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Dense wavelength division multiplexing 1
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- 2. EAGLE PHOTONICS Multiplexing • Multiplexingis a process of putting all the signals into a common channel in different ways and the component is called multiplexer • De-multiplexing is a process which separates out all the multiplexer signals and the component is called de-multiplexer • This is required to increase system overall capacity
- 3. EAGLE PHOTONICS Multiplexing Techniques •Time-division multiplexing (TDM) • Wavelength-division multiplexing (WDM) • Subcarrier multiplexing (SCM) • Code-division multiplexing (CDM) • Polarization-division multiplexing (PDM) • Hybrid Types: WDM/TDM, WDM/SCM, etc
- 4. EAGLE PHOTONICS TDM -Time Division Multiplexing • Combines traffic from multiple inputs onto one common high capacity output • Requires electrical mux/demux function
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- 7. EAGLE PHOTONICS What isWave Division Multiplexing ? • Data from each TDM channel is loaded on one optical frequency (or wavelength, ) of a particular wavelength band • These wavelengths are then multiplexed onto one fiber with the help of WDM multiplexers • Other side of the network these wavelengths are demultiplexed by using either optical filters, gratings or WDM demultiplexers
- 8. EAGLE PHOTONICS Increased NetworkCapacity Independence Of Bit Rates And Formats DWDM = Dense WDM
- 9. EAGLE PHOTONICS DWDM • Canachieve high system capacity by multiplexing more WDM channels, each with relatively low data rate • Consist of a WDM combined with an optical amplifier, to allow multiple wavelengths on a single fiber and also avoid individual regeneration equipment for each wavelength by use of line amplifiers
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- 11. EAGLE PHOTONICS Why WDM? •Better utilization of fiber • Overcome ‘fiber exhaust’, lack of fiber availability • Low unit cost of bandwidth in high capacity systems • Easily integrated with existing equipment in the network • Bit-rate and protocol independent interface • Wavelength leasing instead of Bandwidth leasing
- 12. EAGLE PHOTONICS Limitations ofTDM Technology • Maximum TDM line rate one can get in market to date is 10 Gbps • 40 Gbps is undergoing field trails, but associated with lot of problem – In making systems – Testing and measurement equipments – Degradation of fiber transmission – Cost in having protection system – Overall cost of the network
- 13. EAGLE PHOTONICS Limitations ofTDM Technology • Line cards and other hardware required to be changed to increase capacity • Operators tend to build TDM networks with higher capacity than required, considering future capacity requirements • TDM data rates stagnating at 10 Gbps – Beyond 10 Gbps capacity increase is realized by building parallel SDH networks – Each SDH/SDH system required 2 fiber without protection and 4 fibers with protection – So fiber exhaust?
- 14. EAGLE PHOTONICS Limitations ofTDM Technology Optical Fiber Data Channel (Bit Rate=x) Data Channel (Bit Rate = x) TX TX RX RX Optical fiber Optical fiber New TX/RX required
- 15. EAGLE PHOTONICS Limitations ofTDM Technology Regenerators 40 Gbps (4 x 10 Gbps) Capacity LTE LTE LTE LTE LTE LTE LTE LTE
- 16. EAGLE PHOTONICS Limitations ofTDM Technology • Propagation delays due to O-E-O conversion – All SDH NEs do O-E-O conversion for processing of overhead information – O-E-O conversion slows down the signal • SDH not the ideal carrier for data traffic – Data traffic has its own overheads – SDH overheads are partly redundant while carrying data traffic
- 17. EAGLE PHOTONICS Purpose ofWDM LTE LTE LTE LTE LTE LTE LTE LTE Traditional Network with Repeaters, no WDM 75% fewer fibers WDM Network with Repeaters LTE LTE LTE LTE LTE LTE LTE LTE 75% less equipment WDM Network with Optical Amplifiers LTE LTE LTE LTE LTE LTE LTE LTE
