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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 λsFiber CompatibilityFiber Capacity ReleaseFast Time to MarketLower Cost of OwnershipUtilizes existing TDM Equipment
WDM
Faster Electronics(TDM)
Higher bit rate, same fiberElectronics more expensive
© 2001, Cisco Systems, Inc. All rights reserved. 4© 2001, Cisco Systems, Inc. All rights reserved. 4© 2001, Cisco Systems, Inc. All rights reserved. 4
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 – 1360“New 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:1460–1530nm
L-Band:1565–1625nm
C-Band:1530–1565nm
Fibre Attenuation Curve
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Agenda
• Introduction• Components• Forward Error Correction• DWDM Design
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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
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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
λ1
OEO
OEO
OEO
λ2
λn
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)
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
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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 (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
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Principles of Er3+ Emission
980nmSource
1480nmSource
E0
EM (~10msec)
~1usec
Stimulated Emission(1520–1620 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 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
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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
© 2001, Cisco Systems, Inc. All rights reserved. 43© 2001, Cisco Systems, Inc. All rights reserved. 43© 2001, Cisco Systems, Inc. All rights reserved. 43
Agenda
• Introduction• Components• Forward Error Correction• DWDM Design• Summary
© 2001, Cisco Systems, Inc. All rights reserved. 44© 2001, Cisco Systems, Inc. All rights reserved. 44© 2001, Cisco Systems, Inc. All rights reserved. 44
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
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
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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
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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
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
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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
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
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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
© 2001, Cisco Systems, Inc. All rights reserved. 62© 2001, Cisco Systems, Inc. All rights reserved. 62© 2001, Cisco Systems, Inc. All rights reserved. 62
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
© 2001, Cisco Systems, Inc. All rights reserved. 63© 2001, Cisco Systems, Inc. All rights reserved. 63© 2001, Cisco Systems, Inc. All rights reserved. 63
Agenda
• Introduction• Components• Forward Error Correction• DWDM Design• Summary
© 2001, Cisco Systems, Inc. All rights reserved. 64© 2001, Cisco Systems, Inc. All rights reserved. 64© 2001, Cisco Systems, Inc. All rights reserved. 64
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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