optical Transport
Routing
IP
Services
Swit
chin
g
Transm
ission
Optical Transport
Swit
chin
g
Transm
ission
Optical Transport
Routing
IP
Services
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Typical Super POP
OXC
Core IP
router
Interconnection
Network
LargeMulti-serviceAggregation
Switch
Voice Switch
CoreATM
Switch
SONET
Coupler&
Opt.amp
DWDM+
ADM
DWDM Metro Ring
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Typical POP
OXCDWDM
VoiceSwitch
SONET-XC
DWDM
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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
No converters
1
2 3
New request 1 3
1
2 3
New request 1 3
With converters
WC
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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
CSIT5600 by M. Hamdi15
Wavelength Router
Wavelength Router
Control Plane:Wavelength Routing
Intelligence
Data Plane:Optical Cross
Connect Matrix
Single Channel Links to IP Routers, SDH
Muxes, ...
Unidirectional DWDM Links to
other Wavelength Routers
Unidirectional DWDM Links to
other Wavelength Routers
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Optical Network Architecture
IP Router
Optical Cross Connect (OXC)
OXC Control unit
Control Path
Data Path
UNIUNIMesh Optical
NetworkIP Network IP Network
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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)
Optical SwitchFabric
3
2
2
4
4
1
1
3
All Optical Cross-connect (OXC) Also known as PhotonicCross-connect (PXC)
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Optical Cross-Connect with Full Wavelength Conversion
• M demultiplexers at incoming side• M multiplexers at outgoing side• Mn x Mn optical switch has wavelength converters at switch
outputs
1,2, ... ,n
1,2, ... ,n
1,2, ... ,n
1
2
M
Optical CrossBarSwitch
WavelengthConverters
WavelengthMux
WavelengthDemux
1,2, ... ,n
1,2, ... ,n
1,2, ... ,n
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.
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12n
12n
12n
1
2
n
12n
n12
1
2
M
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Wavelength Router with O/E and E/O
Cross-Connect
1
3
Outgoing InterfaceOutgoing Wavelength
Incoming InterfaceIncoming Wavelength
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Demux1
Incoming fibers
OE
OIndividual wavelengths
Mux
Outgoing fibers
O-E-O Crossconnect Switch (OXC)
O/EO/EO/E
O/EO/EO/E
O/EO/EO/E
N
2
E/OE/OE/O
E/OE/OE/O
E/OE/OE/O
Switches information signal on a particular wavelength on anincoming fiber to (another) wavelength on an outgoing fiber.
1
N
2WDM(many λs)
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Optical core networkOpaque (O-E-O) and transparent (O-O) sections
E/OClientsignals
O/E
to other nodesfrom other nodes
E E O
O
Transparentoptical island
O O
OOE
OO
O O
EO
Opaque optical network
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OEO vs. All-Optical Switches
• Capable of status monitoring
• Optical signal regenerated – improve signal-to-noise ratio
• Traffic grooming at various levels
• Less aggregated throughput
• More expensive
• More power consumption
• 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
• Higher aggregated throughput
• ~10X cost saving
• ~10X power saving
OEO All-Optical
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Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.
cross-connect
opticalfibers
RepeaterOne fiber
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Hierarchical switching: Node with switches of different granularities
FibersOA. Entire fibers
Fibers
O O
OB. Wavelength subsets
O O
“Express trains”
OC. Individual wavelengths
E O“Local trains”
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Wide Area Network (WAN)
GAN
links
OXC: Optical Wavelength/Waveband Cross Connect
WAN : Up to 200-500 wavelengths40-160 Gbit/s/wavebands (> 10 )
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Packet (a) vs. Burst (b) Switching
Incomingfibers
Fixed-length(but unaligned) FDL’s
Synchronizer
Header
Payload
Setup
Header recognition,processing, and generation
Switch1
B
C
DNewheaders
2
1
2 2
1
(a)
A
Switch
2
1 1
2
(b)
O/E/O
Control packet processing(setup/bandwidth reservation)
2 2
1 1
Controlpackets
Data bursts
Controlwavelengths
A
B
C
D
Datawavelengths
Offset time
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MAN (Country / Region)
opticalburst
formation
IPpackets
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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, Maturity of technology, Switching speed, Scalability, Cost, Relaiability (moving components or not), etc.Relaiability (moving components or not), etc.
