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Börje Josefsson SUNET [email protected] Börje Josefsson SUNET [email protected] Lumos Börje Josefsson <[email protected]> 2017-05-24 Page # 4 Lumos !?! lüm\O\s It’s a wizard-spell that generates LIGHT Wish optical networking companies could do that ;-) (After this presentation you’ll see what I mean…) Börje Josefsson <[email protected]> 2017-05-24 Page # 5 Terminology Transmission Technology The simplest transmission system is a fiber jumper from one piece of client equipment to another Data goes in, data comes out No complex operations, no settings Just a piece of glass 10GE “Transmission System” Fiber pair < 80 km (Transmit and Receive) Börje Josefsson <[email protected]> 2017-05-24 Page # 7 Technical Design Elements: Terminology Decibels (dB) – used for power gain or loss Decibels-milliwatt (dBm) – used for output power and receive sensitivity Attenuation – loss of power in dB/km Chromatic dispersion – spreading of the light pulse in ps/nm*km Bit Error Rate (BER) – typical acceptable rate is 10-12 Optical Signal to Noise Ratio (OSNR) – ratio of optical signal power to noise power for the receiver ITU Grid Wavelength – standard for the lasers in DWDM systems Börje Josefsson <[email protected]> 2017-05-24 Page # 8 dB versus dBm dBm used for output power and receive sensitivity (Absolute Value) dB used for power gain or loss (Relative Value) Börje Josefsson <[email protected]> 2017-05-24 Page # 9 Optical Attenuation Pulse amplitude reduction limits “how far” Attenuation in dB (or dB/km) Power is measured in dBm: Pi Examples 10 dBm 10 mW 0 dBM 1 mW -3 dBm 500 μW -10 dBm 100 μW -30 dBm 1 μW P0 T T Börje Josefsson <[email protected]> 2017-05-24 Page # 10 Caution… Börje Josefsson <[email protected]> 2017-05-24 Page # 11 Optical Budget Basic Optical Budget = Output Power – Input Sensitivity Pout = +6 dBm R = -30 dBm Budget = 36 dB Optical Budget is affected by: Börje Josefsson <[email protected]> Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors) 2017-05-24 Page # 12 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 Börje Josefsson <[email protected]> 2017-05-24 Page # 13 Fiber fundamentals Fiber Fundamentals Attenuation Dispersion Nonlinearity Distortion It may be a digital signal, but It’s analog transmission Transmitted data waveform Börje Josefsson <[email protected]> Waveform after xxx km 2017-05-24 Page # 15 Fiber Geometry Core Cladding 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 Börje Josefsson <[email protected]> 2017-05-24 Page # 16 Propagation in Fiber n2 q0 n1 Cladding q1 Core Intensity Profile Light propagates by total internal reflections at the corecladding interface Total internal reflections are lossless Each allowed ray is a mode Börje Josefsson <[email protected]> 2017-05-24 Page # 17 Different Types of Fiber Multimode fiber Core diameter varies 50 μm for step index 62.5 μm for graded index n2 Cladding Bit rate-distance product >500 MHz*km n1 Core Single-mode fiber Core diameter is about 9 μm Bit rate-distance product >100 THz*km n2 Cladding n1 Börje Josefsson <[email protected]> 2017-05-24 Core Page # 18 Optical Spectrum UV S-Band: 1460–1530nm C-Band: 1530–1565nm L-Band: 1565–1625nm IR 125 GHz/nm l Visible Light Ultraviolet (UV) Visible Infrared (IR) 850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm Communication wavelengths 850, 1310, 1550 nm Low-loss wavelengths Specialty wavelengths 980, 1480, 1625 nm Börje