ONS 15454 MSTP DWDM Networking Primer October 2003 : ONS 15454 MSTP DWDM Networking Primer October 2003
Agenda : Agenda Introduction
Optical Fundamentals
Dense Wavelength Division Multiplexing (DWDM)
Optical Fundamentals : Optical Fundamentals
Slide4 : 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
Slide5 : 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 : 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) : 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 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 : Glass Purity Propagation Distance Need to Reduce the
Transmitted 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
Fiber Fundamentals : Attenuation Dispersion Nonlinearity Waveform After 1000 Km Transmitted Data Waveform Distortion It May Be a Digital Signal, but It’s Analog Transmission Fiber Fundamentals
Analog Transmission Effects : 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
Fiber Geometry : Cladding Core Coating Fiber Geometry 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
Propagation in Fiber : q1 n2 n1 Cladding q0 Core Intensity Profile Propagation in Fiber 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 Fiber : n2 n1 Cladding Core n2 n1 Cladding Core Different Types of Fiber Multimode fiber
Core diameter varies
50 mm for step index
62.5 mm for graded index
Bit rate-distance product >500 MHz-km
Single-mode fiber
Core diameter is about 9 mm
Bit rate-distance product >100 THz-km
Optical Spectrum : 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 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm l 125 GHz/nm Optical Spectrum
Optical Attenuation : Optical Attenuation Specified in loss per kilometer (dB/km)
0.40 dB/km at 1310 nm
0.25 dB/km at 1550 nm
Loss due to absorption by impurities
1400 nm peak due to OH ions
EDFA optical amplifiers available in 1550 window 1310
Window 1550
Window
Optical Attenuation : T T P i P 0 Optical Attenuation Pulse amplitude reduction limits “how far”
Attenuation in dB
Power is measured in dBm: )
Types of Dispersion : 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
A Snapshot on Chromatic Dispersion : 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
A Snapshot on Chromatic Dispersion
Limitations From 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 : 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 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 : Dispersion Compensation Transmitter
Dispersion Compensators Dispersion Shifted Fiber Cable +100
0
-100
-200
-300
-400
-500 Cumulative Dispersion (ps/nm) Total Dispersion Controlled Distance from Transmitter (km) No Compensation
With Compensation
How Far Can I Go Without Dispersion? : 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 : 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
Polarization Mode Dispersion (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 ny
Combating Polarization Mode Dispersion : Combating Polarization Mode Dispersion Factors contributing to PMD
Bit Rate
Fiber core symmetry
Environmental factors
Bends/stress in fiber
Imperfections in fiber
Solutions for PMD
Improved fibers
Regeneration
Follow manufacturer’s 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 90’s
For better performance with high capacity DWDM systems
MetroCor, WideLight…
Low PMD ULH fibers Types of Single-Mode Fiber
Different Solutions for Different Fiber Types : The primary Difference is in the Chromatic Dispersion Characteristics Different Solutions for Different Fiber Types
The 3 “R”s of Optical Networking : The 3 “R”s of Optical Networking A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations: Pulse as It Enters the Fiber Pulse as It Exits the Fiber
The 3 “R”s of Optical Networking (Cont.) : 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 t ts Optimum Sampling Time Phase Variation
DWDM : DWDM
Agenda : Agenda Introduction
Components
Forward Error Correction
DWDM Design
Summary
Increasing Network Capacity Options : Increasing Network Capacity Options Faster Electronics
(TDM) Higher bit rate, same fiber
Electronics more expensive More Fibers
(SDM) Same bit rate, more fibers
Slow Time to Market
Expensive Engineering
Limited Rights of Way
Duct Exhaust WDM Same fiber & bit rate, more ls
Fiber Compatibility
Fiber Capacity Release
Fast Time to Market
Lower Cost of Ownership
Utilizes existing TDM Equipment
