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2003 Oct.
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DWDM Networking PrimerOctober 2003

ONS 15454 MSTP

Agenda

Introduction Optical Fundamentals Dense Wavelength Division Multiplexing (DWDM)

Optical Fundamentals

Some terminology 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 milliwattX 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 2003, Cisco Systems, Inc. All rights reserved.

x =C

Some more terminology Attenuation = Loss of power in dB/kmThe 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 inThe separation of light into its different coloured rays.

ps/nm-km

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 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)

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 BudgetBasic Optical Budget = Output Power Input SensitivityPout = +6 dBm R = -30 dBm

Budget = 36 dB

Optical Budget is affected by:Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors) 2003, Cisco Systems, Inc. All rights reserved.

Glass Purity

Fiber Optics Requires Very High Purity GlassWindow Glass Optical Quality Glass Fiber Optics 1 inch (~3 cm) 10 feet (~3 m) 9 miles (~14 km)

Propagation Distance Need to Reduce the Transmitted Light Power by 50% (3 dB) 2003, Cisco Systems, Inc. All rights reserved.

Fiber FundamentalsAttenuation Dispersion Nonlinearity Distortion It May Be a Digital Signal, but Its Analog Transmission

Waveform After 1000 Km

Analog Transmission EffectsAttenuation:Reduces power level with distance

Dispersion and Nonlinearities:Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

An optical fiber is made of three sections:The core carries the light signals The cladding keeps the light in the core The coating protects the glass

Coating

Propagation in Fibern20n1 1 Intensity Profile

Light propagates by total internal reflections at the core-cladding interface Total internal reflections are lossless Each allowed ray is a mode

Different Types of Fibern2 Multimode fiberCore diameter varies 50 mm for step index 62.5 mm for graded index Bit rate-distance product >500 MHz-kmn1 Core

Single-mode fiberCore diameter is about 9 mm Bit rate-distance product >100 THz-km

n2n1

Optical SpectrumUV Visible IR 125 GHz/nm

LightUltraviolet (UV) Visible Infrared (IR)

850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm

Communication wavelengths850, 1310, 1550 nm Low-loss wavelengths

Wavelength:

(nanometers) Frequency: (terahertz)

C = x

Optical Attenuation

Specified in loss per kilometer (dB/km)0.40 dB/km at 1310 nm 0.25 dB/km at 1550 nm1550 Window

Loss due to absorption by impurities1400 nm peak due to OH ions

1310 Window

EDFA optical amplifiers available in 1550 window

Optical Attenuation Pulse amplitude reduction limits how far Attenuation in dB Power is measured in dBm:Examples10dBm 0 dBM -3 dBm -10 dBm -30 dBm 10 mW 1 mW 500 uW 100 uW 1 uW

)

P0 T

Types of Dispersion

Chromatic DispersionDifferent wavelengths travel at different speeds Causes spreading of the light pulse

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 2003, Cisco Systems, Inc. All rights reserved.

A Snapshot on Chromatic Dispersion

Interference

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 2003, Cisco Systems, Inc. All rights reserved.

Limitations From Chromatic Dispersion Dispersion causes pulse distortion, pulse "smearing" effects Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion Limits "how fast and how far10 Gbps60 Km SMF-28t

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 Total Dispersion ControlledCumulative Dispersion (ps/nm) +100 0 -100 -200 -300 -400 -500

No Compensation With Compensation

Distance from Transmitter (km) Dispersion Shifted Fiber Cable

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 in the 90s Most older fiber not characterized for PMD 2003, Cisco Systems, Inc. All rights reserved.

Polarization Mode Dispersion (PMD)Ey nx Ex ny Spreaded Pulse As It Leaves the Fiber

Pulse As It Enters the Fiber

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

Combating Polarization Mode Dispersion Factors contributing to PMDBit Rate Fiber core symmetry Environmental factors Bends/stress in fiber Imperfections in fiber

Solutions for PMDImproved fibers Regeneration Follow manufacturers recommended installation techniques for the fiber cable

Types of Single-Mode Fiber 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 90s For better performance with high capacity DWDM systems MetroCor, WideLight Low PMD ULH fibers 2003, Cisco Systems, Inc. All rights reserved.

