This document discusses optical switching techniques and architectures. It describes how optical switching can provide faster switching times compared to electrical switching. Several optical switch fabrics are discussed, including MEMS, electro-optic, and thermo-optic switches. Optical packet switching and optical burst switching are also summarized as approaches to improve network efficiency by switching data directly in the optical domain without conversion to electrical signals.

1. Optical SwitchingOptical Switching
Switch Fabrics, Techniques and ArchitecturesSwitch Fabrics, Techniques and Architectures
원종호 (INC lab)
Oct 30, 2006

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Outline
 Introduction
 Optical Switch
 Optical Packet Switch
 Optical Burst Switch
 GMPLS
 Conclusion

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Introduction
 Internet traffic has doubled per year
 New services like VOD, IPTV
 DWDM (Dense Wavelength Division Multiplexing) is developed
– Can transport tens to hundreds of wavelengths per fiber
 Then, What is problem?
– Slow O/E/O conversion.
– Electronic equipment is dependent on the data rate &
protocol. (non-transparent)
 Goal?
– All optical!

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Optical switch - OXC
 What is Optical Cross-Connect (OXC)?
– Set up light paths
 Electrical XC, All-optical XC, Opaque XC
Switch
Fabric
Electrical XC All-optical XC
Process O/E/O (slow) O/O/O (fast)
Data rate & format
Transparent?
No Yes
Implementation Easy Hard
 Major difficulties of All-optical XC
– The lack of processing at bit level in optical domain
– The lack of efficient buffering in optical domain

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Optical switch - Optical Switch Fabrics
 Optical Switch Fabrics
– Allow switching directly in the optical domain (All-optical)
 Important parameters
– Switching time ( )↓
– Insertion loss ( and loss uniformity at all input-output↓
connections)
– Crosstalk ( )↓
– Extinction ratio (ratio of ON-OFF power) ( )↑
– Polarization-dependent loss ( )↓
– Reliability, energy usage, scalability, temperature resistance

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Optical switch - Optical Switch Fabrics
 Main optical switching technologies
– Opto-mechanical Switch
• Use prisms, mirrors, directional couplers.
• Lack of scalability
– Micro-electro-mechanical System Device (MEMS)
• Use tiny reflective surfaces to redirect the light
• 2D-MEMS(on-off mirror)
• 3D-MEMS(movable mirror)
• Scalability
• low loss
• short switching time
• Low power consumption
• Low crosstalk
• Low polarization effect

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Optical switch - Optical Switch Fabrics
– Electro-optic Switch
• Use a directional coupler
• Its coupling ratio is changed by varying the refractive index
– Thermo-optic Switch
– Liquid-Crystal Switch
– Bubble Switch
– Acousto-optic Switch

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Optical switch - Optical Switch Fabrics
Optomechanical
Switch
MEMS Electro-optic
Switching time Milliseconds Milliseconds Nanoseconds
Insertion loss Low Low High
PDL Low Low high
Scalability Bad Good Bad

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Optical switch – large switches
 Main considerations in building Large switch
– Number of small switches required
– Loss uniformity
– Number of crossovers
• cause power loss, crosstalk
– Blocking Characteristics
• Blocking vs. non-blocking

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Optical Packet Switching
 Optical Circuit Switching
– Limited circuit
– Low efficiency (due to fixed bandwidth)
 Optical Packet Switching
– Using Packet ( = Header (for routing) + Data )
 If Optical Packet Switching is realized, it can
– allocate WDM channels on demand (microsecond)
– share network resource efficiently
– support burst traffic efficiently
– offer high-speed data rate/format transparency &
configurability

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Optical Packet Switching
 What is the problem in implementing OPS?
– Long Switching time
– Buffer is needed
 Long switching time is due to
– Extracting the routing information from the header
– Controlling switching matrix electronically
– Performing the switching and buffering functions
 Buffer at Optical domain is needed
– Data should be buffered while header is processed
– When a contention is occurred
– When the bandwidth is not sufficient
 We don’t have perfect solutions yet.

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Optical Packet Switching
 Contention Resolution
– Buffering
• Using FDL (Fiber Delay Line)
– bulky, expensive, indefinite, Quality degradation
• Solutions to reduce the number of FDLs,
– synchronous manner
– Use TOWC (Tunable optical wavelength converter)
– Deflection routing
• Only one packet – desired link, others – longer links
• There can be the looping of packets

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Optical Packet Switching
 Architecture
Output
Input
O/E interface:
extract the header info.
FDL

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Optical Packet Switching
 Shared Wavelength Converters
Reduce Tunable
Wavelength
converter

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Optical Burst Switching
 Switch the channels entirely in the optical domain
using electronic tech.
 Process
– Assemble the packets (have same destination) -> make bursts
at the edge
– Bursts are assigned to wavelength channels
– Switched through transparently without any conversion
– Disassemble into the original packets
 No need for Optical buffer.

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Optical Burst Switching
 How is it possible?
– reservation request
(control packet)
– Using offset-time
reservation

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Optical Burst Switching
 QoS Support Bigger offset, lower
probability of discard
 Optical Composite Burst Switching
– Minimize packet loss
Low priority burst
Not discarded

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The future of Optical switching

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GMPLS - Generalized Multiprotocol Label Switching
 Extends common control plane to support various
interfaces.
 GMPLS can support
– Packet switching
– Time-division
(SONET/SDH) +
– MPλS (wavelength
switching) +
– Spatial-switching
(OXC)
 It can support integrated control and management

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Conclusion
 Optical fiber is not fully exploited
 Switching functions must be executed optically
 Two obstacles
– The lack of optical memory
– processing capabilities in optical domain
 In the future, breakthroughs may counteract
the fundamental limitations of optics
 Then, current network is completely changed