Switching Architectures for Optical Networks

CSIT5600 by M. Hamdi1

Switching Architectures for Optical Networks


SONET

DataCenter SONET

SONET

SONET

DWDM DWD

M

AccessLong HaulAccess MetroMetro

Internet Reality


Hierarchies of Networks: IP / ATM / SONET / WDM

IP

ATM

SONET

WDM


Why Optical?• Enormous bandwidth made available

– DWDM makes ~160 channels/ possible in a fiber

– Each wavelength “potentially” carries about 40 Gbps

– Hence Tbps speeds become a reality

• Low bit error rates – 10-9 as compared to 10-5 for copper wires

• Very large distance transmissions with very little amplification.


Dense Wave Division Multiplexing (DWDM)

Multiple wavelength bands on each fiberTransmit by combining multiple lasers @ different

frequencies

Output fibers

Long-haul fiber

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CSIT5600 by M. Hamdi

Anatomy of a DWDM System

Terminal A Terminal B

Post-Amp

Pre-Amp

Line Amplifiers

MUX

DEMUX

TransponderInterfaces

TransponderInterfaces

DirectConnections

DirectConnections

Basic building blocks• Optical amplifiers• Optical multiplexers• Stable optical sources


Core Transport Services

OC-3

OC-3

OC-12

STS-1STS-1STS-1

• ProvisionedSONET circuits.

• Aggregated intoLamdbas.

• Carried overFiber optic cables.

CircuitOrigin

Circuit Destination


WDM Network: Wavelength View

WDM link

Optical Switch

Edge Router

Legacy

InterfacesLegacy

Interfaces

Legacy

Interfaces

( e.g., PoS, Gigabit

Ethernet, IP/ATM)

Interfaces


Relationship of IP and Optical

• Optical brings

–Bandwidth multiplication

–Network simplicity (removal of redundant layers)

• IP brings

–Scalable, mature control plane

–Universal OS and application support

–Global Internet

• Collectively IP and Optical (IP+Optical) introduces a set of service-enabling technologies

Swit

chin

g

Transm

ission

Optical Transport

Routing

IP

Services

Swit

chin

g

Transm

ission

Optical Transport

Swit

chin

g

Transm

ission

Optical Transport

Routing

IP

Services


Typical Super POP

OXC

Core IP

router

Interconnection

Network

LargeMulti-serviceAggregation

Switch

Voice Switch

CoreATM

Switch

SONET

Coupler&

Opt.amp

DWDM+

ADM

DWDM Metro Ring


Typical POP

OXCDWDM

VoiceSwitch

SONET-XC

DWDM


What are the Challenges with Optical Networks?

• Processing: Needs to be done with electronics

– Network configuration and management

– Packet processing and scheduling

– Resource allocation, etc.

• Traffic Buffering

– Optics still not mature for this (use Delay Fiber Lines)

– 1 pkt = 12 kbits @ 10 Gbps requires 1.2 s of delay => 360 m of fiber)

• Switch configuration

– Relatively slow


Wavelength Converters

• Improve utilization of available wavelengths on links

• All-optical WCs being developed

• Greatly reduce blocking probabilities

No converters

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2 3

New request 1 3

1

2 3

New request 1 3

With converters

WC


Wavelength Cross-Connects (WXCs)• A WDM network consists of wavelength cross-connects (WXCs)

(OXC) interconnected by fiber links.

• 2 Types of WXCs

– Wavelength selective cross-connect (WSXC)

• Route a message arriving at an incoming fiber on some wavelength to an outgoing fiber on the same wavelength.

• Wavelength continuity constraint

– Wavelength interchanging cross-connect (WIXC)

• Wavelength conversion employed

• Yield better performance

• Expensive


Wavelength Router

Wavelength Router

Control Plane:Wavelength Routing

Intelligence

Data Plane:Optical Cross

Connect Matrix

Single Channel Links to IP Routers, SDH

Muxes, ...

Unidirectional DWDM Links to

other Wavelength Routers

Unidirectional DWDM Links to

other Wavelength Routers


Optical Network Architecture

IP Router

Optical Cross Connect (OXC)

OXC Control unit

Control Path

Data Path

UNIUNIMesh Optical

NetworkIP Network IP Network


OXC Control Unit• Each OXC has a control unit

• Responsible for switch configuration

• Communicates with adjacent OXCs or the client network through single-hop light paths– These are Control light paths

– Use standard signaling protocol like GMPLS for control functions

• Data light paths carry the data flow– Originate and terminate at client networks/edge routers

and transparently traverse the core


Optical Cross-connects (No wavelength conversion)

Optical SwitchFabric

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2

4

4

1

1

3

All Optical Cross-connect (OXC) Also known as PhotonicCross-connect (PXC)


Optical Cross-Connect with Full Wavelength Conversion

• M demultiplexers at incoming side• M multiplexers at outgoing side• Mn x Mn optical switch has wavelength converters at switch

outputs

1,2, ... ,n

1,2, ... ,n

1,2, ... ,n

1

2

M

Optical CrossBarSwitch

WavelengthConverters

WavelengthMux

WavelengthDemux

1,2, ... ,n

1,2, ... ,n

1,2, ... ,n

.