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- 20. WDM Classifications • Classificationof WDM is based on the channel spacing between the two wavelengths • Channel spacing > 200 GHz called CWDM • Channel spacing < 100 GHz called DWDM • Channel spacing < 25 GHz called UDWDM 0.8 nm = 100 GHz 0.8 nm = 100 GHz
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- 24. EAGLE PHOTONICS DWDM Bands Wavelengthband available for communication • C band (1530nm - 1565nm, 35 nm) • L band (1565nm - 1610nm, 45 nm)
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- 26. EAGLE PHOTONICS Wavelength (nm) Wavelength(nm) OSC 1510 OSC 1625 OSC 1510 OSC 1625 1547.72 nm – 1559.79 nm –( band) 1528.77 nm – 1540.56 nm –( band) channel channel 1 2 3 4 5 6 7 8 9 11 10 13 12 14 15 16 1 2 3 4 5 6 7 8 9 11 10 13 12 14 15 16 ITU Wavelength Grid ITU Wavelength Grid
- 27. EAGLE PHOTONICS Wavelength spacing 1528.77 nm 196.1THz 1562.23 nm 191.9THz 1528.77 nm 196.1 THz 1562.23 nm 191.9THz 40-Channels, 100 GHz Spacing 80-Channels, 50 GHz Spacing
- 28. EAGLE PHOTONICS DWDM Types •Unidirectional & Bi-Directional • Transponder based systems • Passive & Active
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- 33. EAGLE PHOTONICS Unidirectional versusBidirectional • Unidirectional – More popular – Ideal for high capacity growth • Bidirectional system – Ideal when there are fiber constraints – Unsuitable for large capacity
- 34. EAGLE PHOTONICS Transponder BasedDWDM • Transponder is a device that performs an optical-electrical-optical conversion to a specific wavelength • Allows the input of any wavelength to DWDM • Allows better performance due to control of input power, dispersion matching of transmitters, allows use of non-ITU grid • Better for wavelength leasing, as customer can send any wavelength and any wavelength pipe in the network can be used, requires a bit-rate flexible transponder
- 35. EAGLE PHOTONICS Transponders 1310nm 1550nm 1530nm 1540nm Transponders (Wavelength Translators) Transpondersin Terminal Transponders in OADM OADM 1310nm 1550nm 1600nm 1560nm Any wavelength from 1300- 1600nm Any one wavelength from ITU-grid
- 36. EAGLE PHOTONICS Types ofTransponders • Protocol specific transponders – SONET, GigE Transponders available • Transponders with open interfaces – Protocol independent, hence flexibility of application – Incoming signals transparently transported over DWDM – Does not take care of OAM & P Functionality provided in protocols like SONET
- 37. EAGLE PHOTONICS Types ofTransponders • FEC Transponders –Suitable for error prone links, systems • High Dispersion Tolerant Transponders –Uses narrow pulse width laser / modulation –Used to increase tolerance to dispersion
- 38. EAGLE PHOTONICS DWDM -Non-Transponder Based DWDM - Non-Transponder Based • Non-transponder system have the light wave system transmitter directly input to the DWDM • Cheaper than transponder based systems, do not have to buy the transmitter twice (once in LTE and once in DWDM) • Requires LTE to be equipped with laser TX of the exact wavelength • More flexible for wavelength leasing, as long as customer supplies proper wavelength can use any bit-rate any protocol
- 39. EAGLE PHOTONICS Active vs.Passive WDM • Active means optical amplifiers • Allows long spans without regeneration equipment • One line amp can do the work of many regenerators • Passive means only the WDM equipment, no amplification • Useful for short distances where amplification is not needed • Avoids expensive OP-Amps • Adds attenuation loss to the span, shortens maximum span for non-amplified equipment
- 40. EAGLE PHOTONICS Fiber usedfor DWDM Applications • DSF (Dispersion shifted fiber) • + NZ-DSF (Positive dispersion shifted fiber ) • - NZ-DSF (Negative dispersion shifted fiber) • LEAF (Larger effective area fiber) • G.653 – Characteristics of a dispersion-shifted single-mode optical fiber cable • G.654 – Characteristics of a 1550 nm wavelength loss-minimized single-mode optical fiber cable • G.655 – Characteristics of a non-zero dispersion single-mode optical fiber cable
