An Overview of All-optical Switching

8/13/2019 An Overview of All-optical Switching

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2001 Marconi plc. The Copyright in this document belongs to Marconi plc and no

part of this document should be used or copied without their prior written permission.

An Overview of All-Optical Switching

Geoff Bennett

VP Technology Advocacy, Marconi plc

Email: [email protected]


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Agenda

Definitions of Switching How are switching decisions made both electronically and optically?

Define a structure to aid understanding of the topic

Describe the Taxonomy chosen in the GMPLS Architecture draft

The primary application areas for optical switching

Protection switching, Optical Add-Drop Mux, OXC, Burst/Packet switch

The primary switching techniques Lambda switching, Optical Burst, Optical Packet

An incomplete list of switching technologies

A look at selected technologies

Matching it up; applications, techniques & technologies


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Agenda











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Your Basic Electronic Switch

Space Switching

Direct a packet, cell or timeslot from an input port to an output port

Input port

Output port

Electronic switchfabric or backplane


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Your Basic Optical Switch

Space Switching

Direct a beam of light from a given input port to a given output port

Input port

Output port

Optical backplane


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Electronic vs Optical Switching

Data FCSIPS IPDL1L2L3MACD MACS

In an Ethernet Switch we use the MAC address to make a

switching decision

In an IP switch (ie. router) we use the Destination IP address tomake the switching decision

In an MPLS LSR we use the outmost label in the stack to make a

switching decision

In a Lambda Switch we use the value of the wavelength to make

a switching decision


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switching decision



switching decision




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switching decision



switching decision




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switching decision



switching decision



Demux

Detector 4

Detector 3

Detector 2

Detector 1These detectors are

wideband, and so the

Demux must create

a spatial separation


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Marketing Goes Wild

In the past 2 years, adding the word optical to any

of your products was a surefire way to add to your

stock price

So Marketings depts. got to work


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OEO Switch

Pro: Known technology

3R regeneration for free

Opportunity for

Sub-lambda grooming

Statistical multiplexing

Con: Moores Law scalability limits

Not bit-rate transparent

Not service/protocol transparent

Optical

Electronic


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OOO Switch

Pro: Optical scalability

Bit-rate transparent

Service/protocol transparent

Con: Emerging technologies

Lambda granularity

BER monitoring is hard

Uses amplification, notregeneration

Optical

Electronic


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OEO-O-OEO Switch!!!

Pro: Combines scalability of optics

with 3R regeneration andwavelength translation ofelectronics

May allow BER visibility

Can choose this as an optionper-port

Con: Increased complexity over both

designs

Still doesnt allow stat muxing or

grooming

Optical

Electronic


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Agenda











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A Basic Structure to understand optical

switching

A 3-part structure

Application areas The purpose for which we will use this switch will have a critical impact

on the features & properties we expect of it

Techniques

How is the switching decision made? In other words, on what basis isthe traffic directed through the switch?

Switching implies muxing as well as directing, so how does the muxingwork?

Technologies Switches need a backplane, or at least some way to connect the input

port to the output port. What technologies are available to do this?


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What Is Optical Switching

In the context of GMPLS?

Must match expectation with product capability, AND

work within limitations of legacy equipment Packet-Switch Capable Interfaces (PSC)

Time-Division Multiplex Capable Interfaces (TDM)

Lambda Switch Capable Interfaces (LSC)

Fibre-Switch Capable Interfaces (FSC)

Routers, ATM

Switches, LSRs

SONET/SDH

ADMs & DXCs

DWDM OADMs

& OXCs


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Packet-Switch Capable (PSC)

Interfaces that recognise bits

recognise packet or cell boundaries

can make forwarding decisions based on the content of the

appropriate MPLS header (eg. shim, VPI/VCI, DLCI)

are capable of receiving and processing routing & signallingmessages on in-band channels

Examples:

Interfaces onRouters, ATM Switches, Frame

Relay switches that have been enabled with an

MPLS control Plane


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Time-Division Multiplex Capable (TDM)

Interfaces that recognise bits

recognise repeating, synchronous frame structure

forward data on the basis of a timeslot within structure

are capable of receiving and processing Control Plane

information sent in-band with the synchronous frames

Examples:Interfaces onSONET/SDH Add-Drop Mux (ADM),

Digital Cross-Connect (DXC), Terminal Mux ,

OEO Optical Add-Drop Mux, OEO Optical Cross-

Connect (OXC)


