DWDM 101 Rodger Nutt High-End Routing and Optical BU Technical Leader
Agenda Introduction What is DWDM Fiber Types Linear Effects The BIG Three: Attenuation, Chromatic Dispersion, OSNR Solutions to the BIG Three: Optical Amplifiers (EDFA, RAMAN), Dispersion Compensators, FEC Non-Linear Effects Components Transponders / Muxponders / Xponders Pluggable Optics Amplifiers Filters: OADMs / ROADMs, OSC Protection Schemes DWDM Software Automatic Node Setup Automatic Power Control Automatic Laser Shutdown WSON/GMPLS 3
What is DWDM?
Wavelength Division Multiplexing DWDM systems use optical devices to combine the output of several optical transmitters TX TX TX TX Optical transmitters Transmission Optical fiber pair DWDM devices RX RX RX RX Optical receivers 5
ITU-T Grid ITU wavelengths = lambdas = channels center around 1550 nm (193 THz) 0.4 nm spacing 1528.77 nm 1552.52 nm 1578.23 nm (Center channel) 50 GHz spacing Wavelength (nm) Frequency (THz) 196.2 THz 193.1 THz 190.1 THz (Center channel) 6
Dense vs. Coarse (CWDM vs. DWDM) DWDM CWDM Application Long Haul Metro Amplifiers Typically EDFAs Almost Never # Channels Up to 80 Up to 8 Channel Spacing 0.4 nm 20nm Distance Up to 3000km Up to 80km Spectrum 1530nm to 1560nm 1270nm to 1610nm Filter Technology Intelligent Passive 7
Optical Fiber
Fiber Geometry and Dimensions The core carries the light signals The refractive index difference between core & cladding confines the light to the core The coating protects the glass Core SMF 8 microns Cladding 125 microns Coating 250 microns 9
Optical Spectrum UltraViolet Visible InfraRed λ 850 nm 1310 nm 1550 nm 1625 nm Communication Wavelengths in the InfraRed 850 nm Multimode 1310 nm Singlemode C-band:1550 nm Singlemode L-band: 1625 nm Singlemode C =ƒ x λ Wavelength: λ (nanometers) Frequency: ƒ (terahertz) 10
Applications for the Different Fiber Types SMF (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 primary difference is in the Chromatic Dispersion Characteristics 11
Linear Effects
Transmission Impairments Attenuation Loss of Signal Strength Chromatic Dispersion (CD) Distortion of pulses Optical Signal to Noise Ratio (OSNR) Effect of Noise in Transmission Loss (db/km) 2.0 0.5 S-band:1460 1530nm C-band:1530 1565nm L-band:1565 1625nm 0.2 800 900 1000 1100 1200 1300 1400 1500 1600 Wavelength (nm) Time Slot 2.5Gb/s Fiber 10Gb/s Fiber S+N N 13
Attenuation With enough attenuation, a light pulse may not be detected by an optical receiver Attenuation (db) Distance (km) Optical device Insertion loss (db) 14
Fiber Attenuation (Loss) Characteristic Loss(dB)/km vs. Wavelength S-band:1460 1530nm L-band:1565 1625nm 2.0 db/km OH - Absorption Peaks in Actual Fiber Attenuation Curve 0.5 db/km 0.2 db/km 800 900 1000 1100 1200 1300 1400 1500 1600 Wavelength in Nanometers (nm) OH: Hydroxyl ion absorption is the absorption in optical fibers of electromagnetic waves, due to the presence of trapped hydroxyl ions remaining from water as a contaminant. C-band:1530 1565nm 15
Laser Output Power and Receiver Sensitivity and dbm Fiber loss expressed in db but transmitter/receiver power is expressed in dbm This is why both the transmitter output power and the receiver sensitivity is expressed in dbm: Power dbm =10log(P mw /1mW) db and dbm are additive, hence the simplification Example: Power dbm = 10log(2mW/1mW)=3dBm Power dbm = 10log(1mW/1mW)=0dBm 16
Gain and Decibels (db) P in Amp P out Gain can be expressed by the ratio of P out /P in Gain is measured more conveniently in db, calculated by 10 log 10 P out /P in If the power is doubled by an amplifier, this is +3 db 17
Attenuation: Optical Budget Basic Optical Budget = Output Power Input Sensitivity Pout = +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) 18
Attenuation Solution: EDFA Erbium doped fiber amplifies optical signals through stimulated emission using 980nm and 1480nm pump lasers Signal Input Isolator Erbium Doped Fiber Isolator Amplified Signal Output 980 or 1480 nm Pump Laser WDM Coupler for pump and signal Basic EDFA configuration 19
Chromatic Dispersion (CD) Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2 The Optical Pulse tends to Spread as it propagates down the fiber generating Inter-Symbol-Interference (ISI) Total dispersion is a function of the length of fiber and it s dispersion factor Limits transmission distance for 10G and above wavelengths Can be compensated by using negative dispersion fiber or electronically through modulation schemes 20