- 41. EAGLE PHOTONICS Why DWDM- Incremental Capacity Growth
- 42. EAGLE PHOTONICS WDM NetworksEvolution • First Generation First Generation: : Dense WDM networks with linear architecture used in point-to- point link, as a high bandwidth pipes between two network elements. These systems are integrated with optical amplifiers and electronic regenerators
- 43. EAGLE PHOTONICS WDM NetworksEvolution • Second Generation Second Generation: : WDM networks with ring and mesh architectures. These systems are integrated with optical amplifiers, OADM’s, Dispersion compensators, OXC’s and electronic regenerators
- 44. EAGLE PHOTONICS WDM NetworksEvolution • Third Generation Third Generation: : DWDM and OTDM networks (All-optical networks )with linear, ring and mesh architectures. These systems are integrated with optical 3R regeneration, OADM, OXC (which supports Photonic packet switching)
- 45. EAGLE PHOTONICS WDM NetworksEvolution
- 46. EAGLE PHOTONICS 1×40G up to 65 km (Alcatel’98) PMD 1×40 G up to 65 km (Alcatel’98) PMD Limited. Limited. 32× 5 G to 9300 km (1998) 32× 5 G to 9300 km (1998) 64× 5 G to 7200 km (Lucent’97) 64× 5 G to 7200 km (Lucent’97) 100×10 G to 400 km (Lucent’97) 100×10 G to 400 km (Lucent’97) 16×10 G to 6000 km (1998) 16×10 G to 6000 km (1998) 132×20 G to 120 km (NEC’96) 132×20 G to 120 km (NEC’96) 70×20 G to 600 km (NTT’97) 70×20 G to 600 km (NTT’97) 1022 Wavelengths on one fiber (Lucent 99) 1022 Wavelengths on one fiber (Lucent 99) Recent WDM Records Recent WDM Records Source From Internet
- 47. EAGLE PHOTONICS Max DWDMThroughput Achieved • NEC :10.9 Tbps over a single fiber; 273 channels, each at 40 Gbps over 117 km, Used S-band, C- and L-bands for amplification; ultra-dense channel multiplexing scheme (March 2002) • Alcatel -- 256 wavelengths at 40 Gbps for 10.2 Tbps, March 2001 • Siemens/Optisphere -- 176 wavelengths at 40 Gbps for 7.04 Tbps, October 2000
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- 49. EAGLE PHOTONICS Post AmplifierPre Amplifier Line Amplifier Wavelengths Wavelengths Optical Multiplexer Optical Demultiplexer Linear Backbone Link Linear Backbone Link
- 50. EAGLE PHOTONICS Backbone LinkWith OADM Backbone Link With OADM OADM Mux Mux Demux Demux Add Wavelengths Drop Wavelengths
- 51. EAGLE PHOTONICS Optical Add/DropMultiplexer Optical Add/Drop Multiplexer Source From Internet
- 52. EAGLE PHOTONICS Backbone LinkWith OXC Backbone Link With OXC OXC Mux Mux Demux Demux Add/Drop Ports Add/Drop Ports
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- 54. EAGLE PHOTONICS WDM Systems •3R Compensators • Optical Amplifiers • Optical Add/Drop Multiplexers • Optical Cross Connects
- 55. EAGLE PHOTONICS 3R -Regeneration 3R regeneration means: First R :Re-amplification Second 2R: R + Re-shaping Third 3R :2R + Re-timing
- 56. EAGLE PHOTONICS 3R Regenerators TheseRegeneration done by R- Done by Optical amplifiers 2R- Done by dispersion compensation or OEO 3R- By using PLL and optical clock recovery
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- 59. EAGLE PHOTONICS Purpose ofDCM • Dispersion is the function of the length of the optical fiber and thus with respect of the length it increases • This accumulated dispersion lead to ISI and the loss of the data in the transmission • To overcome this accumulated dispersion and increase the length of the transmission we need a module called Dispersion Compensating Module (DCM) • DCM generally consist of optical elements having high negative dispersion coefficient
- 60. EAGLE PHOTONICS Dispersion CompensationModule DCM with ILA DCM with Terminal
- 61. EAGLE PHOTONICS Dispersion Compensation Methods •The problem of dispersion-compensation can be solved by one of way such as; • Dispersion Compensating fiber (DCF) • Chirped Fiber Bragg Grating • Mid-span spectral inversion • Multilevel coding • First two approaches are more practical and implemented in the field while last two has only academic interests