Fiber Networks : Single Fiber (One Wavelength) Channel 1 Channel n Single Fiber
(Multiple Wavelengths) l1 l2 ln Fiber Networks Time division multiplexing
Single 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
Wave division multiplexing
Multiple wavelengths per fiber
4, 16, 32, 64 channels per system
Multiple channels per fiber
TDM and DWDM Comparison : DS-1
DS-3
OC-1
OC-3
OC-12
OC-48 OC-12c
OC-48c
OC-192c Fiber DWDM
OADM SONET
ADM Fiber 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)WDM
Takes multiple optical signals and multiplexes onto a single fiber
No signal format conversion
DWDM History : 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 systems
64 to 160 channels in 1550 nm window
50 and 25 GHz spacing
Why DWDM—The Business Case : TERM TERM TERM Conventional TDM Transmission—10 Gbps 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR TERM 40km 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR TERM 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR TERM 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR 1310
RPTR TERM 120 km OC-48 OA OA OA OA 120 km 120 km OC-48 OC-48 OC-48 OC-48 OC-48 OC-48 OC-48 DWDM Transmission—10 Gbps 1 Fiber Pair
4 Optical Amplifiers Why DWDM—The Business Case TERM 4 Fibers Pairs
32 Regenerators 40km 40km 40km 40km 40km 40km 40km 40km
Drivers of WDM Economics : 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 POP
Re-shape, re-time and re-amplify
Simpler network management
Delayering, less complexity, less elements
Characteristics of a WDM Network Wavelength Characteristics : 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 Network Wavelength Characteristics
Optical Transmission Bands : Optical Transmission Bands
ITU Wavelength Grid : ITU Wavelength Grid ITU-T l grid is based on 191.7 THz + 100 GHz
It is a standard for laser in DWDM systems l 1530.33 nm 1553.86 nm 0.80 nm 195.9 THz 193.0 THz 100 GHz
Fiber Attenuation Characteristics : Wavelength in Nanometers (nm) 0.2 dB/Km 0.5 dB/Km 2.0 dB/Km Attenuation vs. Wavelength S-Band:1460–1530nm Fiber Attenuation Characteristics Fibre Attenuation Curve
Characteristics of a WDM Network Sub-wavelength Multiplexing or MuxPonding : Ability to put multiple services onto a single wavelength Characteristics of a WDM Network Sub-wavelength Multiplexing or MuxPonding
Why DWDM? The Technical Argument : Why DWDM? The Technical Argument DWDM provides enormous amounts of scaleable transmission capacity
Unconstrained by speed of available electronics
Subject to relaxed dispersion and nonlinearity tolerances
Capable of graceful capacity growth
Agenda : Agenda Introduction
Components
Forward Error Correction
DWDM Design
DWDM Components : Optical Multiplexer Optical De-multiplexer Transponder DWDM Components l1 l2 l3 l1 l2 l3 850/1310 15xx l1 l2 l3 l1...n l1...n
More DWDM Components : Optical Amplifier
(EDFA) Optical Attenuator
Variable Optical Attenuator Dispersion Compensator (DCM / DCU) More DWDM Components
Typical DWDM Network Architecture : VOA EDFA DCM VOA EDFA DCM Service Mux
(Muxponder) Service Mux
(Muxponder) DWDM SYSTEM DWDM SYSTEM Typical DWDM Network Architecture
Transponders : 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 l1 From Optical OLTE To DWDM Mux
Performance Monitoring : Performance Monitoring Performance monitoring performed on a per wavelength basis through transponder
No modification of overhead
Data transparency is preserved
Laser Characteristics : Laser Characteristics DWDM Laser
Distributed Feedback (DFB) Non DWDM Laser Fabry Perot Spectrally broad
Unstable center/peak wavelength Dominant single laser line
Tighter wavelength control
DWDM Receiver Requirements : DWDM Receiver Requirements Receivers Common to all Transponders
Not Specific to wavelength (Broadband) I
Optical Amplifier : Optical Amplifier Pout = GPin Pin EDFA amplifiers
Separate amplifiers for C-band and L-band
Source of optical noise
Simple G
OA Gain and Fiber Loss : OA Gain Typical
Fiber 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 : 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 Isolator Coupler Erbium-Doped
Fiber (10–50m) Pump
Laser Pump
Laser
Slide57 : 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
Loss Management: Limitations Erbium Doped Fiber Amplifier : Loss Management: Limitations Erbium 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 dB
Typically between 4 and 6
