Optical switching. UNSW School of Electrical Engineering and Telecommunications

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School of Electrical Engineering and Telecommunications

UNSW

Optical switching

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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, Nov.-Dec. 2008

• 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, 1994

• M. Borella: 'Optical components for WDM lightwave networks', Proc. IEEE 85(8):1274-307, 1997

LightReading.com – Industry news

Later lecture on MPLS will discuss GMPLS for optical routing control & connection management

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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

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References about

specific technologies

P. Ball: “The next generation of optical fibers”, Technology Review, 104(4):55, 7 pgs, May 2001

J. Kiniry: 'Wavelength Division Multiplexing: Ultra high speed fiber optics', IEEE Internet Comp. 2(2):13-5, 1998

X. Ma and G.-S. Kuo: 'Optical switching technology comparison: optical MEMS vs. other technologies', IEEE Comm. Mag. 41(11):S16-S23, Nov. 2003

P. Chu et al: 'MEMS: the path to large optical crossconnects', IEEE

Comm. Mag. 40(3):80-7, 2002

S. Yao et al: 'Advances in photonic packet switching: an overview', IEEE

Comm. Mag. 38(2):84-94, 2000

T. Battestilli and H. Perros: 'An introduction to Optical Burst Switching',

IEEE Optical Communications 41(8):S10-5, Aug. 2003

H. Zang et al: “A review of routing and wavelength assignment

approaches for wavelength-routed optical WDM networks”, Optical

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Outline

Optical components • fibres • amplifiers • transmitters, receivers Optical switching overview Wavelength-routed networks

• Optical demultiplexing / filtering • Switch construction

• Wavelength assignment problem & conversion All-wavelength switching (MEMS)

Photonic packet switching

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Optical fibre

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

Fibre consists of:

• Glass (silica) core: 2 - 125μm in diameter. • Glass cladding:

Thickens fibre, making it stronger.

n_cladd < n_core => 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.

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Fibre – Attenuation in silica

Monochrome figure from Cisco

approximate wavelengths of visible light

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

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

0.85μm band (cheap LEDs)

1.3μm and 1.5μm bands (lasers)

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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 switching.

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Mismatch between fibre and transceivers

40Tb/s of fibre >> 10Gb/s optoelectronics (today) + fibre is often scarce

=> use multiple optoelectronic systems tx/rx @ different λs => Wavelength Division Multiplexing

(optical equivalent of FDM in the RF domain)

Wavelengths = “lambdas” (λ)

(sometimes spelled lamda)

Figure from Cisco Figure from Cisco

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WDM technology

DWDM = Dense WDM, e.g. 0.8nm spacing => about 64λs (Coarse DWM: 12λs, Ultra Dense WDM >100λs)

Example of state of the art (world record) [NTT press release, 2006sep29]: 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.

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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

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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

√ Transparency: Payload rate and format can change (with OE

progress) 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

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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) • Uncertainty about provisioning rings (e.g. how much BW needed?) can be

accommodated by subdividing rings & adding lambdas to serve new rings.

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

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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 Lkm Lkm 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 (advanced technology)

√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,

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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 Lkm Lkm 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 (advanced technology)

√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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Optical switching applications

OADM = Optical Add-Drop Multiplexer OXC = Optical Cross-Connect

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Optical switching technologies

MEMS = MicroElectroMechanical Systems

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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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Wavelength routed networks

• Motivated by WDM – not all wavelengths may have same destination.

• Directing signal propagation alleviates power problems of broadcast-and-select network.

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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

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Static demultiplexing ...

Static demultiplexers: Physical configuration assigns λs to ports.

Generally require precise physical alignment, e.g. by using photolithography

- need to assess thermal stability

Figure from Cisco

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... using diffraction

Figure from Cisco

Diffraction grating

Diffraction grating

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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)

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

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... using (interference) 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

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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

• Silica has lower optical loss than polymer, but requires more heat and conducts heat more (=> switch requires more space)

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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 (2×2)

× Crosstalk between outputs

+ =

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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

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Wavelength assignment problem

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

Q: What

λ

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: 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

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MicroElectroMechanical Systems

(MEMS)

It’s all done with mirrors…

Mechanical => switching time ∝ 4_{√I =>}_{ms switching times}

(too slow for burst or packet switching) Loss of approx 1.25dB / mirror

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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) Current 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', Jan. 05

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MEMS switches

Figures from Chu02

2D

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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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Electro-optic

†

_{packet switching}

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.

Payload Control

Figure from Blumenthal94

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Electro-optic

†

_{packet switching}

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.

Payload Control

Figure from Blumenthal94

† Distinct from optoelectronic

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Ways to deal with contention

• Time domain: optical buffering … • Space domain: deflection routing …

• Frequency domain: Wavelength conversion

• Creates more options, but may still have contention for specific frequency on output port.

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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

In silica: slower than in a vacuum: n_SiO2=1.5,

v_SiO2=2E8m/s

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

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Optical buffering: Problems

× Attenuation in delay lines

× Complexity (to implement variable delay →)

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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

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Optical buffering for TSI

Switching time

1 2 3 4

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Deflection routing

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.

Assume that the ports are bidirectional but can only support one flow

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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