Optical switching - subjects.ee.unsw.edu.au · X. Ma and G.-S. Kuo: 'Optical switching technology comparison: optical ... • Network must recognize control rate and format for packet

School of Electrical Engineering and Telecommunications

UNSW

Copyright © 30/03/2017 Tim Moors

Optical switching

1 Figures from http://www.leehansen.com/clipart/Seasons/Spring/pages/rainbow.htm and Lucent

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Copyright © Copyright © 2 30/03/2017 Tim Moors

References

Keshav & Varghese doesn’t cover this (except brief reference on K p. 15) :-( Other courses: e.g. ELEC9350/9355 (optical fibres/comms), PHTN4662 (Photonic

Networks) Overviews: • M. Maier and M. Reisslein: “Trends in Optical Switching Techniques: A Short Survey”,

IEEE Network, 22(6):42-7

• Cisco’s Fundamentals of DWDM Technology http://www.cisco.com/univercd/cc/td/doc/product/mels/dwdm/dwdm_ovr.htm

• D. Blumenthal et al: 'Photonic packet switches: Architectures and experimental implementations', Proc. IEEE 82(11):1650-67

• M. Borella: 'Optical components for WDM lightwave networks', Proc. IEEE 85(8):1274-307

• R. Ramaswami: Optical networking technologies: What worked and what didn't, IEEE Communications Magazine

LightReading.com – Industry news Later lecture on MPLS will discuss GMPLS [1RF] for optical routing control &

connection management

9751QW Copyright ©

http://dx.doi.org/10.1109/MNET.2008.4694173

http://www.cisco.com/univercd/cc/td/doc/product/mels/dwdm/dwdm_ovr.htm

http://dx.doi.org/10.1109/5.333744

http://dx.doi.org/10.1109/5.333744

http://dx.doi.org/10.1109/5.622506

http://dx.doi.org.wwwproxy0.library.unsw.edu.au/10.1109/MCOM.2006.1705990


http://www.lightreading.com/


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Easy-going Wikipedia pages

http://en.wikipedia.org/wiki/Multi-mode_optical_fiber

http://en.wikipedia.org/wiki/Single-mode_optical_fiber

http://en.wikipedia.org/wiki/Transverse_mode

http://en.wikipedia.org/wiki/Optical_amplifier

http://en.wikipedia.org/wiki/Wavelength-division_multiplexing

http://en.wikipedia.org/wiki/Tunable_laser

http://en.wikipedia.org/wiki/Routing_Wavelength_Assignment_%28RWA%29

http://en.wikipedia.org/wiki/Arrayed_Waveguide_Grating

http://en.wikipedia.org/wiki/Four_wave_mixing

http://en.wikipedia.org/wiki/Optical_switching

http://en.wikipedia.org/wiki/Optical_burst_switching

http://www.opticsjournal.com/tunablelasers.htm

9751Q4 Copyright ©







http://en.wikipedia.org/wiki/Transverse_mode

http://en.wikipedia.org/wiki/Optical_amplifier




http://en.wikipedia.org/wiki/Tunable_laser

http://en.wikipedia.org/wiki/Routing_Wavelength_Assignment_%28RWA%29

http://en.wikipedia.org/wiki/Arrayed_Waveguide_Grating

http://en.wikipedia.org/wiki/Four_wave_mixing

http://en.wikipedia.org/wiki/Optical_switching

http://en.wikipedia.org/wiki/Optical_burst_switching

http://www.opticsjournal.com/tunablelasers.htm


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References about

specific technologies P. Ball: “The next generation of optical fibers”, Technology Review,

104(4):55, 7 pgs J. Kiniry: 'Wavelength Division Multiplexing: Ultra high speed fiber

optics', IEEE Internet Comp. 2(2):13-5 X. Ma and G.-S. Kuo: 'Optical switching technology comparison: optical

MEMS vs. other technologies', IEEE Comm. Mag. 41(11):S16-S23 P. Chu et al: 'MEMS: the path to large optical crossconnects', IEEE

Comm. Mag. 40(3):80-7 S. Yao et al: 'Advances in photonic packet switching: an overview', IEEE

Comm. Mag. 38(2):84-94 T. Battestilli and H. Perros: 'An introduction to Optical Burst Switching',

