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– Electronic packet switches are power-hungry
– Optical technologies are not power-hungry
– Electronic packet switches have reached their limit of scaling
– Optical packet switches can scale to massive throughput
– The “electronic bottleneck” constrains electronic throughput
– Optical to electronic (O/E) and electronic to optical (E/O) converters need to be eliminated
Packet Switching “Myths”
Reduced bit rate (i.e. parallel processing)
Electronic Packet Switch
Speed (throughput) is not a limitation
Fibers
Optics
Switch Fabrics Buffers
DEMUXsMUXs
Forwarding Engine
J
m x mSwitch
m O/E Converters
Electronics
k:1 DEMUX
1: k MUX
12
k
Optical Packet Switch
Demutiplexers
m x m Wavelength-Interchanging
Switch
Output Buffers
Forwarding Engine
Input SynchronizersMutiplexers
Electronics Optics
Optical synchronization and buffering not viable in practice. See, for example, Tucker JLT, pp. 4655-4673, 2008.
m
m
E/O
12
12
k:1 MUX
k
1
Input wavelengths
(B)Output
wavelengths (B)
Forwarding Engine
m x mElectronic
Switch(B/k)
k
2
k x kElectronic Switches
(B/k)
Forwarding Engine
1:k DEMUX
1
m
2
O/E
2
m
1
Electronic Packet Switch (Wavelength I/O)No buffers
(45pJ/b)
(30 pJ/b/stage)
(20 pJ/b/stage)
(30 pJ/b)
Optical Frequency
Optical Frequency
1 2 3 k 1 2 3 k 1 2 3 k
Wavelength
Waveband
Bit rate per wavelength = B
Bit rate per wavelength = B/k
Aggregate bit rate per
waveband = B
Electronic Packet Switch (Wavelength I/O)
Wavelength (WL)
Waveband (WB)
m
1
E/O
1 k:1 MUX
1:k DEMUX
Input wavebands
(B)
Output wavebands
(B)
O/E
2
m
1
Forwarding Engine
21
k
1
k
1
k
k
m x mElectronic
Switch(B/k)
k
2
Forwarding Engine
k x kElectronic Switches
(B/k)
Electronic Packet Switch (Waveband I/O)No buffers
(30 pJ/b)
(45pJ/b)
Integration: see, for example, R. Nagarajan et al., JSTQE 2010
Power consumption = 21 W 20bitE pJ→ =
Assuming Benes architecture, 22 log 144 1 14 1.4 /bitp E pJ stage= − = → =
10-12
10
2010
100 1000
10-7
10-8
10-9
10-10
10-11Waveband
Ene
rgy
per B
it pe
r Sw
itch,
Ebi
t(J
)
Bit Rate per Wavelength or Waveband, B (Gb/s)
Switch Capacity: mB = 1000 Tb/s
Wavelength
Input-to-Output Energy per Bit
Limited by O/E/O and MUX/DEMUX speed
Excludes Forwarding
Engine
m
m
E/O
12
12
k:1 MUX
Input wavelengths
(B)
Output wavelengths
(B)
1
m
2
O/E
2
m
1
m x mOptical
Switch (B)
Forwarding engine, header replacement (B/k)
1:k DEMUX
Optical Packet Switch
Power off during payload ?
Driver
Optical Packet Switch
Consider four switch fabric technologies:
– Arrayed-waveguide-gratings with tuneable wavelength converters
– Semiconductor optical amplifier (SOA) gate arrays
– Arrays of 1 X N electro-optic switches
– Micro-ring resonator switches
m TWCs m TWCs m FWCsm TWCs1
m
AWG AWG AWG
AWGs
1
m
AWG-Based Switch FabricThree-stage Benes
AWGs AWGs
SOA Gate
FWC
m
1
m3-dB Coupler
1
2
3
2
m x m Benes SOA Gate Array
2Number of stages 2log 1m= − 1024 19 stagesm = →8192 25 stagesm = →
4mlog2 m -1 control lines
R. F. Kalman et al., , IEEE Photonics Tech. Lett., vol. 4, p. 1048, 1992.
1 2
3 dB
3 dB
3 dB 3 dB22 log 1m −
ER ER ER
Crosstalk in SOA Gate Array
22 log 1mCrosstalkSignal ER
−=
Psignal
Pcrosstalk
Idrive ~ 100 mA
Extinction Ratio
Array of 1 X N E/O Switches (Spanke Architecture)
61000 ~ 2 10 control linesm x= →810,000 ~ 2 10 control linesm x= →
n (> m) control lines
FWC
1
m
12
m
n control lines
12
12
m1
m
1 x mswitch
1 x mswitch
m x 1 switch
n control lines
m
12
mm x 1 switch
T. Tanemura and Y. Nakano, IEICE Electron. Express, 2008.
FWCs
m
1
m
2
FWCs
1
2
Crosspoints
Dilated Micro-Ring Resonator Switch
control lines
4 resonance controllers
Lee et al., IEEE Photonics Society Annual meeting, 2010
CMOS driver
10-12
10 100 1000
10-7
10-8
10-9
10-10
10-11
E-WB
O-Spanke
O-Benes/Spanke
Ene
rgy
per B
it pe
r Sw
itch,
Ebi
t(J
)
Bit Rate per Wavelength or Waveband, B (Gb/s)
O-SOA
O-AWG O-Micro-Ring
E-WL
Energy per Bit per SwitchExcludes Forwarding Engine
2010 Switch Capacity: mB = 1000 Tb/s
O- AWG
E-WB
E-WL
O-SOA
O-Benes/Spanke O-Spanke
10-12
10-7
10-8
10-9
10-10
10-11
Ene
rgy
per B
it pe
r Sw
itch,
Ebi
t(J
)
10 100 1000
Bit Rate per Wavelength or Waveband, B (Gb/s)
O-Micro-Ring
Energy per Bit per Switch (2020)Electronics: ITRS Roadmap. Optics: 10% p.a. improvement.
2020 Switch Capacity: mB = 1000 Tb/s
103
107
106
105
104
Sw
itch
Pow
er C
onsu
mpt
ion
(W)
100 1000Switch Throughput, mB (Tb/s)
O-Benes/Spanke
O-SOA
E-WLO-AWG
E-WB
B = 100 Gb/s
2020
Switch Power Consumption(2020)
– Electronic packet switches are power-hungry
– Optical technologies are not power-hungry
– Electronic packet switches have reached their limit of scaling
– Optical packet switches can scale to massive throughput
– The “electronic bottleneck” constrains electronic throughput
– Optical to electronic (O/E) and electronic to optical (E/O) converters need to be eliminated
Packet Switching “Myths”
?
Conclusion: “Don’t underestimate the potential of electronics”
- Optical interconnects