- 62. EAGLE PHOTONICS Where DCMis Deployed? DCM are deployed at various places in the network –After the transmitters –With in Line amplifiers –Before post amplifier –After pre amplifier
- 63. EAGLE PHOTONICS Optical Amplifier Typesof optical amplifiers Principle of operation of amplification Amplifier vs. regenerators
- 64. EAGLE PHOTONICS Introduction • Inany link, optical power pumped and the receiver sensitivity is limited and can only support for a limited distance • To over come the losses in the network, either electrical or optical amplification is required • Optical amplification is more cost effective over electrical one • An optical amplifier is a device which amplifies the optical signal directly without ever changing it to electricity
- 65. EAGLE PHOTONICS Types ofOptical Amplifiers Two Types of optical amplifiers available • Solid state Optical Amplifiers Semiconductor Optical Amplifiers • Fiber Amplifiers Erbium Doped Fiber Amplifiers ( EDFAs ) Raman Amplification ( RA )
- 66. EAGLE PHOTONICS Amplifiers inTransmission Three type of fiber amplifier used in transmission • Pre Amplifier • In-line Amplifier • Post Amplifier
- 67. EAGLE PHOTONICS Rx Tx Signal Power Link Length ReceiverSensitivity Post Amplifier Line Amplifier Pre Amplifier Typical Point To Point Optical Link Typical Point To Point Optical Link
- 68. EAGLE PHOTONICS In LineAmplifier ILA
- 69. EAGLE PHOTONICS Pre andPost Amplifiers • Post Amp is used to amplify the output of a Multiplexer to a sufficient level to take care of the link losses • Preamp is used for amplifying the incoming signal to a sufficient level for the detectors to sense the signal
- 70. EAGLE PHOTONICS Unidirectional versusBi-directional Terminal ILA Bidirectional Coupler Terminal ILA Bidirectional System Unidirectional System
- 71. EAGLE PHOTONICS Erbium DopedFiber Amplifiers • An Erbium Doped Fibre Amplifier consists of a short length of optical fibre doped by small controlled amount of the rare earth element erbium • This rare earth element contributes in the amplification process in presence of pump signal • Pump laser excites erbium ions which give extra energy to signal • Principle of operation is similar to principle of a laser
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- 73. EAGLE PHOTONICS Configuration ofEDFA The typical configuration of the EDFA consists of: – Optical pump source – WDM coupler – Er+ doped fiber – Isolators
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- 75. EAGLE PHOTONICS Erbium DopedFiber Amplifier • Pumping with 980nm laser is more effective than 1480nm pumping • Commonly used in submarine systems, and increasingly on land • Amplification possible at many wavelengths around 1550nm • Gain profile is not flat from the EDFA and need some flatting mechanism
- 76. EAGLE PHOTONICS Principle ofEDFA Amplification
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- 78. EAGLE PHOTONICS Principle ofOperation • An optical amplification is done with the help of an optical pump laser of selective wavelength • Erbium ions are excited by the pump signal and reached to the higher energy states • Erbium ion at high-energy state will stimulated by the signal needs amplification leads these ion return to a lower-energy called ground energy state • During this transition these ion emits a radiation of similar to the signal
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- 80. EAGLE PHOTONICS Spectrum ofa 32 Ch. DWDM System 1528.77 nm 1562.23 nm 196.1 THz 191.9THz
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- 82. EAGLE PHOTONICS C &L band of EDFA C & L band of EDFA