IEEE Optical Communications 41(8):S10-5 H. Zang et al: “A review of routing and wavelength assignment

approaches for wavelength-routed optical WDM networks”, Optical Networks

9751R9 Copyright ©

http://www.technologyreview.com/featuredstory/400981/the-next-generation-of-optical-fibers/

http://dx.doi.org/10.1109/4236.670678

http://dx.doi.org/10.1109/4236.670678

http://dx.doi.org/10.1109/MCOM.2003.1244924


http://dx.doi.org/10.1109/35.989762

http://dx.doi.org/10.1109/35.819900


http://www.cs.sfu.ca/research/groups/NML/archive/pdf00002.pdf





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Outline

Optical components

• fibres

• amplifiers

• transmitters, receivers

Optical switching overview

All-wavelength switching (MEMS)

Wavelength-routed networks • Optical demultiplexing / filtering • Switch construction • Wavelength assignment problem & conversion

Photonic packet switching

Dealing with contention

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Optical fibre

Photo from Kurose & Ross. T Fig. 2.5 and 2.7

Polymer fibres are thicker, e.g. with cores of 1mm to 3mm, see P. Polishuk: Plastic optical fibers branch out, IEEE Communications Magazine

Fibre aka fiber.

Fibre consists of:

• Glass (silica) core: 8 – 62.5mm in diameter.

• Glass cladding: 125mm in diameter

Thickens fibre, making it stronger.

ncladd < ncore => internal reflection

• Plastic jacket: Protection

Rays (“modes”) of light are confined to fibre by total internal reflection

Using a thin core restricts propagation to one mode only. Such “single mode fibre” offers higher data rates/distance than multi-mode fibre (but is more expensive)

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Evaluation of optical fibre

• Fibre doesn’t conduct electricity

× Often need separate cable to provide power to devices.

√ Good for insulating, e.g.

√ protect computer from lightning striking outdoors

antenna by connecting with fibre

√ comms cables won’t spread high voltages from

failing machines in factories

√ Immune to electromagnetic noise

√ Low attenuation for wide bandwidth …

Note: Optical networks are not “fast” because the speed of light is fast (indeed, in

glass fibre it is comparable to the speed of electrical signals) – that would only

affect propagation delays. They are fast because of their wide bandwidth => high

transmission rates. 9751M1 Copyright ©


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Fibre – Attenuation in silica

approximate wavelengths of visible light

Bandwidth of tens of THz (tens of Tb/s with simple modulation) 121 THz 167 THz

Monochrome figure from Cisco http://www.cisco.com/univercd/illus/4/87/48087.gif

0.85mm band

(cheap LEDs)

1.3mm and 1.5mm

bands (lasers)

9751CV Copyright © 2012: Japan's NTT breaks fibre optic data speed record 1Pb/s over 50km over 1 fibre

http://www.abc.net.au/technology/articles/2012/09/26/3598036.htm


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Erbium Doped Fibre Amplifiers (EDFAs) • Amplify (30dB+) signals (including noise, no regeneration)

• Cover a broad bandwidth, e.g. 35nm

Operation: pump @ 980 or 1480nm, signal @ 1525-1560nm

Switching relevance:

• Many fabrics exhibit high loss => amplify with EDFA after switching

• EDFA forms a basic electronically-controlled on/off gate for switching

• EDFAs are the original “transparent” all-optical network element for trans-oceanic links=>motivate all-optical networks & all-optical switches

More on EDFAs at http://www.rp-photonics.com/erbium_doped_fiber_amplifiers.html & benefits of 2 pumps in R.Deepa and R. Vijaya: "Influence of bidirectional pumping in high-power EDFA on single-channel, multichannel and pulsed signal amplification", Optical Fiber Technology 14, pp. 20–26

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http://www.rp-photonics.com/erbium_doped_fiber_amplifiers.html




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Mismatch between fibre and transceivers

40Tb/s of fibre >> 100Gb/s optoelectronics (today)

+ fibre is often scarce

=> use multiple optoelectronic systems tx/rx @ different s

=> Wavelength Division Multiplexing

(optical equivalent of FDM in the RF domain)

Wavelengths =

“lambdas” () (sometimes spelled lamda)

Figure from Cisco Figure from Cisco 9751X6 Copyright ©


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WDM technology

DWDM = Dense WDM, e.g. 0.8nm spacing => about 64s (Coarse WDM:

12s, Ultra Dense WDM >100s)

Example of state of the art (world record) [NTT press release]:

140 channels @ 111 Gb/s (14Tb/s)

Tunable transmitters (multiple or tunable laser)

Tuning times range from ns (Distributed feedback) to ms

(mechanical/acousto)

Tunable receivers – “Burst mode” receivers can synchronize quickly, unlike

“continuous mode” receivers. Figure from Cisco 9751XP Copyright ©

http://www.ntt.co.jp/news/news06e/0609/060929a.html


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Broadcast and select network

using tunable tx/rx

Tunable lasers Broadcast medium Tunable filters Photodiodes

Combiner Fibre Splitter

× poor signal power because of broad splitting Note that a splitter doesn’t demultiplex or switch: input is sent to all outputs.