.

.

.

.

.

12n

12n

12n

1

2

n

12n

n12

1

2

M


Wavelength Router with O/E and E/O

Cross-Connect

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3

Outgoing InterfaceOutgoing Wavelength

Incoming InterfaceIncoming Wavelength


Demux1

Incoming fibers

OE

OIndividual wavelengths

Mux

Outgoing fibers

O-E-O Crossconnect Switch (OXC)

O/EO/EO/E

O/EO/EO/E

O/EO/EO/E

N

2

E/OE/OE/O

E/OE/OE/O

E/OE/OE/O

Switches information signal on a particular wavelength on anincoming fiber to (another) wavelength on an outgoing fiber.

1

N

2WDM(many λs)


Optical core networkOpaque (O-E-O) and transparent (O-O) sections

E/OClientsignals

O/E

to other nodesfrom other nodes

E E O

O

Transparentoptical island

O O

OOE

OO

O O

EO

Opaque optical network


OEO vs. All-Optical Switches

• Capable of status monitoring

• Optical signal regenerated – improve signal-to-noise ratio

• Traffic grooming at various levels

• Less aggregated throughput

• More expensive

• More power consumption

• Unable to monitor the contents of the data stream

• Only optical amplification – signal-to-noise ratio degraded with distance

• No traffic grooming in sub-wavelength level

• Higher aggregated throughput

• ~10X cost saving

• ~10X power saving

OEO All-Optical


Large customers buy “lightpaths”

A lightpath is a series of wavelength links from end to end.

cross-connect

opticalfibers

RepeaterOne fiber


Hierarchical switching: Node with switches of different granularities

FibersOA. Entire fibers

Fibers

O O

OB. Wavelength subsets

O O

“Express trains”

OC. Individual wavelengths

E O“Local trains”


Wide Area Network (WAN)

GAN

links

OXC: Optical Wavelength/Waveband Cross Connect

WAN : Up to 200-500 wavelengths40-160 Gbit/s/wavebands (> 10 )


Packet (a) vs. Burst (b) Switching

Incomingfibers

Fixed-length(but unaligned) FDL’s

Synchronizer

Header

Payload

Setup

Header recognition,processing, and generation

Switch1

B

C

DNewheaders

2

1

2 2

1

(a)

A

Switch

2

1 1

2

(b)

O/E/O

Control packet processing(setup/bandwidth reservation)

2 2

1 1

Controlpackets

Data bursts

Controlwavelengths

A

B

C

D

Datawavelengths

Offset time


MAN (Country / Region)

opticalburst

formation

IPpackets


Optical Switching Technologies

• MEMs – MicroElectroMechanical• Liquid Crystal• Opto-Mechanical• Bubble Technology• Thermo-optic (Silica, Polymer)• Electro-optic (LiNb03, SOA, InP)• Acousto-optic• Others…

Maturity of technology, Switching speed, Scalability, Cost, Maturity of technology, Switching speed, Scalability, Cost, Relaiability (moving components or not), etc.Relaiability (moving components or not), etc.


MEMS Switches for Optical Cross-Connect

M o v e a b le M ic ro m irro r

Proven technology, switching time (10 to 25 msec), moving mirrors is a Proven technology, switching time (10 to 25 msec), moving mirrors is a reliability problem.reliability problem.


WDM “transparent” transmission system

Wavelengthsaggregator

multipleλs

Fibers

(O-O nodes)

Wavelengthsdisaggregator

O O O O OO

Optical switching fabric (MEMS devices, etc.)

