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Cisco Optical WorkshopDWDMJanuary 31, 2004
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
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Increasing Network Capacity OptionsSame bit rate, more fibersSlow Time to MarketExpensive EngineeringLimited Rights of WayDuct Exhaust
More Fibers(SDM)
Same fiber & bit rate, more sFiber CompatibilityFiber Capacity ReleaseFast Time to MarketLower Cost of OwnershipUtilizes existing TDM Equipment
WDM
Faster Electronics(TDM)
Higher bit rate, same fiberElectronics more expensive
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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
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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
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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
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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
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Optical Transmission Bands
Band Wavelength (nm)820 - 900
1260 1360New Band 1360 1460
S-Band 1460 1530C-Band 1530 1565L-Band 1565 1625U-Band 1625 1675
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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:14601530nmL-Band:15651625nm
C-Band:15301565nm
Fibre Attenuation Curve
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Agenda
Introduction Components Forward Error Correction DWDM Design
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DWDM Components123
1...n15xx850/1310
TransponderOptical Multiplexer
Optical Add/Drop Multiplexer(OADM)
(Band and Channel)
123
1...n 123
Optical De-multiplexer
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More DWDM Components
Optical Amplifier(EDFA)
Optical AttenuatorVariable Optical Attenuator
Dispersion Compensator (DCM / DCU)
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Typical DWDM Network Architecture
DWDM SYSTEM DWDM SYSTEM
VOA EDFA DCM
VOAEDFADCM
Service Mux(Muxponder)
Service Mux(Muxponder)
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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
1
OEO
OEO
OEO
2
n
From Optical OLTE
To DWDM Mux
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Performance Monitoring
Performance monitoring performed on a per wavelength basis through transponder
G.709 based No modification of overhead Data transparency is preserved
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Laser Characteristics Non DWDM Laser
Fabry Perot DWDM Laser
Distributed Feedback (DFB)
Power c
cPower
Dominant single laser line Tighter wavelength control
Spectrally broad Unstable center/peak wavelength
Active medium
MirrorPartially transmitting
Mirror
Amplified light
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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
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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
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Optical Amplifier
Pout = GPinPinGG
EDFA amplifiers Separate amplifiers for C-band and L-band Source of optical noise
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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
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Erbium Doped Fiber Amplifier
Isolator Isolator
PumpLaserPumpLaser
Coupler Coupler
Erbium-DopedFiber (1050m)
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
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Principles of Er3+ Emission
980nmSource
1480nmSource
E0
EM (~10msec)
~1usec
Stimulated Emission(15201620 nm)
EH
SIGNAL PHOTON1550 nm
PUMPPHOTON
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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
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1550nm Output
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1550nm with 15db Attenuator
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EDFA with No Input Signal
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EDFA Output with 1550nm Input
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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
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Optical Thin Film Filter Technology
Dielectric Filter1,2,3,...n
2 1, ,3,...n
Thin Film Filter (TFF) Dielectric material on substrate Photons of a specific wavelength pass through Others are reflected Integrated to demux multiple wavelengths
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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
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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
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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
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Agenda
Introduction Components Forward Error Correction DWDM Design Summary
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Agenda
Introduction Components Forward Error Correction DWDM Design Summary
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DWDM Design Topics
DWDM Challenges Unidirectional vs. Bidirectional Protection Capacity Distance
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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
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Solution for Attenuation
OpticalAmplification
OpticalAmplificationLossLoss
OA
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Solution For Chromatic Dispersion
Saw ToothCompensationSaw ToothCompensationDispersionDispersion
Dispersion
Length
+D -DTotal dispersion averages to ~ zero
Fiber spool Fiber spoolDCU DCU
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Uni Versus Bi-directional DWDM
DWDM systems can be implemented in two different ways
Uni-directional:
Uni -directional
1 3 5 7 Fiber
Fiber 1 3 5 7
2 4 6 8
2 4 6 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
5 6 7 8
Fiber
1 2 3 4
duplex system
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Uni Versus Bi-directional DWDM (cont.) Uni-directional 32 channels system
32
32
Full band
Full band
ChannelSpacing100 GHz
16
16 Blue-band
Red-band
ChannelSpacing100 GHz
16
16
Bi-directional 32 channels system
32 chfull
duplex
16 chfull
duplex
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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
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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..
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2 Transponders
2 Clientinterfaces
2 client & 2 trunk lasers (two transponders) needed, two optically unprotected paths
Protection via higher layer protocol
Client Protected Mode
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Only 1 client & 1 trunk laser (single transponder) needed
Protects against Fiber Breaks
Optical Splitter Switch
Workinglambda
protectedlambda
Optical Splitter Protection
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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
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Designing for Capacity
Distance
SolutionSpaceB i
t
R
a
t
e
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
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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
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Link Distance vs. OA Spacing
W
a
v
e
l
e
n
g
t
h
C
a
p
a
c
i
t
y
(
G
b
/
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
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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
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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
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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
~~~~~~ ~~~~~~
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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
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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)
OADM OADM (Amplified)(Amplified)
OADMOADM(Passive)(Passive)
OADM OADM (Amplified)(Amplified)
OSCOSC
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Agenda
Introduction Components Forward Error Correction DWDM Design Summary
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DWDM Benefits
DWDM systems provide hundreds of Gbps of scalable transmission capacity today
Protocol and bit rate transparency Provides capacity beyond TDMs capability Less fiber deployment Less hardware deployment Supports incremental, modular growth
F0_5585_c2 65 1999, Cisco Systems, Inc.
Cisco Optical WorkshopDWDMJanuary 31, 2004AgendaIncreasing Network Capacity OptionsFiber NetworksTypes of WDMDWDM HistoryWavelength Characteristics for DWDMOptical Transmission BandsFiber Attenuation CharacteristicsAgendaDWDM ComponentsMore DWDM ComponentsTypical DWDM Network ArchitectureTranspondersPerformance MonitoringLaser CharacteristicsTransponder: Direct vs. External ModulationDWDM Receiver RequirementsOptical AmplifierOA Gain and Fiber LossErbium Doped Fiber AmplifierPrinciples of Er3+ Emission1550nm Output1550nm with 15db AttenuatorEDFA with No Input SignalEDFA Output with 1550nm InputLoss Management: LimitationsErbium Doped Fiber AmplifierOptical Thin Film Filter TechnologyFiber Bragg GratingsMultiplexer / DemultiplexerOptical Add/Drop Filters (OADMs)AgendaTransmission ErrorsForward Error CorrectionErrorsReed-Solomon CodesRS(255, 239) ExampleG.709 FECFEC Sub-RowsFEC Performance, TheoreticalFEC in DWDM SystemsAgendaDWDM Design TopicsTransmission EffectsSolution for AttenuationSolution For Chromatic DispersionUni Versus Bi-directional DWDMUni Versus Bi-directional DWDM (cont.)Optical Protection SchemesUnprotectedClient Protected ModeOptical Splitter ProtectionLine Card / Y- Cable ProtectionDesigning for CapacityDesigning for DistanceLink Distance vs. OA SpacingOEO Regeneration in DWDM Networks3R with Optical Multiplexor and OADMSynchronization over DWDMNetwork Topologies and Node TypesNetwork Topologies and Node TypesAgendaDWDM Benefits