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~ ~

~ ~

~ ~


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Outline

• Optical switching overview

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Optoelectronic networks

• Simplest use of optics to networking merely involves

replacing wired links with fibre. Where link joins switch,

line interface card translates signal between optical and

electronic forms.

× Expensive optoelectronic parts are replicated (on switch

interfaces, not just terminal interfaces)

× Electronics (particularly energy flow: power in, heat out)

limits switch throughput

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Motivation for all-optical networks

Benefits:

√ High transmission speed (switching speed may not be as impressive, e.g. Gb/s throughput on each

port, but reconfiguration (changing port mappings) only once per ms)

√ Eliminate optoelectronics from network √ Low cost network

√ Environmentally friendly: Lower power consumption

√ Transparency: Payload rate and format can change (with new optoelectronic technology) without changing network elements.

• Network must recognize control rate and format for packet switching. • Particularly for trans-oceanic cables – original motivation for EDFA

amplifiers

Costs: × Add optical switching equipment × Analog: Signals are amplified, not regenerated × Error rate monitoring is hard

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Copyright © Copyright ©

Photonics: fast when going, slow to change

• “Photonic”/“all-optical” systems: keep info in optical

form; no optoelectronic conversion in the network

• Fast transmission (e.g. Tb/s) due to fibre.

• Often slow to change

• e.g. shift input port 1 between output ports 2 & 3.

• Switch needs physical change, e.g. mechanical mirrors

[E7>

• e.g. us/ms reconfiguration speed [WF>

=> New “burst switching” [RJ> modes

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Copyright © Copyright ©

Layers of optical switching

• Digital

(e.g. OEO switching)

• Wavelength

(e.g. WDM)

• Waveband

(e.g. Broad amplifiers)

• Fibre / All-wavelength

(e.g. MEMS)

17 30/03/2017 Tim Moors 97516R Copyright ©

Figure from X. Cao et al: “Waveband switching in optical networks”,

IEEE Communications Magazine, 41(4):105-12



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Where are optical networks used?

LANs: Rarely is optical switching needed (exception: CERN, ?NSA?!)

Access networks: Passive Optical Networks (PONs) (see next slide)

Metropolitan Area Networks

• Traditionally SDH – optoelectronic rings (s.t. can adapt around single failure)

Long haul (e.g. inter-city): Optical transmission preferable because of high bandwidth for low attenuation.

Core networks (e.g. switch in Sydney switching traffic between Melbourne, Canberra, Brisbane):

√ High transmission speeds

√ But reconfiguration needn’t be frequent.

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Passive Optical Networks (PONs)

Image from slides by Vijay Sivaraman

For more, see www.ponforum.org

Curb switch

CO

CO

CO

(a) Point-to-point network

N fibers

2N transceivers

(b) Curb-switched network

1 fiber

2N+2 transceivers

(c) Passive optical network

1 fiber

N transceivers

N subscribers

L km

L km

L km

Passive

optical

splitter

N subscribers

N subscribers

Why?: Higher rates to subscribers than wire

Options:

×Individual fibres from Exchange to N subscribers

×N fibres to Exchange (Central Office – CO)

×2N transceivers

×Fibre to the curb, switch to fibres to subscribers

•1 fibre to Exchange

×2N+2 transceivers

×Either optoelectronic switch (may fail, expensive optoelectronic tx/rx) or photonic switch (expensive)

√Fibre → curb + passive optical splitter to subs

√1 fibre to Exchange

√N+1 transceivers

×Power loss from splitting limits fan-out; need to deal with shared medium (security, MAC)

Terms: Fibre To The x (FTTx): Home, Building, Premises, Curb, Node, ...