Different Solutions for Different Fiber TypesSMF (G.652) DSF (G.653) NZDSF (G.655) Extended Band (G.652.C) (suppressed attenuation in the traditional water peak region) Good for TDM at 1310 nm OK for TDM at 1550 OK for DWDM (With Dispersion Mgmt) OK for TDM at 1310 nm Good for TDM at 1550 nm Bad for DWDM (C-Band) OK for TDM at 1310 nm Good for TDM at 1550 nm Good for DWDM (C + L Bands) 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 Rs of Optical NetworkingA Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations:Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Loss of Energy

Shape DistortionPhase Variation

Loss of Timing (Jitter)(From Various Sources)

ts Optimum Sampling Time

t

ts Optimum Sampling Time

t

The 3 Rs 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

Re-ShapePhase Variation

DCU

Phase Re-Alignment

Re-Generate

O-E-Ots Optimum Sampling Time

t

ts Optimum Sampling Time

t

Re-gen, Re-shape and ts Optimum Remove Optical Noise Sampling Time

t

DWDM

Agenda

Introduction Components Forward Error Correction DWDM Design Summary

Increasing Network Capacity OptionsMore Fibers (SDM)Same bit rate, more fibers Slow Time to Market Expensive Engineering Limited Rights of Way Duct Exhaust

W D M

Same fiber & bit rate, more s Fiber Compatibility Fiber Capacity Release Fast Time to Market Lower Cost of Ownership Utilizes existing TDM Equipment

Higher bit rate, same fiber Electronics more expensive

Fiber Networks Time division multiplexingSingle wavelength per fiber Multiple channels per fiber 4 OC-3 channels in OC-12 4 OC-12 channels in OC-48 16 OC-3 channels in OC-48 Channel 1 Channel nSingle Fiber (One Wavelength)

Wave division multiplexingMultiple wavelengths per fiber 4, 16, 32, 64 channels per system Multiple channels per fiber 2003, Cisco Systems, Inc. All rights reserved.

l1 l2 Single Fiber (Multiple Wavelengths)

ln

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

DS-1 DS-3 OC-1 OC-3 OC-12 OC-48

Fiber

(D)WDMTakes multiple optical signals and multiplexes onto a single fiber No signal format conversionOC-12c OC-48c OC-192c DWDM OADM Fiber

DWDM History Early WDM (late 80s)Two widely separated wavelengths (1310, 1550nm)

Second generation WDM (early 90s)Two to eight channels in 1550 nm window 400+ GHz spacing

DWDM systems (mid 90s)16 to 40 channels in 1550 nm window 100 to 200 GHz spacing

Next generation DWDM systems64 to 160 channels in 1550 nm window 50 and 25 GHz spacing 2003, Cisco Systems, Inc. All rights reserved.

Why DWDMThe Business CaseConventional TDM Transmission10 Gbps40km 40km 40km 40km 40km 40km 40km 40km 40km1310 1310 1310 1310 1310 1310 1310 1310 TERM TERM RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 TERM TERM RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 TERM TERM RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 TERM TERM RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR

OC-48 OC-48 OC-48 OC-48

DWDM Transmission10 GbpsOA 120 km 120 km OA OA 120 km OA

OC-48 OC-48 OC-48 OC-48

1 Fiber Pair 4 Optical Amplifiers

Drivers of WDM Economics Fiber underground/underseaExisting fiber

Conduit rights-of-wayLease or purchase

DiggingTime-consuming, labor intensive, license \$15,000 to \$90,000 per Km

3R regeneratorsSpace, power, OPS in POP Re-shape, re-time and re-amplify

Characteristics of a WDM NetworkWavelength Characteristics

TransparencyCan carry multiple protocols on same fiber Monitoring can be aware of multiple protocols

Wavelength spacing50GHz, 100GHz, 200GHz

0 50 100 150 200 250 300 350 400

Defines how many and which wavelengths can be used

Wavelength capacityExample: 1.25Gb/s, 2.5Gb/s, 10Gb/s

Optical Transmission BandsBand Wavelength (nm) 820 - 900 1260 1360 1360 1460 1460 1530 1530 1565 1565 1625 1625 1675