Incoming fiberTiny mirrors

Outgoing fibers


Upcoming Optical Technologies

• WDM routing is circuit switched

– Resources are wasted if enough data is not sent

– Wastage more prominent in optical networks

• Techniques for eliminating resource wastage

– Burst Switching

– Packet Switching

• Optical burst switching (OBS) is a new method to transmit data

• A burst has an intermediate characteristics compared to the basic switching units in circuit and packet switching, which are a session and a packet, respectively


Optical Burst Switching (OBS)

• Group of packets a grouped in to ‘bursts’, which is the transmission unit

• Before the transmission, a control packet is sent out– The control packet contains the information of burst

arrival time, burst duration, and destination address

• Resources are reserved for this burst along the switches along the way

• The burst is then transmitted

• Reservations are torn down after the burst


Optical Burst Switching (OBS)


Optical Packet Switching

• Fully utilizes the advantages of statistical multiplexing

• Optical switching and buffering

• Packet has Header + Payload– Separated at an optical switch

• Header sent to the electronic control unit, which configures the switch for packet forwarding

• Payload remains in optical domain, and is re-combined with the header at output interface


Optical Packet Switch

• Has– Input interface, Switching fabric, Output interface and

control unit

• Input interface separates payload and header

• Control unit operates in electronic domain and configures the switch fabric

• Output interface regenerates optical signals and inserts packet headers

• Issues in optical packet switches– Synchronization

– Contention resolution


• Main operation in a switch: – The header and the payload are separated.

– Header is processed electronically.

– Payload remains as an optical signal throughout the switch.

– Payload and header are re-combined at the output interface.

payload hdr

Wavelength iinput port j

Opticalpacket

hdr CPU

Optical switch

payload

payload hdr

Re-combinedWavelength ioutput port j


Output port contention

• Assuming a non-blocking switching matrix, more than one packet may arrive at the same output port at the same time.

Output ports

payloadhdr

payloadhdr

payloadhdr

.

.

.

Optical SwitchInput ports

.

.

.. . .

.

.

.


Sync.

•Fixed packet size

•Synchronization stages required

Slotted networks

OPS Architecture: SynchronizationOccurs in electronic switches – solved by input bufferingOccurs in electronic switches – solved by input buffering




Slotted networks

Sync.

OPS Architecture: Synchronization




Slotted networks


Sync.




Slotted networks


Sync.




Slotted networks


Sync.



Sync.


OPS: Contention Resolution

• More than one packet trying to go out of the same output port at the same time– Occurs in electronic switches too and is resolved by

buffering the packets at the output

– Optical buffering ?

• Solutions for contention– Optical Buffering

– Wavelength multiplexing

– Deflection routing


OPS Architecture

Contention Resolutions

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• Optical Buffering– Should hold an optical signal

• How? By delaying it using Optical Delay Lines (ODL)

– ODLs are acceptable in prototypes, but not commercially viable

– Can convert the signal to electronic domain, store, and re-convert the signal back to optical domain

• Electronic memories too slow for optical networks


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•Optical buffering

OPS Architecture



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OPS Architecture



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OPS Architecture




• Wavelength multiplexing– Resolve contention by transmitting on different

wavelengths

– Requires wavelength converters - $$$


•Wavelength conversion

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OPS Architecture



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OPS Architecture



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OPS Architecture



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OPS Architecture



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OPS Architecture



Deflection routing

• When there is a conflict between two optical packets, one will be routed to the correct output port, and the other will be routed to any other available output port.

• A deflected optical packet may follow a longer path to its destination. In view of this:

– The end-to-end delay for an optical packet may be unacceptably high.

– Optical packets may have to be re-ordered at the destination


Electronic Switches Using Optical Crossbars


Scalable Multi-Rack Switch Architecture

Switch Core

Optical links

Line cardrack• Number of linecards is limited in a single rack– Limited power supplement, i.e. 10KW– Physical consideration, i.e. temperature, humidity

• Scaling to multiple racks– Fiber links and central fabrics


Logical Architecture of Multi-rack Switches

• Optical I/O interfaces connected to WDM fibers

• Electronic packet processing and buffering– Optical buffering, i.e. fiber delay lines, is costly and not mature

• Optical interconnect– Higher bandwidth, lower latency and extended link length than copper

twisted lines

• Switch fabric: electronic? Optical?

Crossbar

Scheduler

Switch Fabric System

Framer

Line Card

Laser Laser

Laser

LaserLocal

Buffers

Framer

Line Card

Laser LaserLocal

Buffers

Framer

Line Card

LaserLocal

Buffers

Framer

Line Card

LaserLocal

Buffers

Fiber I/O

Fiber I/O

Fiber I/O

Fiber I/O


Optical Switch Fabric

• Less optical-to-electrical conversion inside switch– Cheaper, physically smaller

• Compare to electronic fabric, optical fabric brings advantages in– Low power requirement, Scalability, Port density, High capacity

• Technologies that can be used– 2D/3D MEMS, liquid crystal, bubbles, thermo-optic, etc.

