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© 2003, Cisco Systems, Inc. All rights reserved. ONS 15454 MSTP DWDM Networking Primer DWDM Networking Primer October 2003
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Page 1: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

ONS 15454 MSTPDWDM Networking PrimerDWDM Networking Primer

October 2003

Page 2: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

• Introduction• Optical Fundamentals• Dense Wavelength Division Multiplexing (DWDM)

Page 3: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Fundamentals

Page 4: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Decibels (dB): unit of level (relative measure) X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501

Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain.

• Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW

• Wavelength (): length of a wave in a particular medium. Common unit: nanometers, 10-9m (nm)

300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm

• Frequency (): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz)

Wavelength x frequency = Speed of light x = C

Some terminology

Page 5: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Attenuation = Loss of power in dB/km The extent to which lighting intensity from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source.

• Chromatic Dispersion = Spread of light pulse in ps/nm-km

The separation of light into its different coloured rays. • ITU Grid = Standard set of wavelengths to be used in Fibre Optic

communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz• Optical Signal to Noise Ration (OSNR) = Ratio of optical

signal power to noise power for the receiver • Lambda = Name of Greek Letter used as Wavelength

symbol ()• Optical Supervisory Channel (OSC) = Management channel

Some more terminology

Page 6: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

dB versus dBm

• dBm used for output power and receive sensitivity (Absolute Value)

• dB used for power gain or loss (Relative Value)

Page 7: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Bit Error Rate ( BER)

• BER is a key objective of the Optical System Design

• Goal is to get from Tx to Rx with a BER < BER threshold of the Rx

• BER thresholds are on Data sheets• Typical minimum acceptable rate is 10 -12

Page 8: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Budget

Optical Budget is affected by: Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors)

Basic Optical Budget = Output Power – Input Sensitivity

Pout = +6 dBm R = -30 dBm

Budget = 36 dB

Page 9: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Glass Purity

Propagation Distance Need to Reduce theTransmitted Light Power by 50% (3 dB)

Window Glass 1 inch (~3 cm)Optical Quality Glass 10 feet (~3 m)Fiber Optics 9 miles (~14 km)

Fiber Optics Requires Very High Purity Glass

Page 10: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

AttenuationDispersion

Nonlinearity

Waveform After 1000 KmTransmitted Data Waveform

Distortion

It May Be a Digital Signal, but It’s Analog Transmission

Fiber Fundamentals

Page 11: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Attenuation: Reduces power level with distance

Dispersion and Nonlinearities: Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

Analog Transmission Effects

Page 12: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

CladdingCore

Coating

Fiber Geometry

• An optical fiber is made ofthree sections:

The core carries thelight signals

The cladding keeps the lightin the core

The coating protects the glass

Page 13: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

n2

n1

Cladding

Core

Intensity Profile

Propagation in Fiber

• Light propagates by total internal reflectionsat the core-cladding interface

• Total internal reflections are lossless• Each allowed ray is a mode

Page 14: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

n2

n1

Cladding

Core

n2

n1

Cladding

Core

Different Types of Fiber

• Multimode fiberCore diameter varies

50 mm for step index62.5 mm for graded index

Bit rate-distance product>500 MHz-km

• Single-mode fiberCore diameter is about 9 mmBit rate-distance product>100 THz-km

Page 15: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Light Ultraviolet (UV) Visible Infrared (IR)

• Communication wavelengths 850, 1310, 1550 nm Low-loss wavelengths

• Specialty wavelengths 980, 1480, 1625 nm

UV IR

Visible

850 nm980 nm

1310 nm1480 nm

1550 nm1625 nm

125 GHz/nm

Wavelength: (nanometers)Frequency: (terahertz)

C =x

Optical Spectrum

Page 16: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Attenuation

• Specified in loss per kilometer (dB/km)

0.40 dB/km at 1310 nm0.25 dB/km at 1550 nm

• Loss due to absorptionby impurities

1400 nm peak due to OH ions

• EDFA optical amplifiers available in 1550 window

1310Window

1550Window

Page 17: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

T T

P i P0

Optical Attenuation

• Pulse amplitude reduction limits “how far” • Attenuation in dB• Power is measured in dBm:

Examples10dBm 10 mW0 dBM 1 mW-3 dBm 500 uW

-10 dBm 100 uW-30 dBm 1 uW

)

Page 18: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization

states Fast and slow axes have different group

velocities Causes spreading of the light pulse

• Chromatic Dispersion Different wavelengths travel at different speeds Causes spreading of the light pulse