Josefsson <[email protected]> C = x l l (nanometers) Frequency: (terahertz) Wavelength: 2017-05-24 Page # 19 Fiber attenuation characteristics Attenuation vs. Wavelength S-Band: 1460–1530nm L-Band: 1565–1625nm 2.0 dB/Km Fibre Attenuation Curve 0.5 dB/Km 0.2 dB/Km 800 900 1000 1100 1200 1300 1400 Wavelength in Nanometers (nm) Börje Josefsson <[email protected]> 2017-05-24 1500 1600 C-Band: 1530–1565nm Page # 20 Glass Purity Fiber optics requires very high purity glass Window glass ~ 3 cm Optical quality glass ~3m Fiber optics ~ 14 km Propagation distance needed to reduce the transmitted light power by 50% (3 dB) Börje Josefsson <[email protected]> 2017-05-24 Page # 21 Fraction of Power Remaining Fiber loss 1 0.8 0.6 80 km 100x loss 0.4 0.2 0 0 20 40 60 80 100 Distance (km) Börje Josefsson <[email protected]> 2017-05-24 Page # 22 Optical amplifiers compensate loss Input After Loss After Amplifier Added Noise Power Wavelength Need to maintain adequate signal-to-noise ratio, so more distance means more signal power Börje Josefsson <[email protected]> 2017-05-24 Page # 23 DWDM To get more information on a single fiber Use more wavelengths (DWDM) Power (dBm) 0 -10 -20 -30 1570 1580 1590 1600 W a v el e n gt h ( n m ) Börje Josefsson <[email protected]> 2017-05-24 Page # 25 Increasing Network Capacity Options Same bit rate, more fibers Slow Time to Market Expensive Engineering Limited Rights of Way Duct Exhaust More Fibers Same fiber & bit rate, more ls Fiber Compatibility Fiber Capacity Release Fast Time to Market Lower Cost of Ownership Utilizes existing TDM Equipment W D M Faster Electronics (TDM) Börje Josefsson <[email protected]> Higher bit rate, same fiber Electronics more expensive 2017-05-24 Page # 26 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 Upcoming DWDM systems 64 to 160 channels in 1550 nm window 50 and 25 GHz spacing Börje Josefsson <[email protected]> 2017-05-24 Page # 27 ITU Wavelength Grid 1530.33 nm 195.9 THz 0.80 nm 100 GHz ITU-T l grid is based on 191.7 THz + 100 GHz It is a standard for laser in DWDM systems Börje Josefsson <[email protected]> 2017-05-24 1553.86 nm l 193.0 THz Freq (THz) 192.90 192.85 192.80 192.75 192.70 192.65 192.60 ITU Ch 29 28 27 26 Page # 28 Wave (nm) 1520 1554.13 1554.54 1554.94 1555.34 1555.75 1556.15 1556.55 DWDM Components l1 850/1310 15xx l1...n l2 l3 Transponder Optical Multiplexer l1 l2 l1 l1...n l2 l3 l3 Optical De-multiplexer Optical Add/Drop Multiplexer (OADM) Börje Josefsson <[email protected]> 2017-05-24 Page # 29 More DWDM Components Optical Amplifier (EDFA) Optical Attenuator Variable Optical Attenuator Dispersion Compensator (DCM / DCU) Börje Josefsson <[email protected]> 2017-05-24 Page # 30 Typical DWDM Network Architecture DWDM SYSTEM DWDM SYSTEM VOA DCM EDFA EDFA DCM VOA Service Mux (Muxponder) Börje Josefsson <[email protected]> Service Mux (Muxponder) 2017-05-24 Page # 31 Sub-wavelength Multiplexing or MuxPonding Ability to put multiple services onto a single wavelength Börje Josefsson <[email protected]> 2017-05-24 Page # 32 Optical Signal-to Noise Ratio (OSNR) Signal Level X dB Noise Level • Depends on : Optical Amplifier Noise Figure: (OSNR)in = (OSNR)outNF EDFA Schematic (OSNR)out (OSNR)in Pin • Target : Large Value for X NF • Don’t confuse this with attenuation etc! Börje Josefsson <[email protected]> 2017-05-24 Page # 33 Optical Signal to Noise Ratio OSNR = 58 – 10 * log(N) – 10 * log(M) – NF -10log(L) + Pout - The optical signal to