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Photonics in Switching
Lena Wosinska, Assoc. Professor
Royal Institute of Technology KTH
Electrum 229, 164 40 Kista, Sweden
The aim of this tutorial
To s ow princip es or optica circuitswitching and optical packet switching
To highlight the main technologicalproblems
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school 2
To give some examples of the opticalswitching node architectures
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Outline
Introduction
Photonic Circuit Switching WDM network design
WDM network elements
Photonic Packet Switching
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Contention resolution
Summary
How to connect end users ?
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Switched networks
Switching nodes not concerned with contents
of data purpose: provide switching
facility in general not fully connected
End nodes provides data to transfer
1
A
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nodesLinks
physical connections betweennodes
Switching
Circuit
Switching
Packet
Switching
Switching
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Connection Oriented
(Virtual Circuit)
Datagram
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Development ofoptical networks
First-generation optical networks
Transmission in the optical domain (to provide capacity)
Example: SONET network (Synchronous Optical Network)
Second-generation optical networks
More functionality in the optical domain (optical networking)
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Some of routing, switching and intelligence is moving into theoptical layer
Third-generation optical networks (?)
SDH/SONET Networking
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Photonics in switching
O tical circuit switchin OCS Wavelength-routed networks
Relatively mature technology today
Providing lightpaths
WDM network elements: OLT, OADM, OXC
(All)optical packet switching (OPS) Not available today due to some technological problems
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on ro a e op ca memory or op ca u er ng Control functions in the optical domain
Synchronization, etc
Optical burst switching (OBS) A feasible solution?
Optical Circuit Switching
W ave leng th -Rou t ed Ne tw o rk s
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-
Wavelength-RoutedNetworks
Provide lightpaths
Problems:
Low bandwidthefficiency
Large granularity
Network
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1
A
Wavelength-routing switch
Access (client) node (e.g., IP router):
contains (tunable) transmitters and receivers
Optical circuit switching
Solvin LTD and RWA roblems
A lightpath corresponds to a circuit
Set-up a lightpath
The whole lightpath is available during the connection Disconnect
Network elements
Fiber
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Optical line amplifier (OLA)
Optical line terminal (OLT)
Optical add-drop multiplexer (OADM)
Optical cross-connect (OXC)
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Definitions
Logical (virtual) topology
Physical topology: Fiber topology
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Kind of time multiplexing: packing a low speed
channels into higher speed channels.
ObjectiveSeattle
New York
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school 14
Given a traffic matrix (a forecast) and a fiber (physical) topology:design the networkthat fits the traffic forecast or/andoptimize the (existing) network
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Network design
the fiber (physical) topology and
traffic matrix (obtained by forecasting): in packets/second
Determine
the lightpath (virtual or logical) topology (Lightpath topologydesign: LTD): grooming
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physical routes through the network and wavelengthassignment (RWA): map the LTD into the physical topology
Heuristic solution
ar to eterm ne t e g tpat topo ogyjointly with the routing and wavelength
assignment Split into separate LTD and RWA problems
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obtained LTD within the optical layer (i.e. for theobtained LTD solve RWA problem).
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LTD:Lightpath topology design
ven: The traffic demands
The maximum number of ports per client node
Lightpaths interconnect client nodes bidirectionally
Determine the topology and routing of packets
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Objective (an optimization problem): Minimize the maximum load that any lightpath must
carry
RWA:Routing and wavelength assignment
to the fiber topology
Objective
Offline: for all lightpaths determined by the
LTD minimize the number of wavelen ths
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used per link
Online: for demands coming during
operation minimize the blocking probability
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RWA approaches
Heuristic Routing sub-problem
Wavelength assignment sub-problem
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Routing sub-problem
Fixed shortest-path routing
Fixed-alternate routing Routing table at each node that contains a list of
fixed routes to each destination
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The route is chosen dynamically, depending of the
network state
Fault-tolerant routing
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Routing algorithms
Shortest Path selects the shortest source-destinationpath (# of links/nodes)
Least Loaded Routing avoids the busiest links
Least Loaded Node avoids the busiest nodes
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WA sub-problem
-
Graph coloring
Dynamic WA sub-problem Random (R) Wavelength Assignment
First-Fit (FF)
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Least-Use LU SPREAD
Max-Used (MU)/PACK
Least Loaded (LL)
etc.
