Cisco DWDM

1© 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.

Cisco Optical WorkshopDWDM

January 31, 2004

© 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2

Agenda

• Introduction• Components• Forward Error Correction• DWDM Design• Summary


Increasing Network Capacity OptionsSame bit rate, more fibersSlow Time to MarketExpensive EngineeringLimited Rights of WayDuct Exhaust

More Fibers(SDM)

Same fiber & bit rate, more λsFiber CompatibilityFiber Capacity ReleaseFast Time to MarketLower Cost of OwnershipUtilizes existing TDM Equipment

WDM

Faster Electronics(TDM)

Higher bit rate, same fiberElectronics more expensive


Fiber Networks

Single Single Fiber (One Fiber (One

Wavelength)Wavelength)

Channel 1

Channel n

• Time division multiplexingSingle wavelength per fiberMultiple channels per fiber4 OC-3/STM1 channels in OC-12/STM44 OC-12/STM4 channels in OC-48/STM1616 OC-3/STM1 channels in OC-48/STM16

• Wave division multiplexingMultiple wavelengths per fiber4, 16, 24, 40 channels per systemMultiple channels per fiber

• Hybrid Networks

Single FiberSingle Fiber(Multiple (Multiple

Wavelengths)Wavelengths)

l1l1l2l2

lnln


Types of WDM• Traditional passive systems

Low channel countsLess than 100km

• CWDMDefined in ITU-T G694.2Up to 18 channels with 20nm spacingTarget distances from 40km to ~100km

• DWDMSpacing of 200, 100, 50 or 25 GHzChannel counts of 32 and greaterDistances of 600km and greater


DWDM History• Early WDM (late 80s)

Two widely separated wavelengths (1310, 1550nm)• “Second generation” WDM (early 90s)

Two to eight channels in 1550 nm window400+ GHz spacing

• Current DWDM systems16 to 40 channels in 1550 nm window100 to 200 GHz spacingAutomatic power control schemesHybrid DWDM/TDM systems

• Next generation DWDM systems64 to 160 channels in 1550 nm window50 and 25 GHz spacing


Wavelength Characteristics for DWDM

• TransparencyCan carry multiple protocols on same fiberCan carry multiple TDM channels on a wave (muxponding)Monitoring can be aware of multiple protocols

• Wavelength spacing50GHz, 100GHz, 200GHzDefines how many and which wavelengths can be used

• Wavelength capacity and bit rateExample: 1.25Gb/s, 2.5Gb/s, 10Gb/s


Optical Transmission Bands

Band Wavelength (nm)820 - 900

1260 – 1360“New Band” 1360 – 1460

S-Band 1460 – 1530C-Band 1530 – 1565L-Band 1565 – 1625U-Band 1625 – 1675


Fiber Attenuation Characteristics

800 900 1000 1100 1200 1300 1400 1500 1600

Wavelength in Nanometers (nm)

0.2 dB/Km

0.5 dB/Km

2.0 dB/Km

Attenuation vs. WavelengthAttenuation vs. Wavelength S-Band:1460–1530nm

L-Band:1565–1625nm

C-Band:1530–1565nm

Fibre Attenuation Curve


Agenda

• Introduction• Components• Forward Error Correction• DWDM Design


DWDM Componentsλ1

λ2

λ3

λ1...n15xx850/1310

TransponderOptical Multiplexer

Optical Add/Drop Multiplexer(OADM)

(Band and Channel)

λ1

λ2

λ3

λ1...n λ1

λ2

λ3

Optical De-multiplexer


More DWDM Components

Optical Amplifier(EDFA)

Optical AttenuatorVariable Optical Attenuator

Dispersion Compensator (DCM / DCU)


Typical DWDM Network Architecture

DWDM SYSTEM DWDM SYSTEM

VOA EDFA DCM

VOAEDFADCM

Service Mux(Muxponder)

Service Mux(Muxponder)


Transponders

• Converts broadband optical signals to a specific wavelength via optical to electrical to optical conversion (O-E-O)

• Used when Optical LTE (Line Termination Equipment) does not have tight tolerance ITU optics

• Performs 2R or 3R regeneration function• Receive Transponders perform reverse function

Low Cost IR/SR Optics

Wavelengths Converted

λ1

OEO

OEO

OEO

λ2

λn

From Optical OLTE

To DWDM Mux


Performance Monitoring

• Performance monitoring performed on a per wavelength basis through transponder

• G.709 based• No modification of overhead• Data transparency is preserved


Laser Characteristics Non DWDM Laser

Fabry Perot DWDM Laser

Distributed Feedback (DFB)