noise ratio at the end of the link depends on the number of channels, the number of spans, the noise figure, the OA output power and some other factors, like gain tilt, flatness, etc. The OSNR needed is determined by the receiver BER versus OSNR performance characteristics. This is a measured parameter. M Channels, N Amplifiers, L Loss Per Span, NF: Noise Figure Pout :OA Output Power, : Other Factors Börje Josefsson <[email protected]> 2017-05-24 Page # 34 Loss Management: Limitations 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 Noise Figure > 3 dB Typically between 4 and 6 section span Rule of thumb: Distance of spans are dependant on the number of spans in a section Börje Josefsson <[email protected]> 2017-05-24 Page # 35 Designing for Distance L = Fiber Loss in a Span Pin Pout S Amplifier Spacing D = Link Distance Link distance (D) is limited by the minimum acceptable electrical SNR at the receiver Pnoise G = Gain of Amplifier 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 Börje Josefsson <[email protected]> 2017-05-24 Page # 36 Wavelength Capacity (Gb/s) Link Distance vs. OA Spacing Amp Spacing 20 60 km 10 80 km 100 km 5 120 km 140 km 2.5 0 2000 4000 6000 8000 Total System Length (km) System cost and and link distance both depend strongly on OA spacing Börje Josefsson <[email protected]> 2017-05-24 Page # 37 Error Correction Information Transmitter Transmission Channel Information Receiver “Detect is good, correct is better”. Error correcting codes both detect errors and correct them Forward Error Correction (FEC) is a system that: “Noise” adds additional information to the data stream corrects eventual errors that are caused by the transmission system. Low BER achievable on noisy medium Börje Josefsson <[email protected]> 2017-05-24 Page # 38 FEC performance, theoretical FEC gain 6.3 dB @ 10-15 BER Bit Error Rate 1 BER without FEC 10 -10 Coding Gain BER floor 10 -20 BER with FEC 10 -30 -46 Börje Josefsson <[email protected]> -44 -42 -40 -38 -36 2017-05-24 -34 -32 Received Optical power (dBm) Page # 39 FEC in DWDM systems 9.58 G 10.66 G 9.58 G 10.66 G IP FEC FEC IP SDH FEC FEC SDH . . . . FEC FEC ATM 2.48 G 2.66 G 2.66 G 2.48 G FEC implemented on transponders (TX, RX, 3R) No change on the rest of the system Börje Josefsson <[email protected]> 2017-05-24 ATM Page # 40 Uni- versus bi-directional DWDM DWDM systems can be implemented in two different ways • Uni-directional: l1 l3 l5 l7 wavelengths for one direction travel within one fiber l2 l4 l6 l8 Fiber l1 l3 l5 l7 l2 l4 l6 l8 Fiber two fibers needed for Uni -directional full-duplex system • Bi-directional: a group of wavelengths for each direction single fiber operation for fullduplex system Börje Josefsson <[email protected]> 2017-05-24 Fiber l5 l6 l7 l8 l1 l2 l3 l4 Bi -directional Page # 41 Fiber Anomalies Types of dispersion • Chromatic Dispersion Different 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 Börje Josefsson <[email protected]> 2017-05-24 Page # 43 Data distortion from dispersion 10101 0110 010 Propagation Length 160 km 80 km 1’s 0 km 0’s NRZ distortion very pattern dependent Börje Josefsson <[email protected]> 2017-05-24 Page # 44 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 fiber types Börje Josefsson <[email protected]> 2017-05-24 Page # 45 Solution for chromatic dispersion Dispersion Saw tooth compensation DCU Dispersion +D DCU -D Length Total dispersion averages to ~ zero The precision required in the DCU