Solution: Dispersion Compensating Unit DCUs use fiber with chromatic dispersion of opposite sign/slope and of suitable length to bring the average dispersion of the link close to zero. 21
Optical Signal-to-Noise Ratio (OSNR) OSNR is a measure of the ratio of signal level to the level of system noise As OSNR decreases, possible errors increase OSNR is measured in decibels (db) EDFAs are the source of noise Signal level dbm) Noise level (dbm) Signal level OSNR = ----------------- Noise level 22
Optical Signal Detection Across a fiber span, optical signals encounter attenuation, dispersion, and increased noise levels at amplifiers. Each of these factors causes bit detection errors at the receiver. Low attenuation Low dispersion High OSNR High attenuation High dispersion Low OSNR Transmitting end Distance (km) Receiving end 23
Example: Link Design with Line Amplifiers 10G Xenpak spec: Tx: -1dBm min TX Tx: +3 to -1dBm, Rx min: -21dBm (0ps/nm) CD tolerance: +1600ps/nm @ 2dB penalty OSNR min: 16dB (0.5nm resolution) 25dB DCU -1600 ps/nm 25dB DCU -1600 ps/nm Meets receiver minimum OSNR and power requirement OSNR: 18dB Rx: -9dBm RX Mux +2dBm/ch -23dBm/ch +2dBm/ch -23dBm/ch Demux Time Domain OSNR= 35dB Noise OSNR= 21dB Noise OSNR= 18dB Noise Wavelength Domain -1dBm +2dBm 0ps/nm -23dBm 1600ps/nm +2dBm 0ps/nm -23dBm 1600ps/nm +2dBm 0ps/nm 24
OSNR Solution #1 Raman Amplifier Stimulated Raman Scattering creates the Gain Reduces the effective span loss and increases noise performance Gain is highly dependent on quality of fiber Gain Spectrum ~ 40nm with a single pump 25
OSNR Solution #2: Forward Error Correction FEC extends reach and design flexibility, at silicon cost G.709 (G.709 Annex A) standard improves OSNR tolerance by 6.2 db (at 10 15 BER) Offers intrinsic performance monitoring (error statistics) Higher gains (8.4dB) possible by enhanced FEC (with same G.709 overhead G.975.1 I.4) Log (BER) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Raw Channel BER=1.5e-3 G.709 RS(255,239) EFEC=8.4 db FEC=6.2 db Uncoded No FEC 4 5 6 7 8 9 10 11 12 13 14 15 S/N (db) Benefit: FEC/EFEC Extends Reach and Offers 10 15 BER 26
Non-linear Effects
Non Linear Effects Polarization Mode Dispersion (PMD) Caused by Non Linearity Of Fiber Geometry Effective for Higher Bit rates (10G) Four Wave Mixing (FWM) Effects in multi-channel systems Effects for higher bit rates Self/Cross Phase Modulation (SPM, XPM) Effected by high channel power Effected by neighbor channels Ey nx Ex Pulse As it Enters the Fiber ny Spreaded Pulse As it Leaves the Fiber Power (dbm) -5-10 -15-20 -25-30 -35-40 1542 1543 1544 1545 1546 1547 1548 SPM Distortion Wavelength (nm) Power 28
Polarization Mode Dispersion (PMD) Ey n x Ex Pulse as It Enters the Fiber n y Spreaded Pulse as It Leaves the Fiber It is Relevant at Bit Rates of 10Gb/s or More Pulse broadens as it travels down fiber Mainly a manufacturing/install issue with concentricity of fiber 29
PMD Solutions Increase system robustness with FEC Leverage MLSE Use PMD Compensation (PMDC) Deploy PMD-optimized fibers Advanced Modulation Schemes 10Gb/s QPSK1 Modulator Laser 10Gb/s 10Gb/s 40Gb/s = 10Gbaud QPSK2 Modulator 10Gb/s 30
DWDM Components
Typical Components of DWDM Systems Optical transmitters and receivers DWDM mux/demux filters Optical add/drop multiplexers (OADMs) Reconfigurable OADM (ROADM) Optical amplifiers Transponders/Muxponders 32
Optical Transmitter Block Diagram + - Detects pulses of electrical charge Power measured in watts (W) Amplitude measured in volts (V) + V - 1 0 1 1 Electrical-to-optical (E-O) conversion E-O Creates pulses of light Power measured in decibel-milliwatts (dbm) Relative amplitude measured in decibels (db) + db - 1 0 1 1 Electrical conductor Optical fiber 33
Optical Receiver Block Diagram Detects pulses of light Power measured in decibel-milliwatt (dbm) Relative amplitude measured in decibels (db) Creates pulses of electrical charge Power measured in watts (W) Amplitude measured in volts (V) + db - 1 0 1 1 Optical-to-electrical (O- E) conversion O-E + V - 1 0 1 1 + - Optical fiber Electrical conductor 34
