switching architectures for optical networks · occurs in electronic switches –solved by input...

CSIT5600 by M. Hamdi1

Switching

Architectures for

Optical Networks


SONET

Data

CenterSONET

SONET

SONET

DWD

M DWD

M

AccessLong HaulAccess MetroMetro

Internet Reality


Hierarchies of Networks: 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 fiber

Transmit by combining multiple lasers @ different

frequencies

Output fibers

Long-haul fiber

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2

3

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CSIT5600 by M. Hamdi

Anatomy of a DWDM System

Terminal A Terminal B

Post-

AmpPre-

AmpLine Amplifiers

M

U

X

D

E

M

U

X

Transponder

Interfaces

Transponder

Interfaces

Direct

ConnectionsDirect

Connections

Basic building blocks

• Optical amplifiers

• Optical multiplexers

• Stable optical sources


Core Transport Services

OC-3

OC-3

OC-12

STS-1

STS-1STS-1

• Provisioned

SONET circuits.

• Aggregated into

Lamdbas.

• Carried over

Fiber optic cables.

Circuit

Origin

Circuit

Destination


WDM Network: Wavelength View

WDM link

Optical Switch

Edge Router

Legacy

Interfaces

Legacy

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


Typical Super POP

OXC

Core

IP

router

Interconnectio

n

Network

Large

Multi-service

Aggregation

Switch

Voice

Switch

Core

ATM

Switch

SONET

Coupler

&

Opt.amp

DWDM

+

ADM

DWDM

Metro

Ring


Typical POP

OXC

D

W

D

M

Voice

Switch

SONET-XC

D

W

D

M


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

1

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 Network

IP 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

Switch

Fabric

3

2

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

.

.

.

.

.

.

1

2

n

1

2

n

1

2

n

1

2

n

1

2

n

n

1

2

1

2

M


Wavelength Router with O/E and E/O

Cross-Connect

1

3

Outgoing Interface

Outgoing Wavelength

Incoming Interface

Incoming Wavelength


Demux1

Incoming fibers

OE

O

Individual 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

Repeater

One fiber


Hierarchical switching: Node with switches of different granularities

FibersOA. Entire fibers

Fibers

O O

OB. Wavelengthsubsets

O O

“Expresstrains”

OC. Individualwavelengths

E O

“Localtrains”


Wide Area Network (WAN)

OXC: Optical Wavelength/Waveband Cross

Connect

WAN :

Up to 200-500 wavelengths

40-160 Gbit/s/

wavebands (> 10 )


Packet (a) vs. Burst (b) Switching

Incoming

fibers

Fixed-length

(but unaligned) FDL’s

Synchronizer

Header

Payload

Setup

Header recognition,

processing, and generation

Switch1

B

C

DNew

headers

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

Control

packets

Data bursts

Control

wavelengthsA

B

C

D

Data

wavelengths

Offset time


MAN (Country / Region)

optical

burst

formation

IP

packets


WDM “transparent” transmission system

Wavelengthsaggregator

multipleλs

Fibers

(O-O nodes)

Wavelengthsdisaggregator

O O OO 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 i

input port j

Optical

packet

hdr CPU

Optical switch

payload

payload hdr

Re-combined

Wavelength i

output 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

.

.

.

.

.

.

.

.

.


Syn

c.

•Fixed packet size

•Synchronization stages required

Slotted networks

OPS Architecture: Synchronization

Occurs in electronic switches – solved by input buffering




Slotted networks

Syn

c.





Slotted networks


Syn

c.



Syn

c.


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



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

CSIT5600 by M. Hamdi

THANK YOU

57

switching architectures for optical networks · occurs in electronic switches –solved by input...

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