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Lambda-Switch Capable (LSC)

Interfaces that do not need to recognise bits, or frames

forward light streams on the basis of their wavelength, or range

of wavelengths (waveband)

are not assumed to be able to receive and process Control

Plane information on an in-band channel

Examples:

Interfaces onan all-optical Add-Drop Mux

(OADM), Optical Cross-Connect (OXC)


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Fibre-Switch Capable (FSC)

Interfaces that do not need to recognise bits, or frames

do not necessarily have visibility of individual wavelengths or

wavebands

forward data based on its position in real-world physical space

are not assumed to be able to receive and process ControlPlane information on an in-band channel

Examples:

Interfaces on photonic cross-connects that operate

only at the level of a fibre, an automated optical

patch panel, or a protection switch


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What Do We Call The Circuits?

A packet/cell-switched LSR call it an LSP

A SONET/SDH device might call it a circuit or channel

A DWDM device might call it an Optical Channel Trail

A Fibre Switch might call it a fibre path?

In the context of GMPLS, all of thesecircuits will be referenced by a common

name: Label Switched Path (LSP)


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Maintaining Consistency

End Users /

Applications

IP

SDH

Photonics

Access

Any LSP must begin and end on

the same kind of device (eg. PSC-

PSC, TDM-TDM, etc.)


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Agenda











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All-Optical Switch Applications

Optical Core

Metro Optical Ring

1 x 2

Protection

Switch

OpticalAdd-Drop

Mux

OpticalCross

Connect

(Future) All-

optical packet

switch


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What Do These Switches Do?

Protection switch

Allows us to protect an individual fibre connection by failing over to abackup fibre assuming some condition (eg. loss of signal) is met

OADM Generally used in ring-based systems to add or drop connections

OXC The crossroads of the optical network. Used as an interconnect device

between rings, or to create optical mesh backbones.

Lambda Switch Optical Burst Switch (OBS)

Optical Packet Switch (OPS)

Lets take a closer look at these


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Agenda











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The Two Really Difficult Things In All-

Optical Switching

1: We are using all-optical in order to scale to very high speeds.

But how do we electronically*read bits at these speeds?

* Remember we cant build an optical CPU yet

2: Statistical muxing implies buffering. How do we store photons?


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Switching Techniques

Lambda Switching Manual

Dynamic

Optical Burst Switching

Optical Packet Switching

How does each technique work around The TwoReally Difficult Things?


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Manual Lambda Switchingaka Wavelength Provisioning

Each optical trail is set up, step by step, by Service

Provider NMS

OXC OXC

NMS

OADM

OXC

OADM

OXC

1

23

4

5


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Automatic Lambda Switching

Usually applies to protection switches

Failure trigger simple 1x2 path switch Failure criteria can include LoS (loss of signal),

BER or Digital Wrapper detection

Can apply to entire fibre, waveband or

wavelength

Metro Optical Ring


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How Do Manual & Automatic Lambda

Switching Overcome The Two Really Difficult

Things?

Q: How do we read bits at very high speeds?

A: We dont. Once the wavelength is set up, we just

switch the light. The switch never tries to interpret

bits within the stream of photons.

Q: How do we buffer traffic for statistical muxing?

A: We dont. Once the wavelength is established, it isused exclusively by one input stream, and no

statistical muxing is possible.


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Dynamic Lambda Switching

Edge devices signal to the optical network for a

wavelength to be established

OXC OXC

OADM

OXC

OADM

OXC

1: LSR-A signals for a

wavelength that

connects it to LSR-B

2: Optical network sets

up a wavelength path

3:LSR-B receives theconnection and data

data starts to flow


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How Does Dynamic Lambda Switching

Overcome The Two Really Difficult Things?


A: We dont. The data plane is still service

transparent. The control plane can operate at a

much lower data rate.


A: We dont. Once the wavelength is established, it isused exclusively by one input stream, and no

statistical muxing is possible.


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Switching On Wavelengths

1: Single-wavelength light enters the switch

2: A wavelength selection is made

3: Based on the wavelength selection, the light is switched to a

known output port

Note that a Lambda Switch does not read in-band address on light

1

2

3


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How do we measure a wavelength?

