Considerations in Migrating DWDM Networks to 100G

Considerations in Migrating DWDM Networks to 100G

© 2006 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1

Networks to 100G

Kevin McGrattanConsulting System EngineerJoint Techs Winter 2011

Agenda� Introductory Remarks� Goals and Assumptions� Options for Increasing System Capacity� Impairments


� Impairments� Fiber� Modulation Schemes� Non-linear Effects� Considerations and Trade-offs

Migrating 10G DWDM Networks� Goal

Introduce 40G & 100G to deployed networksPreserve existing 10G investmentMinimize cost to add 40G and 100G

� Assumptions


� AssumptionsExisting 10G DWDM networkSignificant investment in common equipmentLarge investment in 10G transponders

� Challenges Technical, planning, migration

40G/100G Network Design Options� Stay at 10G Channels

Circuit bonding to add capacityAdd capacity by moving from 100GHz to 50GHz spacing

� Greenfield 40G/100G Only Design


Optimize for advanced modulation schemesUse state of the art common equipment

� Add 40G/100G to existing 10G networkSpecifically mixed technologies…this last scenario is the focus of this talk

Increasing Speeds, New Impairments� T1, DS3

Electrical noisePoor quality cable, bad crimps

� OC3 through OC-48 and OC-192 SONET


Fiber lossFew problems up to optics specification limits

� Up to 10G DWDMCD as a function of distance comes into playOSNR limits with large numbers of long spans

DWDM at 10G and Beyond� New modulation formats

DQPSK or equivalent Polarization Multiplexed DPSKOther coherent modulation schemes

� Non-linear Effects more significant


� Non-linear Effects more significant

Fiber Types� Effective Area

How much input power can be launched into fiber without causing non-linear effectsLEAF and SMF-28 roughly equivalent

� NLE are higher with LEAF than SMF-28


� NLE are higher with LEAF than SMF-28Marginally smaller Effective Area (FWM)Reduced CD (FWM and XPM)

� LEAF best for 10G systems� SMF-28 best for high bit rate systems using advanced

modulation schemes

Modulation Schemes


DQPSK – 4-Level Phase Modulation

� Four “signal points” are used� This way, each “phase”, or “signal point”, carries two bits

2π

0

bits “01”bits “11”


bits� Better CD tolerance� Improved DGD characterisitcs

0π

23π bits “00”

bit “10”

40G PM-QPSK SolutionDual Polarization QPSK with Coherent DetectionTransmitter: Two QPSK signals are multiplexed in polarization

+ 40Gb/s‘10’

‘11’

‘00’

‘01’

‘10’

‘11’

‘00’

‘01’

Same Optical Bandwidth as a

10G NRZ!!


+‘11’ ‘01’

20Gb/s‘11’ ‘01’

20Gb/s

Laser

10Gb/s

QPSK1 Modulator10Gb/s

10G NRZ!!

40Gb/s = 10Gbaud10Gb/s

QPSK2 Modulator10Gb/s

Modulation Schemes� Amplitude Modulation (AM)

Chromatic Dispersion (CD) must be compensated to meet the CD tolerance of the receiversSolution is to place compensation (DCU) throughout the networkAdditional loss incurred by DCULEAF and DCU desirable for AM 10G networks


� Polarization Multiplexed Modulation (PM)PM receivers can have much higher CD toleranceCD also washes out NLE allowing for higher launch powerSome dispersion can help performanceSMF-28 and no/reduced DCU desirable for PM 40G/100G networks

� Distributed DCU also minimizes NLE (SPM and XFM)Value and position essential for DCU map that controls NLE

Non-linear Effects

� Must be considered at 10G and higher� Less challenging if all amplitude modulated or all

polarization multiplexed


� More challenging in mixed AM and PM designs� NLE is not always a bad thing

e.g. Raman amplification

Inten

sity Slow Phase

VelocityFast PhaseVelocity

Optical Pulsen = n0 + N2Ι

Index of Refraction

NonlinearCoefficient

Light Intensity

Optical Fiber’s Nonlinear Index


TimeRefraction Coefficient Intensity

Intensity of an optical pulse modulates the index of refraction

From Linear to Non Linear Propagation

� As long as optical power within an optical fiber is small, the fiber can be treated as a linear mediumLoss and refractive index are independent of the