λ

Power λc

λ

λcPower

• Dominant single laser line• Tighter wavelength control

• Spectrally broad• Unstable center/peak wavelength

Active medium

MirrorPartially transmitting

Mirror

Amplified light


Transponder: Direct vs. External ModulationDirect Modulation External Modulation

Electrical Signal in

Electrical Signal in

IinDC Iin

Mod. Optical Signal

Optical Signal out

CW UnmodulatedOptical Signal

External Modulator

• Simple approach• Low cost• Client side• Metro WDM

• Extra components• Higher cost• WDM side• LH WDM

Ex: 1800 ps/nm Dispersion Tolerance Ex: 10,000 ps/nm Dispersion Tolerance


DWDM Receiver Requirements

I

• Receivers Common to all Transponders• Not Specific to wavelength (Broadband)• PIN photodiodes

Simple and fast• Avalanche photodiodes (APD)

Slower, but better sensitivityBetter receiver


Optical Amplifier

Pout = GPinPinGG

• EDFA amplifiers• Separate amplifiers for C-band and L-band• Source of optical noise


OA Gain and Fiber Loss

OA Gain

TypicalFiber Loss

4 THz

25 THz

• OA gain is centered in 1550 window• OA bandwidth is less than fiber bandwidth


Erbium Doped Fiber Amplifier

Isolator Isolator

PumpLaserPumpLaser

Coupler Coupler

Erbium-DopedFiber (10–50m)

PumpLaserPumpLaser

“Simple” device consisting of four parts:• Erbium-doped fiber• An optical pump (to invert the population).• A coupler• An isolator to cut off backpropagating noise


Principles of Er3+ Emission

980nmSource

1480nmSource

E0

EM (~10msec)

~1usec

Stimulated Emission(1520–1620 nm)

EH

SIGNAL PHOTON1550 nm

PUMPPHOTON


Optical Signal-to Noise Ratio (OSNR)

• Ratio of signal power to noise• OSNR = 10 log10(Ps/Pn)• Large OSNR is better• OSNR reduced at each amplifier

Signal Level

Noise Level

X dB

EDFA SchematicEDFA Schematic

(OSNR)out(OSNR)in

NFPin


1550nm Output


1550nm with 15db Attenuator


EDFA with No Input Signal


EDFA Output with 1550nm Input


Loss Management: LimitationsErbium Doped Fiber Amplifier

• Each amplifier adds noise, thus the optical SNR decreases gradually along the chain; we can have only have a finite number of amplifiers and spans and eventually electrical regeneration will be necessary

• Gain flatness is another key parameter mainly for long amplifier chains

Each EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input

Noise Figure > 3 dBTypically between 4 and 6

Noise Figure > 3 dBTypically between 4 and 6


Optical Thin Film Filter Technology

Dielectric Filterλ1,λ2,λ3,...λn

λ2λ1, ,λ3,...λn

• Thin Film Filter (TFF)• Dielectric material on substrate• Photons of a specific wavelength pass through• Others are reflected• Integrated to demux multiple wavelengths


Fiber Bragg Gratings

Core Cladding

Refractive Index Changes

• Small section of fiber modified by UV exposure• Creates periodic changes in refractive index• Light of a specific wavelength is refracted then reflected back• Wavelength is determined by refractive index change and

distance between refraction changes


Multiplexer / Demultiplexer

DWDMDemux

Wavelengths Converted via Transponders

Wavelength Multiplexed Signals

DWDMMux

Wavelength Multiplexed Signals

Wavelengths separated into individual ITU Specific lambdas Loss of power for each Lambda


Optical Add/Drop Filters (OADMs)

OADMs allow flexible add/drop of channels

Drop Channel

Add Channel

Drop & Insert

Pass Through loss and Add/Drop loss


Agenda



Transmission Errors

• Errors happen in the real world• Large BW-delay products in tranport systems• Bursty appearance rather than distributed• Noisy medium (ASE, distortion, PMD…)• TX/RX instability (spikes, current surges…)• Detect is good, correct is better

Transmitter ReceiverTransmission

Channel

Information InformationNoise


Forward Error Correction

• Error correcting codes both detect errors and correct them

• Forward Error Correction (FEC) is a systemadds additional information to the data streamcorrects eventual errors that are caused by the transmission system.