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Static (offline) WA:Graph coloring
NC Wavelen th continuit constraint
Given: all connections and routes
Objective: assign wavelengths (colors) to each lightpath so as tominimize the number of wavelengths used under the wavelengthcontinuity constraint
Constr uct a graph G so that each lightpath in the system isrepresented by a node. Undirected edge between nodes in the
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G if the corresponding lightpaths share a physical link Color t he nods of G such that no two adjacent nodes have the
same color.
Dynamic (online) WAalgorithms
Random (R)
Pick one with uniform probability
First -Fit (FF)
All wavelengths are numbered Assign the first available wavelength
Least -Used (LU)/ SPREAD
Select the wavelength that is the least used in the network
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school 24
Max-Used ( MU)/ PACKED
Select the most used wavelength in the network
Least- Loaded (LL) designed for multi-fiber networks
Select the wavelength with largest residual capacity on the most loadedlink along the connection.
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WDM network elementsWDM network example
Optical line terminals
(OLTs)
Optical add-drop
multiplexers (OADMs)
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p ca cross-connec s(OXCs)
Optical line amplifiers
OLT: Optical line terminal
Trans onders Adaptation from/to access
links Wavelength and fiber
Overhead and FEC BER monitoring
Main cost of OLT
Multiplexer To mer e the incomin
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channels
Amplifier (optional)
Optical supervisory channel(OSC) Added and terminated
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OADMOptical add/drop multiplexer
To and from equipment at local node
Remaining channels pass transparently
Channel selecti on
Any channel or only some
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Reconfigurable: software configurable remotely
One, a few or any number of channels Modularity
Loss dependence on number of dropped channels
OXCOptical Cross-connect
From input to output ports
From input to output wavelengths
Does not include input/output OLTs
Functions
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Protection switching (rerouting)
Performance monitoring
Wavelength conversion
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Types of OXCs
Transparency
Cost
Size
Transparent or opaquecore
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Electronic signal
Signal monitoring
Optical signal
Transparency
All-optical OXCs
Grooming
Higher demand for lightpaths No aggregation of low bitrate demands
Wavelength conversion
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g er oc ng o g pa eman s
Signal regeneration More constrained routing of lightpaths
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All-optical OXCEx. 1: Clos architecture
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Three stage strict internal non-blocking Clos architecture.
Size: 128x128
L. Wosinska, L. Thylen, and R.P. Holmstrom: Large Capacity Strictly Non-Blocking OXCs Based on MEOMS
Switch Matrices. Reliability Performance analysis, IEEE/OSA JLT, Vol.19, No.8, Aug. 2001
All-optical OXCEx. 2: WR architecture
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Strict internal non-blocking wavelength routing architecture.
Size 128x128
L. Wosinska L. Thylen, and R.P. Holmstrom: Large Capacity Strictly Non-Blocking OXCs Based on MEOMS
Switch Matrices. Reliability Performance analysis, IEEE/OSA JLT, Vol.19, No.8, Aug. 2001
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All-optical OXCEx. 3: with TWC
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school
33J. Chen, A. Jirattigalachote, L. Wosinska and L Thyln, Novel Node Architectures for Wavelength-Routed WDMNetworks with Wavelength Conversion Capability, in Proc. of ECOC08, Brussels, Belgium, September 2008
Performance evaluation
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school
34J. Chen, A. Jirattigalachote, L. Wosinska and L Thyln, Novel Node Architectures for Wavelength-Routed WDMNetworks with Wavelength Conversion Capability, in Proc. of ECOC08, Brussels, Belgium, September 2008
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Shortcomings with OCS
Low utilisation of resources
Hard optimization problems need to be solved
(LTD and RWA)
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Solution: Optical Packet Switching (?)
Optical packet switching
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OPS Networks
Large capacity
High bandwidth efficiency
Rich routing functionalities
Great flexibility andreliability
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Optical packet switching
Advanta es Complements WDM
Allows grooming in optical domain
Allows statistical multiplexing
Can improve bandwidth utilization within the optical layer
Increase flexibility
Problems
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Technological problems
Optical control functions
Synchronization
Optical buffering
High complexity of OPS nodes high cost and low reliability
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Optical router (OPS node):Needed functions.