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Passive Optical Networks (PONs)

Image from slides by Vijay Sivaraman

For more, see www.ponforum.org

Curb switch

CO

CO

CO

(a) Point-to-point network

N fibers

2N transceivers

(b) Curb-switched network

1 fiber

2N+2 transceivers

(c) Passive optical network

1 fiber

N transceivers

N subscribers

L km

L km

L km

Passive

optical

splitter

N subscribers

N subscribers

Why?: Higher rates to subscribers than wire

Options:

×Individual fibres from Exchange to N subscribers

×N fibres to Exchange (Central Office – CO)

×2N transceivers

×Fibre to the curb, switch to fibres to subscribers

•1 fibre to Exchange

×2N+2 transceivers

×Either optoelectronic switch (may fail, expensive optoelectronic tx/rx) or photonic switch (expensive)

√Fibre → curb + passive optical splitter to subs

√1 fibre to Exchange

√N+1 transceivers

×Power loss from splitting limits fan-out; need to deal with shared medium (security, MAC)

Terms: Fibre To The x (FTTx): Home, Building, Premises, Curb, Node, ...

Repeat of previous slide without animation, e.g. to facilitate printing

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Optical switching applications

OADM = Optical Add-Drop Multiplexer <AW] [QU>

OXC = Optical Cross-Connect Figure from LightReading

9751WF

Labels in figure: burst switch,

protection switch, packet switch,

reconfiguration speed, port count Copyright ©


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Optical switching technologies

MEMS = MicroElectroMechanical Systems Figure from LightReading

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How the optical domain differs √ High-rate transmission

× Processing e.g. optical address matching

× Buffering

Re-evaluate decisions made in

electronic networks

Electronic: Packet switching uses

processing (routing decisions) to save

transmission capacity

Optical: Circuit switching is currently

more cost effective

Electronic: Route along shortest path

(to save transmission) and buffer when ∃ contention on that path (buffering is

cheap).

Optical: Deflection routing: Use longer

path to avoid buffering

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Outline

• All-wavelength switching (MEMS)

The following devices are wavelength-insensitive.

Colours appear in figures only to distinguish flows/bits, not

to suggest different frequencies

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MicroElectroMechanical Systems

(MEMS) It’s all done with mirrors…

Mechanical => switching speed∝ 4√I => ms switching times (too slow for burst or packet switching)

Loss of approx 1.25dB / mirror

Photos from Lucent Photos from Lucent

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MEMS

Implemented on silicon using photolithography

√ precise positioning of mirrors

√ can integrate mechanical system with control

electronics

√ density should advance with progress in

photolithography technology (driven by VLSI)

Devices exist with densities of 256-1024 mirrors

“We have built MEM mirrors on an 8-in. wafer with a million MEM

mirrors on it, each individually moveable”

– Jeffrey Jaffe, president of research and advanced technologies at Bell

Labs in J. Ribeiro: 'Bell Labs grapples with VoIP, open-source'

Photo from Lucent 9751GK Copyright ©

http://www.infoworld.com/article/2668012/application-development/bell-labs-grapples-with-voip--open-source.html




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MEMS switches

Figures from Chu

2D

3D

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Modern 3D MEMs

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From http://glimmerglass.com/demo-high-resolution/

Multicast presumably uses a splitter & GUI suggests only 2 groups.

http://glimmerglass.com/demo-high-resolution/






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Other switching technologies

• Bubble (see Agilent Video)

• Liquid crystal attenuators/switches

• Liquid crystal gratings

• Holograms

• Wavelength-selective technologies

• electro/thermo/acousto-optics

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Aside: More optical dynamic bubbles in

crossbars from

http://luxemodo.com/best-optical-illusions/

http://uluru.ee.unsw.edu.au/~tim/tmp/agilent300.ram


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Outline


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Wavelength routed networks

• Motivated by WDM – not all wavelengths may have same

destination.

• Directing signal propagation alleviates power problems of

broadcast-and-select network.

• Components are also said to be “wavelength selective”

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Optical (de)multiplexing

Multiplexing is trivial: Combine signals by splicing fibres together

“Optical Add-Drop Multiplexer” (OADMs):

• Drop wavelengths (demux)

• Add wavelengths (mux)

• Many access ports; few wavelengths change (c.f. more change in

OXC)

Figure from Cisco 9751QU Copyright ©


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Static demultiplexing ... Static demultiplexers: Physical configuration assigns s to ports.