• Low BER achievable on noisy medium• Increases system capability – coding gain

Trade off BER vs. distance


Errors

• Symbol error occursIf one bit in a symbol is wrongOr if all bits in a symbol are wrong

• RS(255, 239) can correct 8 symbol errors8 single bit errors each in a separate byte

8 bits corrected8 complete byte errors

8 x 8 = 64 bits corrected

• Can detect up to 2t errors• Well suited for handling burst errors


Reed-Solomon Codes

• Linear block codes (subset of BCH codes)• Specified as RS(n,k) with s-bit symbols• Encoder

Takes k data symbols of s bits eachAdds parity symbols to make an n symbol codewordYields n-k parity symbols of s bits each

• DecoderCorrects up to t symbols that contain errors in the codewordWhere 2t = n-k


RS(255, 239) Example

• 8-bit symbols (i.e. byte)• 255 byte codeword• 239 data bytes• 16 parity bytes• n = 255, k = 239, s = 8

• 2t = 16, t = 8• Errors in up to 8 bytes

anywhere in the codeword corrected automatically

k = 239 2t = 16n = 255

ParityParityDataData


G.709 FEC

• RS(255,239)239 data bytes + 16 bytes FEC = 255 bytes

• OTU row split into 16 sub rows of 255 bytes16 x 255 = 4080 = 1 OTU row

• Sub rows processed separately• FEC parity check bytes

Calculated over 239 bytes of sub rowTransmitted in the last 16 bytes of same sub row


FEC Sub-Rows

InformationInformation ParityParityFEC sub-row #16

FEC sub-row #1

FEC sub-row #2

Information bytesInformation bytes Parity check bytesParity check bytesOTU Row

InformationInformation ParityParity

InformationInformation ParityParity

1, 2 ...16 3824 3825, 3826 ... 3840 4080

1 239 240 255

1 239 240 255

1 239 240 255


FEC Performance, Theoretical

FEC gain ∼ 6.3 dB @ 10-15 BER

Received Opticalpower (dBm)

Bit Error Rate

10-30

10-10

-46 -44 -42 -40 -38

1

10-20

-36 -34 -32

BER without FEC

BER with FEC

Coding Gain

BER floor


FEC in DWDM Systems

• FEC implemented on transponders (TX, RX, 3R)• No change on the rest of the system

IP

SDH

ATM

.

.

FEC

FEC

FEC

2.48 G 2.66 G

9.58 G 10.66 G

IP

SDH

ATM

9.58 G 10.66 G

.

.

FEC

FEC

FEC

2.66 G 2.48 G


Agenda



DWDM Design Topics

• DWDM Challenges• Unidirectional vs. Bidirectional• Protection• Capacity• Distance


Transmission Effects• Attenuation:

Reduces power level with distance

• Dispersion and nonlinear effects: Erodes clarity with distance and speed

• Noise and Jitter:Leading to a blurred image


Solution for Attenuation

OpticalAmplification

OpticalAmplificationLossLoss

OA


Solution For Chromatic Dispersion

Saw ToothCompensationSaw ToothCompensationDispersionDispersion

Dispersion

Length

+D -DTotal dispersion averages to ~ zero

Fiber spool Fiber spoolDCU DCU


Uni Versus Bi-directional DWDM

DWDM systems can be implemented in two different ways

• Uni-directional:

Uni -directional

λ 1λ 3λ 5λ 7

Fiber

Fiberλ 1λ 3λ 5λ 7

λ 2λ 4λ 6λ 8

λ 2λ 4λ 6λ 8

wavelengths for one direction travel within one fibertwo fibers needed for full-duplex system

• Bi-directional:a group of wavelengths for each direction single fiber operation for full-

Bi -directional

λ 5λ 6λ 7λ 8

Fiber

λ 1λ 2λ 3λ 4

duplex system


Uni Versus Bi-directional DWDM (cont.)• Uni-directional 32 channels system

32 λ

32 λ

Full band

Full band

ChannelSpacing100 GHz

16 λ

16 λ

Blue-band

Red-band

ChannelSpacing100 GHz

16 λ

16 λ

• Bi-directional 32 channels system

32 chfull

duplex

16 chfull

duplex


Optical Protection Schemes

Unprotected Client Protected

Single client, single txpdr Two client ports, equipment protected Txpdr

Splitter Protected Y-Cable Protected

Single client, protected WDM fiber Single client port, equipment protected Txpdr


1 Transponder

1 ClientInterface

Unprotected

• 1 client & 1 trunk laser (one transponder) needed, only 1 path available

• No protection in case of fiber cut, transponder failure, client failure, etc..