Types of Dispersion

Page 19: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Affects single channel and DWDM systems• A pulse spreads as it travels down the fiber• Inter-symbol Interference (ISI) leads to

performance impairments• Degradation depends on:

laser used (spectral width)bit-rate (temporal pulse separation)Different SM types

Interference

A Snapshot on Chromatic Dispersion

Page 20: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

60 Km SMF-28

4 Km SMF-28

10 Gbps

40 Gbps

Limitations From Chromatic Dispersion

t

t

• Dispersion causes pulse distortion, pulse "smearing" effects

• Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion

• Limits "how fast“ and “how far”

Page 21: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Combating Chromatic Dispersion

• Use DSF and NZDSF fibers(G.653 & G.655)

• Dispersion Compensating Fiber• Transmitters with narrow spectral width

Page 22: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Dispersion Compensating Fiber

• Dispersion Compensating Fiber:

By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter

Page 23: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Dispersion Compensation

Transmitter

Dispersion Compensators

Dispersion Shifted Fiber Cable

+1000

-100-200-300-400-500

Cum

ulat

ive

Dis

pers

ion

(ps/

nm)

Total Dispersion Controlled

Distance fromTransmitter (km)

No CompensationWith Compensation

Page 24: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

How Far Can I Go Without Dispersion?

Distance (Km) =Specification of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

A laser signal with dispersion tolerance of 3400 ps/nm is sent across a standard SMF fiber which has a Coefficient of

Dispersion of 17 ps/nm*km.It will reach 200 Km at maximum bandwidth.

Note that lower speeds will travel farther.

Page 25: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Polarization Mode Dispersion

• Caused by ovality of core due to:

Manufacturing process

Internal stress (cabling)

External stress (trucks)

• Only discovered inthe 90s

• Most older fiber not characterized for PMD

Page 26: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Polarization Mode Dispersion (PMD)

• The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of little relevance at bit rates of 10Gb/s or less

nx

nyEx

Ey

Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber

Page 27: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Combating Polarization Mode Dispersion

• Factors contributing to PMDBit RateFiber core symmetryEnvironmental factorsBends/stress in fiberImperfections in fiber

• Solutions for PMDImproved fibers RegenerationFollow manufacturer’s recommended installation techniques for the fiber cable

Page 28: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• SMF-28(e) (standard, 1310 nm optimized, G.652)Most widely deployed so far, introduced in 1986, cheapest

• DSF (Dispersion Shifted, G.653)Intended for single channel operation at 1550 nm

• NZDSF (Non-Zero Dispersion Shifted, G.655)For WDM operation, optimized for 1550 nm region– TrueWave, FreeLight, LEAF, TeraLight…

Latest generation fibers developed in mid 90’sFor better performance with high capacity DWDM systems

– MetroCor, WideLight…– Low PMD ULH fibers

Types of Single-Mode Fiber

Page 29: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

The primary Difference is in the Chromatic Dispersion Characteristics

Different Solutions forDifferent Fiber Types

SMF

(G.652)

•Good for TDM at 1310 nm•OK for TDM at 1550•OK for DWDM (With Dispersion Mgmt)

DSF

(G.653)

•OK for TDM at 1310 nm•Good for TDM at 1550 nm•Bad for DWDM (C-Band)

NZDSF

(G.655)

•OK for TDM at 1310 nm•Good for TDM at 1550 nm•Good for DWDM (C + L Bands)

Extended Band

(G.652.C)

(suppressed attenuation in the traditional water peak region)

•Good for TDM at 1310 nm•OK for TDM at 1550 nm•OK for DWDM (With Dispersion Mgmt•Good for CWDM (>8 wavelengths)

Page 30: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

The 3 “R”s of Optical NetworkingA Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations:

Loss of Energy

Loss of Timing (Jitter)(From Various Sources) tts Optimum

Sampling Time

tts Optimum Sampling Time

Phase Variation

Shape Distortion

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Page 31: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Re-Shape DCU

The 3 “R”s of Optical Networking (Cont.)The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are:

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Amplify to Boost the Power

tts Optimum Sampling Time

tts Optimum Sampling Time

Phase Variation

Re-Generate O-E-O

Re-gen, Re-shape andRemove Optical Noise

tts Optimum Sampling Time

Phase Re-Alignment

Page 32: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DWDM

Page 33: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

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

Page 34: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Increasing Network Capacity Options

Faster Electronics(TDM)

Higher bit rate, same fiberElectronics more expensive

More Fibers(SDM)

Same bit rate, more fibersSlow Time to MarketExpensive EngineeringLimited Rights of WayDuct Exhaust

WDM

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

Page 35: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Single Single Fiber (One Fiber (One

Wavelength)Wavelength)