devices scales as the square of the bit rate Börje Josefsson <[email protected]> 2017-05-24 Page # 46 Dispersion limits (without compensation) DL = 16,000 ps/nm at 2.5 Gb/s NRZ 16,000 / 17ps/nm*km ~ 1000 km in SMF 16 DL = 1,000 ps/nm at 10 Gb/s NRZ 1000 / 17 ~ 60 km in SMF 16 DL = 62.5 ps/nm at 40 Gb/s NRZ 62.5 / 17 ~ 3.5 km in SMF (Gb/s)2 Dispersion Distance < 100,000 Börje Josefsson <[email protected]> 2017-05-24 Page # 47 Not all fiber is created equal Fiber Cross Section Polarization mode dispersion is caused by slight fiber asymmetry Elliptical Fiber Core PMD also changes randomly with environmental effects (e.g. temperature), requiring adaptive compensation. Discovered in the 90s (Fibers installed before 1993 may have significant PMD) Most older fiber not characterized for PMD Börje Josefsson <[email protected]> 2017-05-24 Page # 48 Polarization Mode Dispersion (PMD) Fast Side View Different polarizations travel at different speeds Slow PMD = Time Delay 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. The order of magnitude is ~1 ps/√km • 0.5 ps/√km of PMD at 10 Gb/s over 100 km leads to a pulse broadening of 5 ps (5% of “eye opening”.) Börje Josefsson <[email protected]> 2017-05-24 Page # 49 Polarization Mode Dispersion (PMD) Linear i z im ( z ) t rhs 0 Ψ ( z ) Wˆ ( z ) Ψ (0) z Wˆ ( z ) exp i dz ' mˆ ( z ' ) 0 pulse splitting broadening jitter Jˆ ( z; ) Wˆ ( z )Wˆ ( z ) iˆ 1 Poole, Wagner ‘86 Poole ’90;’91 Statistics of PMD vector is Gaussian. Börje Josefsson <[email protected]> Polarization (PMD) vector (of first order) 2 4 3 P( | |) 3 exp 2 2 Dm z 2017-05-24 Differential group delay (DGD) Page # 50 Four Wave Mixing (FWM) Beating between two signals generates a signal at the difference frequency: 1 2 Into the fiber… 1 Out of the fiber… 212 2 221 n(t) = no + n2[E12+E22+2E1*E2cos(w1-w2)t] Börje Josefsson <[email protected]> 2017-05-24 Page # 51 The 3 “R”s of optical networking The options to recover the signal from degradation are: Pulse as it enters the fiber Pulse as it exits the fiber Re-amplify Re-shape DCU Phase Re-Alignment Phase Variation O-E-O Re-time (Re-generate) t Optimum t s Sampling Time Börje Josefsson <[email protected]> t t ts Optimum Sampling Time 2017-05-24 ts Optimum Sampling Time Page # 52 Different fiber types etc. Conventional Single-Mode Fiber (SMF) 30 S C L Dispersion (ps/nm) 20 D(1530-1565nm) = 16 - 19 ps/nm*km 10 0 D = 0.065 ps/nm2km -10 Aeff = 85 μm2 -20 -30 1250 1350 1450 1550 1650 Wavelength (nm) First single-channel systems operated at 1310 nm (good laser materials) WDM systems moved to 1550 nm: wider loss-window, but higher dispersion Dispersion limit = 1000 km at 2.5Gb/s in SMF. Börje Josefsson <[email protected]> 2017-05-24 Page # 54 Dispersion-Shifted Fiber – Oops! 30 S C L Dispersion (ps/nm) 20 10 0 -10 -20 -30 1250 1350 1450 1550 1650 Wavelength (nm) DSF: Zero dispersion at 1550nm, so no compensation required. However, FWM severely limits optical power levels. Small effective core area, very nonlinear Börje Josefsson <[email protected]> 2017-05-24 Page # 55 Some types of Single-Mode Fiber SMF-28(e) (standard, 1310 nm optimized, G.652) DSF (Dispersion Shifted, G.653) Most widely deployed so far, introduced in 1986, cheapest 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 Fibers before 1993 may have significant PMD Börje Josefsson <[email protected]> 2017-05-24 Page # 56 Now, don’t you wish it was this easy… Questions? Börje Josefsson <[email protected]>