Generally require precise physical alignment

- e.g. by using photolithography

- need to assess thermal stability

Figure from Cisco

... using refraction:

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... using diffraction

Figure from Cisco

Diffraction grating

Diffraction

grating

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Diffraction from grooves on a

CD/DVD

Photo by Luis Fernández García

CC BY-SA 2.1 es

https://commons.wikimedia.org/wiki/User:Luis_Fern%C3%A1ndez_Garc%C3%ADa

http://creativecommons.org/licenses/by-sa/2.1/es/deed.en




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... using interference

Input cavity: splits light into waveguides

Waveguides have differing lengths => delays

Delayed waveforms interfere in output cavity

Position output ports @ points of max. interference

“Arrayed Waveguide Grating” (AWG) aka “optical waveguide router”!

up to 250 ports

Figure from Cisco

e.g. Cisco ONS 15454 based on Reconfigurable Optical Add Drop

Mux (ROADM) using Silicon Array Waveguide Grating (AWG)

Wikipedia:

“The colors seen in a soap

bubble are a common

example of interference. Light

reflecting off the front and

back surfaces of the thin soap

film interferes, resulting in

different colors being

enhanced.” [http://en.wikipedia.org/w/index.p

hp?title=Interference_%28wave_p

ropagation%29&oldid=41970577

3] 97510C Copyright ©

http://en.wikipedia.org/wiki/File:Soap_bubble_sky.jpg







http://en.wikipedia.org/wiki/Soap_bubble

http://en.wikipedia.org/wiki/Soap_bubble

http://en.wikipedia.org/w/index.php?title=Interference_%28wave_propagation%29&oldid=419705773







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Photo from Y. Hibino: An array of photonic filtering advantages: arrayed-waveguide-grating

multi/demultiplexers for photonic networks; IEEE Circuits and Devices Magazine, 16(6):21-7, Nov. 2000 9751F5 Copyright ©


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... using filters

“Interference filters”† transmit

light of one wavelength, and

reflect others.

√ Good thermal stability (variation of vertical/horizontal

dimensions handled by filters

having vertical range) and

isolation between channels, at

moderate cost.

× High loss, e.g. (yellow)

wavelength that is reflected

many times. Figure from Cisco 9751A9 Copyright ©

† The actual interference mechanism that leads to the

name [https://en.wikipedia.org/wiki/Interference_filter] is

not important for this course and should not be confused

with interference in AWGs <0C] or Mach-Zehnder

interferometers [04>

https://en.wikipedia.org/wiki/Interference_filter


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Outline


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Controllable refractive indices

Control the refractive index of the medium by applying:

• Electric field (electro-optic)

e.g. Lithium Niobate (LiNbO3) or liquid crystal

√ ns switching times

× substantial loss

• Sound (acousto-optic), 10us switching times

• Heat (thermo-optic), ms switching times, typically polymer

• Silica has lower optical loss than polymer, but requires more heat and

conducts heat more (=> switch requires more space)

Could use for dynamic demux, e.g. vary prism’s refractive index

Form basic switching element using interferometers...

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Interferometeric filters/gates

delay

Mach-Zehnder interferometer:

Controllable delay element determines relative phase of

combined wavelengths.

180o phase difference => filtered out

Drawbacks:

× Few ports (22)

× Crosstalk between outputs (Bonus marks if you can explain this.

Paper by Blumenthal mentions this issue. Incomplete annulment?)

+ =

+ =

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Interference here is similar to multipath interference in wireless networks, in that if the multiple signals

are out of phase then the signal is weak. Recall also interference patters <0C]


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Building a switch from filters/gates

Splitter Combiner Filter

Power budget: • Each input is split between n outputs (like broadcast&select)

(n=2 in this example) • Combiner has only one active input => readily amplify using EDFA as filter

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Wavelength assignment problem

Can concurrently use the same wavelength in spatially separate areas of the network

Q: What should be used for each lightpath?

A: Complex optimisation problem.

Wavelength assignment can be simplified with expensive wavelength converters.

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Wavelength converters

Better: Analog electrical signal directly modulates laser

Best: Optical conversion using coherence effects

• From non-linear response of medium in the presence of

multiple waves.

• e.g. Four-Wave-Mixing, Difference Frequency Generation

Primitive: Perform opto-electronic-opto conversion, and

use different for output laser

See Recommended Reading website for a presentation on Four-Wave-Mixing 9751U1 Copyright ©


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Outline

• Photonic packet switching

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Electro-optic† packet switching

Payload Control

Figure from Blumenthal † Distinct from optoelectronic

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‡ Separate control & payload paths is like the out-of-band signalling for circuit setup <H1]. WiFi also separates

control & payload s.t. old devices can still understand headers sent by new devices that transmit payload at high speed.

Payload travels over a “lightpath” from source to destination; no

optoelectronic conversion within network switching elements.