2 Transponders

2 Clientinterfaces

• 2 client & 2 trunk lasers (two transponders) needed, two optically unprotected paths

• Protection via higher layer protocol

Client Protected Mode


• Only 1 client & 1 trunk laser (single transponder) needed

• Protects against Fiber Breaks

Optical Splitter Switch

Workinglambda

protectedlambda

Optical Splitter Protection


• 2 client & 2 trunk lasers (two transponders) needed

• Increased cost & availability

2 Transponders

Only oneTX active

workinglambda

protectedlambda

“Y” cable

Line Card / Y- Cable Protection


Designing for Capacity

Distance

SolutionSpaceB

it R

ate

Wavelengths

• Goal is to maximize transmission capacity and system reach

Figure of merit is Gbps • KmLong-haul systems push the envelopeMetro systems are considerably simpler


Designing for Distance

Amplifier SpacingG = Gain of AmplifierS

Pout

Pnoise

Pin

D = Link Distance

L = Fiber Loss in a Span

• Link distance (D) is limited by the minimum acceptable electrical SNR at the receiverDispersion, Jitter, or optical SNR can be limit

• Amplifier spacing (S) is set by span loss (L)Closer spacing maximizes link distance (D)Economics dictates maximum hut spacing


Link Distance vs. OA Spacing

Wav

elen

gth

Cap

acity

(Gb/

s)

2.5

5

10

20

2000 4000 6000 80000

Amp Spacing60 km

80 km

100 km

120 km

140 km

Total System Length (km)

• System cost and and link distance both depend strongly on OA spacing


OEO Regeneration in DWDM Networks

Long Haul

• OA noise and fiber dispersion limit total distance before regenerationOptical-Electrical-Optical conversionFull 3R functionality: Reamplify, Reshape, Retime

• Longer spans can be supported using back to back systems


3R with Optical Multiplexor and OADM

• Express channels must be regenerated

• Two complete DWDM terminals needed

• Provides drop-and- continue functionality

• Express channels only amplified, not regenerated

• Reduces size, powerand cost

Back-to-back DWDM

Optical add/drop multiplexer

7

1234

N

OADM

7

1234

N

7

1234

N7

1234

N


Synchronization over DWDM

Ethernet

GigabitEthernet

Ethernet

DS1T1 OC-12c

OC-48c

Fiber

REGEN

WDM

OC-3c

PRS

SONETNetwork

OC-48c

OC-48c

• Synchronization driven from network

• Router interface timed to PRS via Rx

SONET Network• All links are asynchronous to

each other• Line synchronization

driven from router• Far end derives timing

from line

Point-to-Point DWDM

~~~~~~ ~~~~~~


Network Topologies and Node Types Linear NetworkingLinear Networking

Single SpanSingle Span

Add/DropAdd/Drop

TerminalTerminal TerminalTerminalOADM OADM

(Amplified)(Amplified)OADMOADM

(Passive)(Passive)Line Line

AmplifierAmplifier

OSCOSC

TerminalTerminal TerminalTerminal

OSCOSC


Network Topologies and Node Types

Ring NetworkingRing NetworkingOpen Ring (multiOpen Ring (multi--hub)hub)

Hub Hub (full mux/(full mux/demuxdemux))

Hub Hub (full mux/(full mux/demuxdemux))

Closed RingClosed RingOADM OADM

(Amplified, (Amplified, AntiAnti--ASE)ASE)

Open Ring (single hub)Open Ring (single hub)Hub Hub

(full mux/(full mux/demuxdemux))

OADMOADM(Passive)(Passive)

Line Line AmplifierAmplifier

OADM OADM (Amplified)(Amplified)


OADMOADM(Passive)(Passive)


OSCOSC


Agenda



DWDM Benefits

• DWDM systems provide hundreds of Gbps of scalable transmission capacity today

• Protocol and bit rate transparency• Provides capacity beyond TDM’s capability• Less fiber deployment• Less hardware deployment • Supports incremental, modular growth

F0_5585_c2 65© 1999, Cisco Systems, Inc.

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