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MEMS Switches for Optical Cross-Connect
M o v e a b le M ic ro m irro r
Proven technology, switching time (10 to 25 msec), moving mirrors is a Proven technology, switching time (10 to 25 msec), moving mirrors is a reliability problem.reliability problem.
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WDM “transparent” transmission system
Wavelengthsaggregator
multipleλs
Fibers
(O-O nodes)
Wavelengthsdisaggregator
O O O O OO
Optical switching fabric (MEMS devices, etc.)
Incoming fiberTiny mirrors
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 iinput port j
Opticalpacket
hdr CPU
Optical switch
payload
payload hdr
Re-combinedWavelength ioutput port j
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Output port contention
• Assuming a non-blocking switching matrix, more than one packet may arrive at the same output port at the same time.
Output ports
payloadhdr
payloadhdr
payloadhdr
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Optical SwitchInput ports
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.
.. . .
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Sync.
•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: SynchronizationOccurs in electronic switches – solved by input bufferingOccurs in electronic switches – solved by input buffering
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•Fixed packet size
•Synchronization stages required
Slotted networks
Sync.
OPS Architecture: Synchronization
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•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Sync.
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•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Sync.
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•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Sync.
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OPS Architecture: Synchronization
Sync.
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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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1
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•Optical buffering
OPS Architecture
Contention Resolutions
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•Optical buffering
OPS Architecture
Contention Resolutions
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1
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•Optical buffering
OPS Architecture
Contention Resolutions
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OPS: Contention Resolution
• Wavelength multiplexing– Resolve contention by transmitting on different
wavelengths
– Requires wavelength converters - $$$
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•Wavelength conversion
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1
1
2
1
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OPS Architecture
Contention Resolutions
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1
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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1
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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1
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1
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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1
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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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:
– The end-to-end delay for an optical packet may be unacceptably high.
– Optical packets 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
Switch Core
Optical links
Line cardrack• 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
• 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?
Crossbar
Scheduler
Switch Fabric System
Framer
Line Card
Laser Laser
Laser
LaserLocal
Buffers
Framer
Line Card
Laser LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Fiber I/O
Fiber I/O
Fiber I/O
Fiber I/O
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Optical Switch Fabric
• 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
Crossbar
Scheduler
Switch Fabric System
Framer
Line Card
Laser Laser
Laser
LaserLocal
Buffers
Framer
Line Card
Laser LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Fiber I/O
Fiber I/O
Fiber I/O
Fiber I/O
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Electronic Vs. Optical Fabric
Trans.Line
Buffer
SwitchingFabric
Inter-connection
Trans.Line
BufferInter-connection
Electronic
Trans.Line
Buffer
SwitchingFabric
Inter-connection
Trans.Line
BufferInter-connection
Optical
Optical
Electronic
E/O or O/EConversion
favorred
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Multi-rack Hybrid Packet Switch
OpticalCrossbar
E/OBuf f er O/E Buf f er
E/OBuf f er
E/OBuf f er
E/OBuf f er
O/E Buf f er
O/E Buf f er
O/E Buf f er
Rack
OpticalFiber
OpticalFiber
Switch Core
Linecard
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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
• Low bandwidth usage
Scheduler
Time Line
ScheduleReconfigure Transfer
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Reduced Rate Scheduling
• 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 task1. Find out the matching
2. Determine the holding time
Fabric setup (reconfigure)
Traffic transfer
Time slot
Slot-by-slot Scheduling, zero fabric setup time
Reduced rate Scheduling, each schedule is held for some time
Slot-by-slot Scheduling with reconfigure delay
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Scheduling Under Reconfiguration Overhead
• Reduce the scheduling rate
– Bandwidth Usage = Transfer/(Reconfigure+Transfer)
• Approaches
– Batch scheduling: TSA-based
– Single scheduling: Schedule + Hold
Constant
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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 is limited.
– Physical constraints:
• 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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No Wavelength Converters
POPPOP
Access Fiber
Wavelength 1
Wavelength 2
Wavelength 3
WSXC
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With Wavelength Converters
POPPOP
Access FiberWavelength 1
Wavelength 2
Wavelength 3
WIXC
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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
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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
• 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/
• 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/
• 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/
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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