Loss and refractive index are independent of the signal power

� When optical power levels gets fairly high, the fiber becomes a nonlinear mediumLoss and refractive index depend on the optical

power

A single channel’s pulses are self-distorted as they travel

Interference

Effects of Nonlinearity


Multiple channels interact as they travel

Interference

Non Linear Effects

� The non linearity in fiber cause nonlinear cross-talk

Signal Degradation


� The non linearity in fiber cause nonlinear cross-talk (interference) in the transmitted optical signal

� For single channel systems the major non linear effect is the Self Phase Modulation (SPM)

� For multi channel (DWDM) systems the major effect is the Cross Phase Modulation (XPM)

� For high capacity multi channel systems the Four Wave Mixing (FWM) is a major non linear effect

λFWM

Types of Nonlinearities

� Nonlinear indexFour-wave mixing (FWM)Self-phase modulation (SPM)


λRaman

Self-phase modulation (SPM)Cross-phase modulation (XFM)

� Stimulated scatteringRaman Brillouin

Self Phase Modulation� Single Channel Penalty� AM of channel transformed into phase modulation

of same channel� Not effected by 10G or other channels


http://en.wikipedia.org/wiki/Self-phase_modulation

Cross Phase Modulation� Heavy XPM penalty contribution in mixed 10G AM

and high bit rate PM systems� 10G AM modulation causes phase modulation on

higher bit rate channels� … which are phase modulated


� … which are phase modulated� Co-existence of 10G and high bit rate PM (typically)

requires DCU� In a no DCU system multi-channel effects tend to be

mitigated by accumulated CD …this can help

Out of Fiberω1 ω22ω1-ω2 2ω2-ω1ω1 ω2

Into Fiber

Four-Wave Mixing


Out of FiberInto Fiber� Channels beat against each other to form inter-modulation products

� Creates in-band crosstalk that can not be filtered (optically or electrically)

FWM

Efficie

ncy (

dB)

-100 D=0

Dispersion Washes Out FWM EffectsFWM and Dispersion


Channel Spacing (nm)

FWM

Efficie

ncy (

dB)

0.0 0.5 1.0 1.5 2.0 2.5-50

-30-20

-40D=17

D=2

D=0.2

Optical System Design Types� Power limited designs

High loss single span system (e.g. no ILA)� Noise limited designs

LH, amplified, high capacity systems


This is what we are concerned with here

Optical Amplifier

Pout = GPinPin

• EDFA amplifiers

GG


• EDFA amplifiers• Separate amplifiers for C-band and L-band• Amplify everything in the band• …including optical noise• A source of optical noise

OSNR and Long Chains of AmplifiersEach 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


� 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

Conflicting Requirements� Optical Launch Power

Higher power for reachLower power to constrain NLE

� Dispersion Compensation


Leave in for 10GRemove for 40G and 100G PM designs

� Cost ConsiderationsMaintain 10G investmentOptimize 40G and 100G performance

Considerations� Analyze current OSNR margins

Launch power modifications possible?� Check residual dispersion

PM typically has high CD tolerance


A little dispersion may help reduce NLE� Identify non-Raman, high loss spans

…but beware of Raman spans at terminals (reflections)� Track 40G/100G power & cooling requirements

High complexity, advanced TXP/MXP technology

What Can You Do Now?� Channel plan grooming

Group 10G at end of spectrum� Allocate guard band if necessary� Consider Raman amplification on long, high loss

spans


spans� Determine realistic 40G/100G requirement� Create transition plan

Crossover from mixed with DCU’s to all coherent technology

Other (not so great) Possibilities� Stay at 10G� Regeneration everywhere� Increase channel spacing from 100 GHz to 200 GHz� Leave DCU’s


� Leave DCU’sOptimize for 10GRegen 40G/100G everywhere

� Pull DCU’sOptimize for 40G/100GUse muxponders to increase circuit capacityRedeploy available 10G transponders with regen (CD limit)

Summary� Mixed systems can and do work� It will take some engineering effort� Some trade-offs may be necessary


Considerations in Migrating DWDM Networks to 100G

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