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- 84. EAGLE PHOTONICS Raman FiberAmplifier • Basic principle of Raman fiber amplifier is Stimulated Raman Scattering (SRS) • When stronger optical pump interacts with the medium generates new signal (a Stokes wave) in same direction • New generated frequency is lesser then the pump frequency by13.2 THz • In normal fiber this effect is very small and it takes a relatively long length to have significant amplification
- 85. EAGLE PHOTONICS Raman FiberAmplifier • From this phenomenon signal of lower frequency then pump gets amplified and the optimal amplification occurs when the difference in wavelengths is around 13.2 THz • Any signal lower then pump can be amplified but the efficiency will not be the same for all • Efficiency can be improved by adding an FBG (Fibre Bragg Grating) reflector for the pump wavelength • Thus any frequency can be generated from this phenomenon
- 86. EAGLE PHOTONICS Amplification inDifferent Bands Amplification in Different Bands
- 87. EAGLE PHOTONICS Amplifiers atDifferent Bands TDFA: Thulium doped fiber amplifier EDTFA:Erbium doped tellurite based fiber amplifier FRA: Fiber Raman amplifier GS-EDFA:Gain shifted Erbium doped fiber amplifier
- 88. EAGLE PHOTONICS Amplifier Vs.Repeaters • Optical amplifier, amplifies an optical signal without changing it to electrical signal • Repeaters, Amplifies the optical signal after converting back to electronics and generates a new optical signal of the same format • Reshaping & timing of data stream
- 89. EAGLE PHOTONICS Amplification Vs.Regeneration
- 90. EAGLE PHOTONICS Amplifier Vs.Repeaters Cont. • Optical amplifiers are required typical after 30 to 100 km depends on the losses in the link • Optical amplifier are very cost effective fo DWDM systems • Regenerations are typically necessary after about 600 km (at 2.5 Gbps) • Regenerations operation become very cumbersome for DWDM systems
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- 92. EAGLE PHOTONICS Optical Add/DropMux • System made of optical Mux & Demux components • It selects the dropping wavelengths from the incoming DWDM signals • Adds the same wavelengths to the outgoing DWDM signals • It is a passive system and everything add/drop wavelengths are fixed at the designing of this system
- 93. EAGLE PHOTONICS Optical AddDrop Multiplexer OADM OADM along with ILA without MSA OADM OADM along with ILA having MSA
- 94. EAGLE PHOTONICS Optical Add/DropMux • Allows a few wavelengths to drop out of fiber path, not all will need LTE equipment • Useful at sites where a small number of signals need to drop, not all wavelengths
- 95. EAGLE PHOTONICS OADM EXAMPLE ATMIP Terminal OADM Terminal Site Site Site ATM IP
- 96. EAGLE PHOTONICS Optical CrossConnects
- 97. EAGLE PHOTONICS Optical CrossConnect • It is consist of Optical Mux/Demux, Optical switch • Required this device where multiple rings are interconnection to each other • Switching can be done in fiber, wavelength and packet level • Packet level switching is performed in electronics domain • Costly devices, but gives flexible networks can be made intelligent networks
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- 99. EAGLE PHOTONICS Hybrid Switching HybridSwitching Source From Internet
- 100. EAGLE PHOTONICS DWDM NetworkConfiguration
- 101. EAGLE PHOTONICS Typical DWDMNetworks Termin al Termin al OADM OADM Termin al OADM OADM OADM
- 102. EAGLE PHOTONICS Typical DWDMNetworks Term inal Termi nal Termi nal Term inal Regenera tor Site A Site B OA DM OA DM OA DM OA DM
- 103. EAGLE PHOTONICS Cross ConnectingDWDM Networks Termina l Termina l OAD OAD Termina l Termina l OXC OADM OADM OADM OXC: Optical Cross Connect
- 104. EAGLE PHOTONICS Overlay ofSONET over DWDM Termin al OADM OADM ILA OADM OADM SONET ADM SONET ADM SONET ADM SONET ADM
- 105. EAGLE PHOTONICS SONET ADM SONET ADM SONET ADM SONET TM Terminal Physical Ring,Logical Star • Overlaying of Point-to-Multipoint SONET Network using one wavelength for every link • Route diverse protection could be implemented using extra wavelengths • No reuse of wavelengths • Underutilization of capacity Terminal OADM SONET ADM