Conventional receivers are wideband, they are not

wavelength-selective Tuneable filters and receivers are appearing

We can choose a wavelength separation technology

that turns different wavelengths through differentangles Prisms, filters, gratings

We can send an engineer to change the component (slowswitching time)

We can choose a dynamic technology


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Dynamic Lambda Switching with GMPLS

Many selection technologies are based on static

wavelength muxing and demuxing All 1st generation DWDM

Dynamic data plane technologies must have a

corresponding dynamic control plane

The current proposal for a standardiseddynamic

control plane for optical switching is based on

Generalised MPLS


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Optical Burst Switching

Looks a lot like a Lambda Switch Network superficially

OBS OBS

OBS

OBS

OBS

OBS

1: LSR-A receives a burst of data

2: A connection setup message is

sent along the desired path

3: LSR-A buffers burst

4: Optical trail is established as

the message passes

5: Signalling is simplexburst is

sent along the trail without

confirmation that it is in place


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A Closer LookLambda Switch vs Optical Burst Switch

In a Lambda Switch (eg. GMPLS LSC interface) the goal is to be able to set up the wavelength in a matter of minutes

and the wavelength will stay in place until signalling tears it down (eg.

hours, weeks, years) signalling is quite traditional (ie. have time for reliable, duplex

handshake techniques)

Signalling can be entirely out of band (eg. Ethernet overlay)

In an Optical Burst Switch the goal is to set up the wavelength for the duration of a data burst

(note that 1MByte sent at 10Gbps takes less than 1ms)

the burst will be buffered by an electronic edge device while the

wavelength is being set up signalling has to be veryfast. No time for duplex handshakes.

Signalling may be out of band, but follows same topology as data


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How Does Optical Burst Switching Overcome

The Two Really Difficult Things?


A: We dont. The data plane is still servicetransparent. The control plane can operate at amuch lower data rate, but must follow the path of the

data plane (eg. use Optical Supervisory Channel).


A: By holding the burst at the ingress, and allowing forsetup processing delays it may be possible to build aswitch that does not need buffers.


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Why OBS?

Using OBS we can share a given wavelength in the

time domain Its a form of dynamic TDM for optical networks

We only use the wavelength for the time that we need it (plus the

time taken to set it up and release it)

We get a statistical gain on the efficiency of backbone

utilisation

We can try to avoid the Lambda Tax


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Optical Packet Switching

Looks similar to a router or MPLS network

OPS OPS

OPS

OPS

OPS

OPS


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A Closer LookOptical Packet Switch

Essentially the optical equivalent of an electronicpacket switch

An OPS will read the imbedded header information inthe Optical Packet, and use this information to make aswitching decision

Can be connectionless (use IP address), orconnection-oriented (use GMPLS label)

Problems: How do we read header information at very high speeds?

Remember Moores Law is much slower than optical capacityscaling

Packet switches need buffershow do we build an optical buffer?


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How Does Optical Packet Switching Overcome

The First Really Difficult Thing?


A1: For a connectionless OPS network

We send the packet header in-band with the data, but at a lower

bit rate than the data

A2: For a GMPLS OPS network

We send signalling messages and the label headers on data

packets at a lower bit rate than the data


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How Does That Work?

Optical Packet

Label Optical Packet

1Gbps

10Gbps

ControlExamples: OSPF, ISIS routing messages;

RSVP-TE, CR-LDP signalling messages


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How Does Optical Packet Switching Overcome

The Second Really Difficult Thing?

Q: Do Optical Packet Switches need buffers?

A: Yes, theres no way around this one

OPS devices must make use of input buffers in orderto give address processing circuits the time to do

their job. Like Ethernet cut through switches that

had to buffer the first 64 bytes of the packet.

OPS devices must make use of output buffers so that

they dont drop packets


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Agenda





Protection switching, Optical Add-Drop Mux, OXC, Burst/Packet switch The primary switching techniques

Lambda switching, Optical Burst, Optical Packet





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Candidate TechnologiesIncomplete list!

Bulk mechanical

2-D slow MEMS

3-D slow MEMS

Beam steering moving fibre

Beam steering moving image

2.5D slow MEMS

1-D fast MEMS

3-D fast MEMS

Electro-optic (bulk)

Electro-optic (waveguide)Thermo-optic (waveguide)

Gratings

Bubble

FTIR

Broadcast & selectWavelength routing

Shutter gating

Amplifier gating

Acousto-optic

Polarisation switching

There are new technologies

appearing all the time Old technologies are being

rediscovered as network

requirements and switchingtechniques evolve Eg. LiNbO3 switches (electro-optic

waveguide) for OPS


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Agenda











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Frustrated Total Internal Reflection (FTIR)

Reflect or transmit at mechanically translated prism

interface

~25ms latency Good for simple 1x2

Can cascade


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Gratings & Circulators

What is a grating?