New Band S-Band C-Band L-Band U-Band

ITU Wavelength Grid

1530.33 nm 195.9 THz

0.80 nm 100 GHz

1553.86 nm 193.0 THz

ITU-T grid is based on 191.7 THz + 100 GHz It is a standard for laser in DWDM systemsFreq (THz) 192.90 192.85 192.80 192.75 192.70 192.65 192.60 ITU Ch 29 28 27 26 Wave (nm) 15201/252 1554.13 x 1554.54 1554.94 x 1555.34 1555.75 x 1556.15 1556.55 x 15216 x x x x 15800 x x x x 15540 x x x x 15454 x x x x

Fiber Attenuation CharacteristicsAttenuation vs. Wavelength2.0 dB/Km Fibre Attenuation Curve S-Band:14601530nm L-Band:15651625nm

0.5 dB/Km

0.2 dB/Km 800 900 1000 1100 1200 1300 1400 1500 1600 C-Band:15301565nm

Wavelength in Nanometers (nm)

Characteristics of a WDM Network

Sub-wavelength Multiplexing or MuxPonding Ability to put multiple services onto a single wavelength

Why DWDM? The Technical Argument DWDM provides enormous amounts of scaleable transmission capacityUnconstrained by speed of available electronics Subject to relaxed dispersion and nonlinearity tolerances Capable of graceful capacity growth

Agenda

Introduction Components Forward Error Correction DWDM Design

DWDM Components1 850/1310 15xx 2 3 1...n

Transponder Optical Multiplexer

1 2 3 1...n

1 2 3

More DWDM Components

Optical Amplifier (EDFA)

Optical Attenuator Variable Optical Attenuator

Dispersion Compensator (DCM / DCU)

Typical DWDM Network ArchitectureDWDM SYSTEM VOA EDFA DCM DWDM SYSTEM

DCM

EDFA

VOA

Service Mux (Muxponder)

Service Mux (Muxponder)

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 functionOEO 1 2 OEO n OEO

FromOptical OLTE

ToDWDMMux

Wavelengths Converted

Performance Monitoring

Performance monitoring performed on a per wavelength basis through transponder No modification of overheadData transparency is preserved

Laser CharacteristicsNon DWDM Laser Fabry PerotPower c

DWDM Laser Distributed Feedback (DFB)Power c

Spectrally broad Unstable center/peak wavelengthMirror Partially transmitting Mirror

Dominant single laser line Tighter wavelength control

Active medium

Amplified light

Optical AmplifierPin Pout = GPin

G

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

OA Gain and Fiber LossTypical Fiber Loss 25 THz 4 THz

OA Gain

OA gain is centered in 1550 window OA bandwidth is less than fiber bandwidth 2003, Cisco Systems, Inc. All rights reserved.

Erbium Doped Fiber AmplifierIsolator Coupler Coupler Erbium-Doped Fiber (1050m) Pump Laser Pump Laser Isolator

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 2003, Cisco Systems, Inc. All rights reserved.

Optical Signal-to Noise Ratio (OSNR)SignalLevel

X dBNoiseLevel

Depends on : Optical Amplifier Noise Figure:(OSNR)in = (OSNR)outNF

EDFA Schematic (OSNR)in Pin NF (OSNR)out

Loss Management: Limitations Erbium Doped Fiber AmplifierEach EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input

Noise Figure > 3 dB Typically 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

Optical Filter Technology

Dielectric Filter

1 , 2 , 3 ,... n 2 1 , , 3 ,... n

Well established technology, up to 200 layers

Multiplexer / Demultiplexer

DWDM Mux

DWDM Demux

Wavelength Multiplexed Signals Wavelengths Convertedvia Transponders

Wavelength Multiplexed Signals Wavelengths separatedinto individualITU Specific lambdas

Drop& Insert

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 betterInformation Transmitter 2003, Cisco Systems, Inc. All rights reserved.

Noise Transmission Channel

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, TheoreticalFEC gain 6.3 dB @ 10-15 BERBit Error Rate

1

BER without FEC10 -10

Coding Gain10 -20

BER floor

BER with FEC10 -30 -46 -44 -42 -40 -38 -36 -34 -32

FEC in DWDM Systems9.58 G IP SDH FEC FEC . . ATM 2.48 G FEC 2.66 G 2.66 G 10.66 G 10.66 G FEC FEC . . FEC ATM 2.48 G 9.58 G IP SDH

FEC implemented on transponders (TX, RX, 3R) No change on the rest of the system 2003, Cisco Systems, Inc. All rights reserved.