- 106. EAGLE PHOTONICS Physical Ring,Logical Mesh Terminal Terminal OADM SONET ADM SONET ADM SONET ADM SONET ADM • Multiple SONET Rings are overlaid on the DWDM Ring • Reuse of wavelengths • Optimum utilization of capacity
- 107. EAGLE PHOTONICS Signal Velocityin Various Media Signal Velocity in Various Media Material Propagation Velocity (fraction of speed of light in a vacuum) Index of refraction Velocity of signal (km/s) Optical Fiber Flint glass Water Diamond Air Copper Wire (Category 5 cable) .68 .58 .75 .41 .99971 .77 1.46 1.71 1.33 2.45 1.00029 N / A 205,000 175,000 226,000 122,000 299,890 231,000
- 108. EAGLE PHOTONICS Chromatic Dispersion •Optical Amplifiers does not correct the dispersion of the fiber it only amplify the optical pulses
- 109. EAGLE PHOTONICS Type ofChromatic Dispersion • When velocity variation is caused by some property of the wave guide materials - Effect is called “Material Dispersion” • When velocity variation is caused by structure of the wave guide itself - Effect is called “Wave guide Dispersion” • When velocity variation is caused by refractive index profile of the wave guide itself - Effect is called “Profile Dispersion”
- 110. EAGLE PHOTONICS Chromatic Dispersionvs. Bit rate Not significant effect at OC- 48 Significant at OC-192
- 111. EAGLE PHOTONICS Polarization ModeDispersion • Light traveling in single mode fiber vibrate in two polarization states called modes, represents by x and y axis of the optical fiber • Two modes of polarization are at right angle (i.e. orthogonal to each other) • Refractive indices of the two polarization modes are different due to imperfect circular symmetry of optical fiber
- 112. EAGLE PHOTONICS Polarization ModeDispersion • Difference in refractive indices lead to variation in the velocity of these modes through the fiber, causing a delay in time domain • This delay in time domain between the optical pulses is known as polarization mode dispersion (PMD) • PMD is defined as this difference in arrival times in pico-seconds, normalized to the square root of the fiber length (ps/ Km)
- 113. EAGLE PHOTONICS Polarization ModeDispersion
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- 115. EAGLE PHOTONICS Nonlinear Effectsin Fiber • Kerr Effects – FWM – SPM – XPM • Scattering effects – Stimulated Raman Scattering – Stimulated Brillouin Scattering
- 116. EAGLE PHOTONICS Non-linear Effects KerrEffects Scattering Effects Cross phase modulation Four Wave Mixing Self Phase Modulation Stimulated Raman Scattering Stimulated Brillouin Scattering
- 117. EAGLE PHOTONICS Degradation Dueto Non-linear Effects Channel Spacing Span Length Capacity Power Output Limitation s Signal Losses Noise Cross Talk Pulse broadening Lim itations
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- 120. EAGLE PHOTONICS Four WaveMixing • Also known as four photon mixing • Combination of three optical wave produced a new optical wave • The frequency of the new optical wave will be f FWM = f1 + f2 - f3 • This effect dominates when the spacing of channels are equal because the mixing products can fall directly into other channel • This increased the cross talk between the channels
- 121. EAGLE PHOTONICS Four WaveMixing (FWM) f132 f312 f321 f113 f112 f123 f213 f223 f221 f332 f331 f231 Optical frequency f113 f213 f123 f112 f223 f132 f312 f221 f231 f321 f332 f331 FWM optical power generated by three equally spaced signals f1 f2 f3 Optical frequency FWM optical power generated by three unequally spaced signals f1 f2 f3
- 122. EAGLE PHOTONICS Stimulated RamanScattering Energy Level Time SW Source LW Emission Residue Emission
- 123. EAGLE PHOTONICS Stimulated RamanScattering (contd.) • Short wavelength stimulates long wavelength emission • If the long wavelength emission falls within the usable signal spectrum cross talk will occur • Cross talk becomes significant when source power crosses a threshold • Example: In a 10 channel system with a channel spacing of 1.3THz, the max power per channel is 3 mw • In Raman amplification the short wavelength source acts as a pump