Channel 1

Channel n

Single FiberSingle Fiber(Multiple (Multiple

Wavelengths)Wavelengths)

l1l1

l2l2

lnln

Fiber Networks

• Time division multiplexingSingle wavelength per fiberMultiple channels per fiber4 OC-3 channels in OC-124 OC-12 channels in OC-4816 OC-3 channels in OC-48

• Wave division multiplexingMultiple wavelengths per fiber4, 16, 32, 64 channels per systemMultiple channels per fiber

Page 36: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DS-1DS-1DS-3DS-3OC-1OC-1OC-3OC-3

OC-12OC-12OC-48OC-48

OC-12cOC-12cOC-48cOC-48c

OC-192cOC-192c

FiberFiber

DWDMDWDMOADMOADM

SONETSONETADMADM

FiberFiber

TDM and DWDM Comparison

• TDM (SONET/SDH)Takes sync and async signals and multiplexes them to a single higher optical bit rate

E/O or O/E/O conversion

• (D)WDMTakes multiple optical signals and multiplexes onto a single fiber

No signal format conversion

Page 37: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

• DWDM systems (mid 90s)16 to 40 channels in 1550 nm window100 to 200 GHz spacing

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

Page 38: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

TERMTERM

TERM

Conventional TDM Transmission—10 Gbps

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM

40km1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM

120 kmOC-48

OA OAOA OA120 km 120 km

OC-48OC-48

OC-48

OC-48OC-48

OC-48OC-48

DWDM Transmission—10 Gbps

1 Fiber Pair4 Optical Amplifiers

Why DWDM—The Business Case

TERM

4 Fibers Pairs 32 Regenerators

40km 40km 40km 40km 40km 40km 40km 40km

Page 39: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Drivers of WDM Economics

• Fiber underground/undersea Existing fiber

• Conduit rights-of-way Lease or purchase

• Digging Time-consuming, labor intensive, license $15,000 to $90,000 per Km

• 3R regenerators Space, power, OPS in POPRe-shape, re-time and re-amplify

• Simpler network managementDelayering, less complexity, less elements

Page 40: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• Transparency Can carry multiple protocols on same fiber Monitoring can be aware of multiple protocols

• Wavelength spacing 50GHz, 100GHz, 200GHz Defines how many and which wavelengths can be used

• Wavelength capacity Example: 1.25Gb/s, 2.5Gb/s, 10Gb/s

0 50 100 150 200 250 300 350 400

Characteristics of a WDM NetworkWavelength Characteristics

Page 41: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 42: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

ITU Wavelength Grid

• ITU-T grid is based on 191.7 THz + 100 GHz• It is a standard for laser in DWDM systems

1530.33 nm 1553.86 nm0.80 nm

195.9 THz 193.0 THz100 GHz

Freq (THz) ITU Ch Wave (nm) 15201/252 15216 15800 15540 15454192.90 29 1554.13 x x x x x192.85 1554.54192.80 28 1554.94 x x x x x192.75 1555.34192.70 27 1555.75 x x x x x192.65 1556.15192.60 26 1556.55 x x x x x

Page 43: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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. Wavelength S-Band:1460–1530nm

L-Band:1565–1625nm

C-Band:1530–1565nm

Fiber Attenuation Characteristics

Fibre Attenuation Curve

Page 44: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Ability to put multiple services onto a single wavelength

Characteristics of a WDM NetworkSub-wavelength Multiplexing or MuxPonding

Page 45: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Why DWDM?The Technical Argument

• DWDM provides enormous amounts of scaleable transmission capacity

Unconstrained by speed ofavailable electronics

Subject to relaxed dispersion and nonlinearity tolerances

Capable of graceful capacity growth

Page 46: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

• Introduction• Components• Forward Error Correction• DWDM Design

Page 47: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Multiplexer

Optical De-multiplexer

Optical Add/Drop Multiplexer(OADM)

Transponder

DWDM Components1

2

3

1

2

3

15xx

1

2

3

1...n

1...n

Page 48: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Amplifier(EDFA)

Optical AttenuatorVariable Optical Attenuator

Dispersion Compensator (DCM / DCU)

More DWDM Components

Page 49: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

VOA EDFA DCM

VOAEDFADCM

Service Mux(Muxponder)

Service Mux(Muxponder)

DWDM SYSTEM DWDM SYSTEM

Typical DWDM Network Architecture

Page 50: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

From Optical OLTE

To DWDM MuxOEO

OEO

OEO

2

n

Page 51: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Performance Monitoring

• Performance monitoring performed on a per wavelength basis through transponder

• No modification of overheadData transparency is preserved

Page 52: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Laser Characteristics

cPower

Power c

DWDM Laser Distributed Feedback (DFB)