Control information may incur optoelectronic conversion‡, may

even flow on auxiliary electronic control network.


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Electro-optic† packet switching

Payload Control

Figure from Blumenthal

Repeat of previous slide without animation, e.g. to facilitate printing

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Payload travels over a “lightpath” from source to destination; no

optoelectronic conversion within network switching elements.

Control information may incur optoelectronic conversion, may

even flow on auxiliary electronic control network.

† Distinct from optoelectronic ‡ Separate control & payload paths is like the out-of-band signalling for circuit setup <H1]. WiFi also separates

control & payload s.t. old devices can still understand headers sent by new devices that transmit payload at high speed.


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Optical Burst Switching (OBS)

• If switch can’t reconfigure fast enough for individual packets, then try

switching “bursts” of packets – each going the same way.

• Switching unit (burst) is longer than retransmission unit (packet)

• Ingress edge router:

• Assembles packets into bursts

• Sends control signal into network to set up switches

• Control packet may be processed electronically

• Waits for an offset interval (allowing control signal to take effect)

• Send optical burst of packets

Variations:

• Connectionless: (“Tell and go”) – as above

• Connection-oriented (“Tell and wait”): Wait not just for offset, but also

wait to receive acknowledgement from switches in network 9751RJ Copyright ©


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Outline

• Dealing with contention

• Time domain: optical buffering …

• Space domain: deflection routing …

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Optical buffering

Long loops of fibre form delay lines

How long? “Electricity travels a foot in a nanosecond.” – G. Hopper In vacuum v0=3E8 m/s = 30cm/ns In silica: slower than in a vacuum: nSiO2=1.5, vSiO2=2E8m/s = 20cm/ns e.g. 1500B Ethernet frame (12kb) @ 1Gb/s (1ns/b) = 12us delay = 2.4km

Information continuously physically moves (c.f. electronic buffers)

Memory provides sequential, not random, access But that’s fine for queueing/buffering

spool of 0.9km of fibre

Photo from www.go4fiber.com/bargain/image/products/50ss.jpg 9751NG Copyright ©


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Optical buffering: Problems

× Attenuation in delay lines

× Complexity (to implement variable delay →)

× Some say: unnecessary – modeled on electronic networks

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Alternative design using cavities: Optical memory could ease internet bottlenecks

http://www.nature.com/news/optical-memory-could-ease-internet-bottlenecks-1.10108


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Optical buffer implementation Fibre delay line × Capacity ∝ length ∝ loss

Programmable fibre delay line

√ Enables control of holding time.

× Loss from splitting signal

Active recirculating delay line

√ Enables control of holding time.

× Amp noise limits recirculations

Figure from Blumenthal 9751EK Copyright ©


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Optical buffering for TSI

Switching time 1

2

3

4 Figure from Blumenthal

Bonus mark on offer if you can completely & intuitively describe what’s happening here

e.g. loops essentially serialise slots, which allow downstream switches choice of slot

But what about other slots in switch before those pictured & why 2 loops & 3 switches? 975179 Copyright ©


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Outline

• Dealing with contention

• Time domain: optical buffering …

• Space domain: deflection routing …

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Deflection routing

Assume that the ports are bidirectional

but can only support one flow

in either direction

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Route some inputs ( ) on

shortest path

Deflect others to other outgoing ports (longer paths)

Links between switches

effectively become passive delay

line buffers for deflected packets

Often prioritise packets:

• Better service for originally high priority packets,

• Escalate priority after deflect s.t. not deflected endlessly. (like trap after Banyan <MZ])


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Deflection routing: Evaluation

× Reduces network throughput because packets take longer

paths

Manhattan street network (regular mesh) & deflection:

55-70% of throughput with infinite buffering.

× May later revert to shortest path => mis-sequence packets

√ Deflecting towards source further increases path length, but

creates backpressure & may provide congestion control

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Summary of optical switching

• Switching at the “physical” layer

• Fibre enables massive transmission capacity, but optical

processing and buffering are difficult.

• WDM allows many flows to share one fibre

• Various types of demultiplexers based on refraction,

diffraction, interference

• MicroElectroMechanical Systems: Slow reconfiguration,

but no optoelectronic conversion bottleneck

• Packet switching is possible, but is it out of context?

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Documents

Optical switching - subjects.ee.unsw.edu.au · X. Ma and G.-S. Kuo: 'Optical switching technology comparison: optical ... • Network must recognize control rate and format for packet