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ONS 15454 MSTPDWDM Networking PrimerDWDM Networking Primer

October 2003

Agenda

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

Optical Fundamentals

• 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

• 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

dB versus dBm

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

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

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

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

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

AttenuationDispersion

Nonlinearity

Waveform After 1000 KmTransmitted Data Waveform

Distortion

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

Fiber Fundamentals

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

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

n2

n1

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

n2

n1

Core

n2

n1

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

• 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

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

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

)

• 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

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

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

Interference

A Snapshot on Chromatic Dispersion

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”

Combating Chromatic Dispersion

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

• Dispersion Compensating Fiber• Transmitters with narrow spectral width

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

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

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.

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

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

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

• 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

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)

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

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

DWDM

Agenda

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

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

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

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

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

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

OC-192cOC-192c

FiberFiber

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

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

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

TERM

4 Fibers Pairs 32 Regenerators

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

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

• 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

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

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

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

Ability to put multiple services onto a single wavelength

Characteristics of a WDM NetworkSub-wavelength Multiplexing or MuxPonding

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

Agenda

• Introduction• Components• Forward Error Correction• DWDM Design

Optical Multiplexer

Optical De-multiplexer

Transponder

DWDM Components1

2

3

1

2

3

15xx

1

2

3

1...n

1...n

Optical Amplifier(EDFA)

Optical AttenuatorVariable Optical Attenuator

Dispersion Compensator (DCM / DCU)

More DWDM Components

VOA EDFA DCM

Service Mux(Muxponder)

Service Mux(Muxponder)

DWDM SYSTEM DWDM SYSTEM

Typical DWDM Network Architecture

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

Performance Monitoring

• Performance monitoring performed on a per wavelength basis through transponder

• No modification of overheadData transparency is preserved

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

I

Optical Amplifier

Pout = GPinPin

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

G

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

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

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

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

n

n

Dielectric Filter

• Well established technology, up to 200 layers

Optical Filter Technology

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

Drop Channel

Drop & Insert

Pass Through loss and Add/Drop loss

Agenda

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

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

Channel

Information InformationNoise

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

FEC Performance, Theoretical

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

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

Agenda

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

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

OA

Solution for Attenuation

LossOptical

Amplification

Solution For Chromatic Dispersion

Length

Dispersion

+D -D

Dispersion Saw ToothCompensation

Total dispersion averages to ~ zeroFiber spool Fiber spool

DCU DCU

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

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

DWDM Protection Review

Y-Cable and Line CardProtected

Client ProtectedUnprotected

Splitter Protected

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

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

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

Designing for Distance

Amplifier SpacingG = Gain of AmplifierS

Pout

Pnoise

Pin

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

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

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

• 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

7

1234

N

7

1234

N

7

1234

N7

1234

N

3R with Optical Multiplexor and OADM

Agenda

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

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