cisco systems dwdm primer oct03

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Published on January 11, 2008

Author: Regina1

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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 10log10(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

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