What is a circulator?

How can we combine these components to perform

switching or add-drop?


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Core

Cladding

Refractive Index of

Core (germanium

doped silicate) can be

varied by exposing toUltraviolet light (laser

approx 240 nm)

Periodic spacing produces

Bragg grating within fibre,the period can be precisely

determined by laser

Fibre Gratings


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Lambda 3 isselectively reflected

from Bragg structure

1

2

3

Fibre Gratings


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Can use circulators to

mux and demux

wavelengths onto fibre

Circulators

Ci l t d G ti


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Circulators and Gratings:Muxing

1 - 8Circulator

2 - 8

1

Fibre

Grating

1 - 84:1-8 now

muxed together

1: 7 lambdas from

previous stage

2: One new lambda

to be added

3: Grating is

reflective to 1


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Holographic Switches

Control Electrodes

"A piece of glass"


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Control Electrodes

"A piece of glass"

Electrode causes hologram of

Bragg Grating to appear

This grating is

selective for Red

wavelength


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Control Electrodes

"A piece of glass"

Electrode causes hologram of

Bragg Grating to appear

This grating is

selective for Red

wavelength

This grating is

selective for Yellow

wavelength

W id S i h


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Waveguide SwitchesThermo-Optic and Electro-Optic

Waveguide

Input port

Control point

Output #1

Output #2

W id S it h


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Output #1

Light Enters

1: Light enters. 50% of light follows northern path, 50% follows southern path

2: Arriving at Point X, each light path will have followed a slightly different path

length, and will create interference pattern

3: Result of the interference is that light is switched to Output #1

Point X

50%

50%

W id S it h


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Output #2

Light Enters

1: We now apply a control signal to northern path, refractive index is

changed, which changes the effective velocity of light in this path2: Interference at Point X changes

3: Light is now switched to Output #2

Point X

50%

50%

W id S it h


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Most common material is Lithium Niobate

Limited to 2-port devices, more ports means more stages

Crosstalk at each stage

Control may be applied thermally Changes refractive index through temperature dependence

Relatively slow change

Becomes complex to maintain temperature (Peltier cooler vs electronic

heater with temperature feedback lock)

Control may be applied electrically Electric field causes change in refractive index

Can be very, very fast (ns)


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An AWG is like a prism

Entry wavelength decides the route through the

grating

Possible technology for wavelength multiplexing and

demultiplexing

Can be combined with a fast-tuning laser to create a

switch

Space Switching Mechanism

Arrayed Waveguide Grating


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Inbound Port

Tuneable Laser

B

C

D

E

A

AWG




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Inbound Port

Tuneable LaserC

D

E

A

AWG

B

Tune to "red"wavelength, optical

packet exits at Port B


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Inbound Port

Tuneable LaserC

EAWG

Tune to "blue"wavelength, optical

packet exits at Port D

D

A

B

A d


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Agenda










To Home In On Req irements


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To Home In On Requirements

we must specify the application areas for the switch I will focus on the 4 main application areas mentioned

Two major criteria will then drive the choice oftechnology Ability to scale to large port counts

The speed at which this technology can switch Definition: If a switch has multiple states (A, Bn), the

reconfiguration time is the time required to change (and stabilise)

from one state to another

A

B

n

.

.

.

X

Requirements for Techniques


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OXC

OADM

10ns 1s 1 ms 1 s

Reconfiguration Speed

10,000

1000

100

10

1

Port Count

Requirements for Techniques

Packet

Switch

Protection

Switch

Burst

Switch

Requirements vs Technologies


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OXC

OADM

10ns 1s 1 ms 1 s

Reconfiguration Speed

10,000

1000

100

10

1

Port Count

Requirements vs Technologies

Packet

Switch

Protection

Switch

Burst

Switch3D

Slow

MEMS

2D Slow

MEMS

Bulk

Mechanical

LC Grating

3D Fast

MEMSFast Electro-

optic

Tuneable

Laser

Thank You


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Documents

An Overview of All-optical Switching