Decoding of packet header Could be electronic: header encoded at lower bit rate
Setup of switch fabric Packet delayed until setup done (a fixed delay)
Setup requires scheduling of packets from all inputs Simplified for fixed packet size and synchronized operation
Fast reconfiguration of fabric (200 ns for 250 byte packet at 10 Gb/s)
Synchronization: Elastic buffering of packets to align packets at all inputs
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n y nee e w en sw c a r c s sync ronous Synchronous fabric has better throughput
Multiplexing of lower-speed streams (and reverse operation, i.e.demultiplexing)
Contention resolution (e.g. buffering of packets if output busy)
Contention resolution in OPS
Contention may be dealt with in
Time
Wavelength
Space
Electronic packet switching typically rely on the time
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domain by means of queuing
What about optical packet switching ?
Queuing in optical domain is difficult
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If output busy:Handling packet contention Drop a packet
r y v r
Deflect the packet Send it on a free output
Restrict the deflection Output that leads to destination
Output with a route to the destination that is at most mhops longer
Also called hot-potato routing
Increases delay and network load
Creates variable delays and potentially reordering
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C ange t e wave engt TWC Chose a wavelength available at the output
All-optical tunable wavelength converters not available yet
Buffer the packet
Store the packet until the output is available
Applying TWC may allow for decrease of the buffer size
Contention resolutiontechniques
Bufferless architecturesOCIC
e ec on rou ng
TWC
Electronic buffers
Optical buffers Placement at a node
Output buffer
Input buffer
switchfabric
OC
OC
IC
IC
switchfabric
IC
IC
IC
OC
OC
OC
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ec rcu a on u er
Dedicated or shared buffers
Technology
FDLs
Novel optical memories
EIT or Opt. resonators
buffer
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OPS with electrical buffer
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L. Wosinska and G. Karlsson, A photonic packet switch for high capacity optical networks, in Proc. NFOEC02, Dallas,Texas, September 2002
COMPARISON
Architecture I II
Buffer Dedicated Shared
Packet loss probability High Low
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Scheduling Simple Complex
L. Wosinska and G. Karlsson, A photonic packet switch for high capacity optical networks, in Proc. NFOEC02, Dallas,Texas, September 2002
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Comparison. N=20
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L. Wosinska and G. Karlsson, A photonic packet switch for high capacity optical networks, in Proc. NFOEC02, Dallas,Texas, September 2002
A2A1
Optical buffering
Fiber delay lines (FDLs)
Not random access
Require synchronization
Supported packet format Constant packet size
Some configurations support variable packet size A certain granularity
Not compatible with packet formats of different packet size
Long fiber delay lines Not very practical solution
Ex.: For packets containing 53 bytes (ATM cell) at 2.5 Gb/s the length of fiber in the FDLs needs
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to be the multiples of 640 m
Feed-forward or feed-back configurations
Novel solutions for optical memory
Material subjected to EIT (Electromagnetically induced transparency)
Optical Cavities
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1 km
Fiber delay line
Cheap and easy to manufacture
Several kilometers long
-
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No flexibility in terms of storage time
Requires synchronization
Many architectures proposed to introduce variabledelay
EITNovel type for optical memory
artificially created spectral window of transparencyused to slow and spatially compress light pulses.
Inside the memory cell, light is converted into a spinexcitation of atoms and its velocity drops down tozero once the coupling beam is turned off. After the
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coherence is converted back into light signal.
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EIT, cont.
beam
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Memory cellArriving IP packet
Optical fiber
EIT, cont.
Phase 1 : w r i t i ng
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Light slows down inside the cell andis spatially compressed
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EIT, cont.
Phase 1 : w r i t i ng
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Cell length
The memory cell needs to be long enough to fit the entire packet
IP packet of 1500bytes at 2,5Gb/s is 1,4 km long in free space andabout 1km long in an optical fiber
EIT, cont.
beam is turnedoff
Phase 2 : s tor age
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The light is stored in the material
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EIT, cont.
beam is turnedback on
Phase 3 : read in g
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The light is recoveredand leaves the cell
EIT, cont.