• Hybrid architecture takes advantage of the strengths of both electronics and optics

Crossbar

Scheduler

Switch Fabric System

Framer

Line Card

Laser Laser

Laser

LaserLocal

Buffers

Framer

Line Card

Laser LaserLocal

Buffers

Framer

Line Card

LaserLocal

Buffers

Framer

Line Card

LaserLocal

Buffers

Fiber I/O

Fiber I/O

Fiber I/O

Fiber I/O


Electronic Vs. Optical Fabric

Trans.Line

Buffer

SwitchingFabric

Inter-connection

Trans.Line

BufferInter-connection

Electronic

Trans.Line

Buffer

SwitchingFabric

Inter-connection

Trans.Line

BufferInter-connection

Optical

Optical

Electronic

E/O or O/EConversion

favorred


Multi-rack Hybrid Packet Switch

OpticalCrossbar

E/OBuf f er O/E Buf f er

E/OBuf f er

E/OBuf f er

E/OBuf f er

O/E Buf f er

O/E Buf f er

O/E Buf f er

Rack

OpticalFiber

OpticalFiber

Switch Core

Linecard


Features of Optical Fabric

• Less E/O or O/E conversion

• High capacity

• Low power consumption

• Less cost

However,

• Reconfiguration overhead (50-100ns)

– Tuning of lasers (20-30ns)

– System clock synchronization (10-20ns or higher)


Scheduling Under Reconfiguration Overhead

• Traditional slot-by-slot approach

• Low bandwidth usage

Scheduler

Time Line

ScheduleReconfigure Transfer


Reduced Rate Scheduling

• Challenge: fabric reconfiguration delay– Traditional slot-by-slot scheduling brings lots of overhead

• Solution: slow down the scheduling frequency to compensate– Each schedule will be held for some time

• Scheduling task1. Find out the matching

2. Determine the holding time

Fabric setup (reconfigure)

Traffic transfer

Time slot

Slot-by-slot Scheduling, zero fabric setup time

Reduced rate Scheduling, each schedule is held for some time

Slot-by-slot Scheduling with reconfigure delay


Scheduling Under Reconfiguration Overhead

• Reduce the scheduling rate

– Bandwidth Usage = Transfer/(Reconfigure+Transfer)

• Approaches

– Batch scheduling: TSA-based

– Single scheduling: Schedule + Hold

Constant


Single Scheduling

• Schedule + Hold

– One schedule is generated each time

– Each schedule is held for some time (holding time)

– Holding time can be fixed or variable

– Example: LQF+Hold


Routing and Wavelength Assignment


Optical Circuit Switching• An optical path established between two nodes

• Created by allocation of a wavelength throughout the path.

• Provides a ‘circuit switched’ interconnection between two nodes.

– Path setup takes at least one RTT

– No optical buffers since path is pre-set

Desirable to establish light paths between every pair of nodes.

• Limitations in WDM routing networks,

– Number of wavelengths is limited.

– Physical constraints:

• limited number of optical transceivers limit the number of channels.


Routing and Wavelength Assignment (RWA)

• Light path establishment involves– Selecting a physical path between source and

destination edge nodes

– Assigning a wavelength for the light path

• RWA is more complex than normal routing because– Wavelength continuity constraint

• A light path must have same wavelength along all the links in the path

– Distinct Wavelength Constraint

• Light paths using the same link must have different wavelengths


No Wavelength Converters

POPPOP

Access Fiber

Wavelength 1

Wavelength 2

Wavelength 3

WSXC


With Wavelength Converters

POPPOP

Access FiberWavelength 1

Wavelength 2

Wavelength 3

WIXC


Routing and Wavelength Assignment (RWA)

• RWA algorithms based on traffic assumptions:

• Static Traffic

– Set of connections for source and destination pairs are given

• Dynamic Traffic

– Connection requests arrive to and depart from network one by one in a random manner.

– Performance metrics used fall under one of the following three categories:

• Number of wavelengths required

• Connection blocking probability: Ratio between number of blocked connections and total number of connections arrived


Static and Dynamic RWA

• Static RWA

– Light path assignment when traffic is known well in advance

– Arises in capacity planning and design of optical networks

• Dynamic RWA

– Light path assignment to be done when requests arrive in random fashion

– Encountered during real-time network operation


Static RWA

• RWA is usually solved as an optimization problem with Integer Programming (IP) formulations

• Objective functions

– Minimize average weighted number of hops

– Minimize average packet delay

– Minimize the maximum congestion level

– Minimize number of Wavelenghts


Static RWA

• Methodologies for solving Static RWA– Heuristics for solving the overall ILP sub-optimally

– Algorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set

–http://www.tct.hut.fi/~esa/java/wdm/








Solving Dynamic RWA

• During network operation, requests for new light-paths come randomly

• These requests will have to be serviced based on the network state at that instant

• As the problem is in real-time, dynamic RWA algorithms should be simple

• The problem is broken down into two sub-problems– Routing problem

– Wavelength assignment problem

Documents

Switching Architectures for Optical Networks