100 Gigabit DWDM Transmission Problem: Solution: Transmission impairments increase significantly at higher bit rates (CD, PMD, non-linear effects) Compensate for these impairments with intelligent Digital Signal Processing, enabled by Coherent Detection This is a Demodulation (RX) function Problem: DSP electronics (required for above) not yet capable of processing 100Gb/s serial data rates Solution: Dual Polarization DQPSK Modulation, allows single wavelength 100G transmission with a baud rate of ~28Gb/s This is a Modulation (TX) function 35
100G Technology Coherent Detection Direct Detection Must correct for impairments in the physical domain (insert DCU s) Forced to live with non-correctable impairments via network design (limit distance, regenerate, adjust channel spacing) Dumb detection (OOK), no Digital Signal Processing, only FEC DCU DCU DCU DD DD Coherent Detection Regen Moves impairment correction from the optical domain into the digital domain Allows for digital correction of impairments (powerful DSP) vs. physical correction of impairments (DCU s). Adds advanced FEC. Massive performance improvements over Direct Detection. CD 36
DWDM Mux and Demux Filters Block Diagram N light pulses of different wavelengths 1 2 1, 2,.N Composite signal 1 2 3 3 DWDM fiber N N From N transmitters Multiplexer Demultiplexer To N receivers 37
OADM Block Diagram Original composite signal Pass through path New composite signal DWDM fiber OADM one signal Drop path Signsl 1 drop Signal 2 add Add path New data stream, same wavelength 38
ROADM Architecture Software Controlled Selectors 32 Ch. (Pass-through/Add/Block) West Transponder Module Pass Add Pass Add Add Wavelengths Pass-Through Wavelengths Software Controlled 32 Ch. DeMux Splitter block drop block drop Drop Wavelengths DWDM Signal Network Element λ 1 λ 3 Network Element Network Element λ 3 λ 1 Network Element DWDM Signal Drop Wavelengths drop block drop block Splitter Software Controlled 32 Ch. DeMux Pass-Through Wavelengths Add Wavelengths Pass Add Add Pass Software Controlled Selectors 32 Ch. (Pass-through/Add/Block) Transponder Module East 39
Optical Amplifer Block Diagram Unidirectional operation Extends the reach of a DWDM span Attenuated input composite signal Amplified output composite signal DWDM fiber Power in OA Power out 40
Transponder Block Diagram Non-ITU-T compliant wavelength O-E-O wavelength conversion ITU-T compliant wavelength Tx 850, 1310, 1550 nm 15xx.xx nm Transponder Optical fiber Rx G.709 Enabled 41
Muxponder Block Diagram Multiple Non-ITU-T Compliant Clients Multiplexing and O-E-O wavelength conversion ITU-T compliant wavelength 850, 1310, 1550 nm Muxponder Tx Rx 15xx.xx nm G.709 Enabled Optical fibers 42
Pluggable Optics 10G XENPAK, X2, XFP and SFP+ 40G/100G CFP and CXP Below 10G GBIC and SFP 43
DWDM System Transponder interface Client Tx Rx Tx Rx OEO Direct interface Mux and demux OA OADM OA Rx Tx To client devices Mux and demux OEO Rx Tx Rx Tx Client 44
Optical Protection Scheme Options Platinum-Available network: Combination of multiple protection scheme Gold-Available network: Y-cable protection Silver-Available network: Optical Trunk Protection Bronze-Available network: Multiple Section Protection Available network: Transport Section Protection Platinum-Available Gold-Available Silver-Available Bronze-Available 45
DWDM Software
Intelligent DWDM Modern systems compensate real-time for variations in the network Gain Equalization Amplifier Control Automatic Node Setup Automatic Power Control Allows for less truck rolls and maintenance windows 47
Why Per Channel Equalization AMP Optical Power Equalized Channels Express Path Add/Drop Path AMP OADM Without Power Equalization Channels with Unequal Optical Power Why Per-Channel Optical Power Equalization For amplifiers to operate correctly, all channels must be equalized in power. If channel powers are not equal, more gain will go to the higher powered channels. Channel power is inherently unequal due to different insertion losses, different paths (add path vs. express/pass-through), etc. Controlling the optical power of each channel in an optical network is required. 48
Example AMP Express Path Add/Drop Path AMP OADM Without Power Equalization AMP Express Path Add/Drop Path AMP OADM With Power Equalization 49