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

Solution for Attenuation

Loss

Optical Amplification

OA

Solution For Chromatic Dispersion

DispersionDispersionDCU

Saw Tooth CompensationFiber spool

Fiber spool

DCU

Total dispersion averages to ~ zero+D -D

Uni Versus Bi-directional DWDMDWDM systems can be implemented in two different ways

Uni-directional:wavelengths for one direction travel within one fiber two fibers needed for full-duplex system1 3 5 7 2 4 6 8

Fiber

2 4 6 8

1 3 5 7

Fiber

Uni -directional

Bi-directional:a group of wavelengths for each direction single fiber operation for fullduplex system 2003, Cisco Systems, Inc. All rights reserved.

Fiber5 6 7 8 1 2 3 4

Bi -directional

Uni Versus Bi-directional DWDM (cont.) Uni-directional 32 channels systemFull band

32 ch full duplex

32 32 Channel Spacing 100 GHz

Full band

Bi-directional 32 channels systemBlue-band

16 ch full duplex

16 16 16

16 Channel Spacing 100 GHz

DWDM Protection ReviewUnprotected Client Protected

Splitter Protected

Y-Cable and Line Card Protected

Unprotected1 Transponder 1 Client Interface

1 client & 1 trunk laser (one transponder) needed, only 1 path available No protection in case of fiber cut, transponder failure, client failure, etc.. 2003, Cisco Systems, Inc. All rights reserved.

Client Protected Mode2 Transponders 2 Client interfaces

2 client & 2 trunk lasers (two transponders) needed, two optically unprotected paths Protection via higher layer protocol 2003, Cisco Systems, Inc. All rights reserved.

Optical Splitter ProtectionOptical Splitter Working lambda Switch

protected lambda

Only 1 client & 1 trunk laser (single transponder) needed Protects against Fiber Breaks 2003, Cisco Systems, Inc. All rights reserved.

Line Card / Y- Cable Protection2 Transponders working lambda Only one TX active

Y cable

protected lambda

2 client & 2 trunk lasers (two transponders) needed Increased cost & availability 2003, Cisco Systems, Inc. All rights reserved.

Designing for CapacityBit Rate Distance

Solution SpaceWavelengths

Goal is to maximize transmission capacity and system reachFigure of merit is Gbps Km Long-haul systems push the envelope Metro systems are considerably simpler 2003, Cisco Systems, Inc. All rights reserved.

Designing for DistancePin

L = Fiber Loss in a SpanPout Pnoise

S

G = Gain of AmplifierAmplifier Spacing

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 2003, Cisco Systems, Inc. All rights reserved.

Link Distance vs. OA SpacingWavelength Capacity (Gb/s) 20 60 km

Amp Spacing

10 100 km 120 km 2.5 0 140 km 2000

80 km

5

4000

6000

8000

Total System Length (km)

OEO Regeneration in DWDM Networks

aul Long H

OA noise and fiber dispersion limit total distance before regenerationOptical-Electrical-Optical conversion Full 3R functionality: Reamplify, Reshape, Retime

Longer spans can be supported using back to back systems 2003, Cisco Systems, Inc. All rights reserved.

3R with Optical Multiplexor and OADMBack-to-back DWDM Express channels must be regenerated Two complete DWDM terminals needed1 2 3 4 N 7 1 2 3 4 N 7

Optical add/drop multiplexer Provides drop-and- continue functionality Express channels only amplified, not regenerated Reduces size, power and cost 2003, Cisco Systems, Inc. All rights reserved.

1 2 3 4 N 7

1 2 3 4 N 7

Agenda

Introduction Components Forward Error Correction DWDM Design Summary

DWDM Benefits

DWDM provides hundreds of Gbps of scalable transmission capacity todayProvides capacity beyond TDMs capability Supports incremental, modular growth Transport foundation for next generation networks

Metro DWDM

Metro DWDM is an emerging market for next generation DWDM equipment The value proposition is very different from the long haulRapid-service provisioning Protocol/bitrate transparency Carrier Class Optical Protection

Metro DWDM is not yet as widely deployed

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