Optical Filter Technology : l1,l2,l3,...ln l2 l1, ,l3,...ln Dielectric Filter Well established technology, up to 200 layers Optical Filter Technology
Multiplexer / Demultiplexer : Multiplexer / Demultiplexer Wavelengths Converted via Transponders Wavelength Multiplexed Signals DWDM Mux DWDM Demux Wavelength Multiplexed Signals Wavelengths separated into individual ITU Specific lambdas Loss of power for each Lambda
Optical Add/Drop Filters (OADMs) : Optical Add/Drop Filters (OADMs) OADMs allow flexible add/drop of channels Pass Through loss and Add/Drop loss
Agenda : Agenda Introduction
Components
Forward Error Correction
DWDM Design
Summary
Transmission Errors : 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
Error Correction : Error Correction Error correcting codes both detect errors and correct them
Forward Error Correction (FEC) is a system
adds 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 : FEC Performance, Theoretical FEC gain 6.3 dB @ 10-15 BER
FEC in DWDM Systems : 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 : Agenda Introduction
Components
Forward Error Correction
DWDM Design
Summary
DWDM Design Topics : DWDM Design Topics DWDM Challenges
Unidirectional vs. Bidirectional
Protection
Capacity
Distance
Transmission Effects : 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
Solution for Attenuation : OA Solution for Attenuation Loss Optical Amplification
Solution For Chromatic Dispersion : Solution For Chromatic Dispersion Length Dispersion +D -D Dispersion Saw Tooth
Compensation Total dispersion averages to ~ zero Fiber spool Fiber spool DCU DCU
Uni Versus Bi-directional DWDM : Uni Versus Bi-directional DWDM DWDM systems can be implemented in two different ways Uni-directional:
wavelengths for one direction travel within one fiber
two 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.) : Uni Versus Bi-directional DWDM (cont.) Uni-directional 32 channels system Bi-directional 32 channels system 32 ch
full
duplex 16 ch
full
duplex
DWDM Protection Review : DWDM Protection Review
Unprotected : 1 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.. Unprotected
Client Protected Mode : 2 Transponders 2 Client
interfaces 2 client & 2 trunk lasers (two transponders) needed, two optically unprotected paths
Protection via higher layer protocol Client Protected Mode
Optical Splitter Protection : Only 1 client & 1 trunk laser (single transponder) needed
Protects against Fiber Breaks Optical Splitter Switch Working
lambda protected
lambda Optical Splitter Protection
Line Card / Y- Cable Protection : 2 client & 2 trunk lasers (two transponders) needed
Increased cost & availability 2 Transponders Only one TX active working
lambda protected
lambda “Y” cable Line Card / Y- Cable Protection
Designing for Capacity : Wavelengths Bit Rate Distance Solution
Space Designing for Capacity Goal is to maximize transmission capacity and system reach
Figure of merit is Gbps • Km
Long-haul systems push the envelope
Metro systems are considerably simpler
Designing for Distance : Designing for Distance Amplifier Spacing G = Gain of Amplifier S 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 receiver
Dispersion, 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
Link Distance vs. OA Spacing : Link Distance vs. OA Spacing 2.5 5 10 20 2000 4000 6000 8000 0 Total System Length (km) Wavelength Capacity (Gb/s) Amp Spacing 60 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 : OEO Regeneration in DWDM Networks Long Haul OA noise and fiber dispersion limit total distance before regeneration
Optical-Electrical-Optical conversion
Full 3R functionality: Reamplify, Reshape, Retime
Longer spans can be supported using back to back systems
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, power and cost Back-to-back DWDM Optical add/drop multiplexer 7 1 2 3 4 N OADM 7 1 2 3 4 N 7 1 2 3 4 N 7 1 2 3 4 N 3R with Optical Multiplexor and OADM
Agenda : Agenda Introduction
Components
Forward Error Correction
DWDM Design
Summary
DWDM Benefits : DWDM Benefits DWDM provides hundreds of Gbps of scalable transmission capacity today
Provides capacity beyond TDM’s capability
Supports incremental, modular growth
Transport foundation for next generation networks
Metro DWDM : 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 provisioning
Protocol/bitrate transparency
Carrier Class Optical Protection
Metro DWDM is not yet as widely deployed