ANS Example L1 Express Path VOA Constant Attenuation L3 T3 T4 AMP Express Path AMP Add/Drop Path T1 L4 T2 Add/Drop VOA Constant Power L2 ANS Target Powers T1 Per Channel Power +2dBm Loss L1 (Express Drop) db 2.5dB VOA Express Path VOA db 6dB T2 T3-16dBm -9dBm L2 (Per Ch Add) L3 (Express Add) 5.0dB 2.5dB Add VOA Drop VOA N/A (depends upon laser TX power 12.5dB (Start point) T4 +2dBm L4 (Per Ch Drop) 5.5dB T L VOA Target Power Loss Target Power comes from design tool or Measured Span Loss Values from System Loss values are measured and stored in the OADM(s) / ROADM(s) Constant Attenuation VOA s set via ANS software logic Constant Power VOA s set to close loop 50
Constant Power Mode Add Channels Example Span Loss Increase Example Total Output Power +2dBm Total Output Power +2dBm Per Channel Power -15dBm AMP Per Channel Power -1dBm Per Channel Power -15dBm AMP Per Channel Power -1dBm Initial condition 2 channels Initial condition Gain 14dB Per Channel Power -15dBm AMP Total Output Power +2dBm Per Channel Power -4dBm Per Channel Power -17dBm AMP Total Output Power +2dBm Per Channel Power -1dBm Adding 2 channels Amp set to Constant Power Mode Initial condition Gain 16dB 51
Constant Gain Mode Add Channels Example Span Loss Increase Example Total Output Power +2dBm Total Output Power +2dBm Per Channel Power -15dBm AMP Per Channel Power -1dBm Per Channel Power -15dBm AMP Per Channel Power -1dBm Initial condition Gain 14dB Initial condition Gain 14dB Per Channel Power -15dBm AMP Total Output Power +5dBm Per Channel Power -1dBm Per Channel Power -18dBm AMP Total Output Power -1dBm Per Channel Power -4dBm Gain Stays Constant Gain 14dB Gain stays the Same Gain 14dB 52
Automatic Power Control APC No Human Intervention Required Automatically corrects amplifier power/gain for capacity change, ageing effects, operating conditions Keep traffic working after network failires Prevent BER due to network degrade Keep constant either power or gain on each amplifier No truck rolls No troubleshooting required No operation complexity 53
Automatic Laser Shutdown (ALS) w/ Booster Amplifier Automatic Lasers Shutdown OSCM Automatic Laser Shutdown Amplifier Automatic Lasers Shutdown Node A West side Loss Of Signal (LOS) is declared 1 Payload (LOS-P) & OSC (LOS-O) detected OPT-PRE 1 P P 1 OSCM LOS-O is detected Fiber cut LOS-O is detected OSCM 1 P P 1 1 OPT-PRE Payload (LOS-P) & OSC (LOS-O) detected Loss Of Signal (LOS) is declared Amplifier Automatic Lasers Shutdown OPT-BST ALS is required to decrease the risk of laser damage to the human eye OSCM Automatic Laser Shutdown OPT-BST Node B East side Amplifier Automatic Lasers Shutdown The complete sequence of events is completed within 1s as required by IEC 825-2 This is not possible on passive dwdm systems 54
Dynamic Optical Restoration Touchless Optical Layer + Embedded WSON Intelligence animated slide Client Client IPoDWDM IPoDWDM ONS 15454 MSTP Fiber Cut! Embedded WSON intelligence locates and verifies a new path Edge Nodes instruct client to re-tune its wavelength Colorless, Omni-Directional ROADM switches the path Service is brought back up with the same Client and Optical interfaces, zero touches 55
Session Summary Dramatic increase in Bandwidth has led to the use of DWDM Fiber type effects the quality of transmission Linear Effects are predictable and can be compensated Non-Linear Effects are known but somewhat unpredictable Modern DWDM systems are intelligent and simple to operate Good reference is: http://www.cisco.com/en/us/products/hw/optical/ps2011/products_tech nical_reference_chapter09186a00802342dd.html 56
Glossary Arrayed Waveguide (AWG) Automatic Node Setup (ANS) Automatic Power Control (APC) Chromatic Dispersion (CD) Cross Phase Modulation (XPM) Decibels (db) Decibels-milliwatt (dbm) Dense Wavelength Division Multiplexing (DWDM) Dispersion Compensation Unit (DCU) Dispersion Shifted Fiber (DSF) Erbium Doped Fiber Amplifier (EDFA) Four-Wave Mixing (FWM) 57
Glossary International Telecommunications Union (ITU) Non-Zero Dispersion Shifted Fiber (NZ-DSF) Optical Add Drop Multiplexer (OADM) Optical Signal to Noise Ratio (OSNR) Optical Supervisory Channel (OSC) Optical Supervisory Channel Module (OSCM) Polarization Mode Dispersion (PMD) Reconfigurable Optical Add Drop Multiplexer (ROADM) Self Phase Modulation (SPM) Single Mode Fiber (SMF) Variable Optical Attenuator (VOA) 58
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