Active medium

MirrorPartially transmitting

Mirror

Amplified light

Non DWDM Laser Fabry Perot

• Spectrally broad• Unstable center/peak wavelength

• Dominant single laser line• Tighter wavelength control

Page 53: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DWDM Receiver Requirements

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

I

Page 54: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Amplifier

Pout = GPinPin

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

G

Page 55: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

OA Gain

TypicalFiber Loss

4 THz

25 THz

OA Gain and Fiber Loss

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

Page 56: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Erbium Doped Fiber Amplifier

“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

Isolator Coupler IsolatorCoupler

Erbium-DopedFiber (10–50m)

PumpLaser

PumpLaser

Page 57: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Optical Signal-to Noise Ratio (OSNR)

• Depends on : Optical Amplifier Noise Figure:

(OSNR)in = (OSNR)outNF

• Target : Large Value for X

Signal Level

Noise Level

X dB

EDFA Schematic

(OSNR)out(OSNR)in

NFPin

Page 58: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 59: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

n

n

Dielectric Filter

• Well established technology, up to 200 layers

Optical Filter Technology

Page 60: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Multiplexer / Demultiplexer

Wavelengths Converted via Transponders

Wavelength Multiplexed Signals

DWDMMux DWDM

Demux

Wavelength Multiplexed Signals

Wavelengths separated into individual ITU Specific lambdas

Loss of power for each Lambda

Page 61: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 62: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

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

Page 63: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Transmission Errors

• Errors happen!• A old problem of our era (PCs, wireless…)• 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

Page 64: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Error Correction

• Error correcting codes both detect errors and correct them

• Forward Error Correction (FEC) is a systemadds additional information to the data stream

corrects eventual errors that are caused by the transmission system.

• Low BER achievable on noisy medium

Page 65: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

FEC Performance, Theoretical

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 gain 6.3 dB @ 10-15 BER

Page 66: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

.

.

FEC

FEC

FEC

2.66 G 2.48 G

10.66 G 9.58 G

Page 67: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

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

Page 68: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DWDM Design Topics

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

Page 69: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 70: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

OA

Solution for Attenuation

LossOptical

Amplification

Page 71: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Solution For Chromatic Dispersion

Length

Dispersion

+D -D

Dispersion Saw ToothCompensation

Total dispersion averages to ~ zeroFiber spool Fiber spool

DCU DCU

Page 72: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Uni Versus Bi-directional DWDM

DWDM systems can be implemented in two different ways

Bi -directional

Fiber

Uni -directional

Fiber

Fiber

• Uni-directional: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-duplex system

Page 73: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Uni Versus Bi-directional DWDM (cont.)

32

32

Full band

Full band

ChannelSpacing100 GHz

16

16

Blue-band

Red-band

ChannelSpacing100 GHz

16

16

• Uni-directional 32 channels system

• Bi-directional 32 channels system

32 chfull

duplex

16 chfull

duplex

Page 74: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DWDM Protection Review

Y-Cable and Line CardProtected

Client ProtectedUnprotected

Splitter Protected

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© 2003, Cisco Systems, Inc. All rights reserved.

1 Transponder

1 ClientInterface

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

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

Unprotected

Page 76: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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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© 2003, Cisco Systems, Inc. All rights reserved.

• 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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© 2003, Cisco Systems, Inc. All rights reserved.

• 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

Page 79: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Wavelengths

Bit

Rat

e Distance

SolutionSpace

Designing for Capacity

• Goal is to maximize transmission capacity and system reach

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

Page 80: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 81: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Link Distance vs. OA Spacing

2.5

5

10

20

2000 4000 6000 80000Total System Length (km)

Wav

elen

gth

Cap

acity

(Gb/

s) Amp Spacing60 km

80 km

100 km

120 km

140 km

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

Page 82: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

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

Page 83: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

• 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

3R with Optical Multiplexor and OADM

Page 84: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Agenda

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

Page 85: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

DWDM Benefits

• DWDM provides hundreds of Gbps of scalable transmission capacity today

Provides capacity beyondTDM’s capabilitySupports incremental, modular growth

Transport foundation for nextgeneration networks

Page 86: cisco systems dwdm primer oct03

© 2003, Cisco Systems, Inc. All rights reserved.

Metro DWDM

• Metro DWDM is an emerging market for next generation DWDM equipment

• The value proposition is very different from the long haul

Rapid-service provisioningProtocol/bitrate transparencyCarrier Class Optical Protection

• Metro DWDM is not yet as widely deployed

Page 87: cisco systems dwdm primer oct03

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