Variable couplingpower
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No storage of light
We regulate the slowdown factor by varying the coupling power
The packet is slowed down in the cell
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EIT, cont.
material slow dow n factor storage t ime
Quantum dots 40 in room temperature
107 in very low temperature
8.7ns
Atomic vapor 105 up to 0.5 msdepends on thegas
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Slow down factor and storage time depend on thematerial, temperature, coupling power, bandwidthand wavelength
Optical cavities use optical resonance in photonic structures
Optical cavities
Slow down factor of 104 (depending on the number ofside cavities)
Storage time: 50 ns
Chip scale implementation of the system foreseeable
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Requirementsfor optical memory
Technology Compression rate (cell size) Tuning of the intensity of thecontrol field Temperature and mechanical stress Cost
Te lecommun ica t ions Wavelength Attenuation anddistortion Bandwidth Packet length Control memory cellsseparately
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Qo S The storage time Pulse distortion Priority classes
Comparison
storage t ime cell size temperat ure bandw idt h-wavelength
EIT
Up to 0.5 ms Order of cm Close to 0K or80C
Depends on thematerial
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Opticalcavities
Order of ns Size of a chip Room temp. No limitations
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Parallel electricaland optical buffer
Example 1:OPS with hybrid buffer.
positions
Opticaloutputs
Opticalinputs
Opticaldemultiplexer
Switchingmatrix
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59L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
ASSUMPTIONS The traffic load is uniformly distributed between the outputs.
The traffic load at all inputs is identical.
.
.
.
1
N
1
B u f f e r
1
.
.
.
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The transparency class (i.e. packets that can not beconverted to the electrical signal) represents 20% of the totaltraffic
N + N
.
.
.
.
.
.
N
N + 1
N
L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
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Without output buffer,N > 16
Packet loss probability
0 3 5
0,37
0,39
0,41
0,43
0,45
,
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0,31
0,33
,
0,4 0,5 0,6 0,7 0,8 0,9 1
load
simulated, const. packet l ength simulated. IP-traffic calculated
L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
IP traffic trans arenc class
Simulation results.IP traffic.
1,E-02
1,E-01
1,E+00
packetlossp
r
obability
load 0.2
load 0.5
load 0.7
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The loss probability goes down to a certain point and thanstays constant as the buffer increases.
1,E-03
0 2 4 6 8 10 12 14 16
number of optical buffer positions
L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
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ATM traffic, transparency class
Simulation results.ATM traffic.
1,E-06
1,E-05
1,E-04
1,E-03
1,E-02
1,E-01
1,E+00
loss
probability
3 buffer positions5 buffer positions10 buffer positions
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The lowest achievable packet loss probability for a givennumber of buffer positions reaches a limit that cannot beovercome by increasing the maximum storage time.
0 0,05 0,1 0,15 0,2 0,25
maximum storage time in s
L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
ATM traffic, trasnparency class
Simulation results.ATM traffic.
1,E-06
1,E-05
1,E-04
1,E-031,E-02
1,E-01
1,E+00
packetloss
pro
bability load 0.2
load 0.5
load 0.7
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Storage time of 0.5ms is enough to obtain any value of lossprobability for any traffic load.
0 2 4 6 8 10 12
number of optical buffer positions
L. Wosinska, J. Haralson, L. Thyln, Benefit of Implementing Novel Optical Buffers
in an Asynchronous Photonic Packet Switch, in Porc. ECOC04, Stockholm, Sept. 2004
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Example 2:OPS with TWCs and buffer
Krakow Sept. 29, 2009 L. WosinskaTutorial at BONE Summer school
65JiaJia Chen and L. Wosinska, Novel Architectures of Asynchronous Optical Packet Switch, in Proc. of EuropeanConference on Optical Communication ECOC07, Berlin, Germany, September 2007
Evaluation
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66JiaJia Chen and L. Wosinska, Novel Architectures of Asynchronous Optical Packet Switch, in Proc. of EuropeanConference on Optical Communication ECOC07, Berlin, Germany, September 2007
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Summary Switched networks
Photonic circuit switching WDM network design: solving offline LTD and RWA
Objective: minimize number of wavelengths
Dynamic scenario
Oblective: minimize blocking probability
WDM network elements
Photonic packet switching
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Makes networks more efficient Many technical challenges for switch design
Buffering for contention resolution
Scheduling for contention resolution with possible deflection
Switching speeds
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