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- 125. EAGLE PHOTONICS Stimulated BrillouinScattering (SBS) • Similar to Raman Scattering, but stimulated emission is in a lower wavelength • SBS limits the total power that can be injected into a single-mode fiber • High capacity DWDM systems will have high power output, which can lead to SBS • Using special modulation of signal light, SBS threshold can be raised
- 126. EAGLE PHOTONICS SPM: SelfPhase Modulation • Refractive index of fiber varies with intensity (Kerr effect) • Hence different intensity components of the signal travels at different speeds, leading to different phase delays for the components • Phase delays cause signal distortion • Predominant in G.652 and G.655 Fibers • Maximum permitted channel power output will depend on the span length, no. of spans etc.
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- 128. EAGLE PHOTONICS CPM: CrossPhase Modulation • Occurs in DWDM systems when power fluctuations of one signal result in distortion on other adjacent channels • Causes problems in systems with very narrow channel spacing • More dominant on G.652 fiber • Maximum permitted channel power output will depend on the span length, no. of spans etc. also
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Editor's Notes
- #4 MUX: Multiplexer / Multiplexing DEMUX: Demultiplexer / Demultiplexing
- #11 By using a single pair of fiber to transport multiple signals, the untapped capacity of the fiber is exploding . In DWDM capacity increases are easy to implement. It can be achieved by adding the additional hardware. In-service upgrading is possible in most cases. Thus the overbuilding of a network can be avoided which in turn reduces costs. In high capacity systems where the utilization of the fiber and the equipment is high the unit cost of bandwidth will be low. In many cases dark fibers are not available for building parallel SDH networks (normally referred to as “fiber exhaust”). If laying of new fiber is not possible or not cost effective, DWDM is the only way out. In SDH, capacity upgrades have to be done for the complete network. In DWDM a network’s capacity upgrade can be focused to any part of the network. Thus the trend is towards having protocol independent interfaces in DWDM systems, which will operate at any bit rate. WDM offers protocol and bit rate independent interfaces or transponders which offers tremendous flexibility and savings in comparison to SDH.
- #13 In SDH, network capacity can be increased by replacing the SDH network element. Even if the NE is upgradeable, hardware changes will be required for increasing the line rate. This may lead to service disruption and also prevent instantaneous addition of capacity. Service providers tend to ‘overbuild’ their networks to accommodate future capacity requirements. This requires increased capital investment.
- #15 SDH data rates are stagnating at 10 Gbps Higher bit rate transmission is difficult to achieve due to a lot of factors like Synchronization problems Processing speed limitations Inter Symbol Interference (ISI) Usually capacity increases beyond 10 Gbps are achieved by building parallel systems. When capacity requirement goes beyond 10 Gbps, parallel SDH systems are built on different fibers to meet the demand.
- #16 Propagation delays occur due to O-E-O conversion at SDH nodes. Signal propagation is faster in optical media compared to electrical. At SDH nodes signals are converted to electrical which slows down the signal. The processing required for removing, analyzing and adding overhead bytes further slows down the signal. SDH not the ideal carrier for data traffic. Wasted bandwidth in terms of overhead bytes and protection schemes in the case of data traffic. Additional interface hardware is required to map data traffic on to SDH.
- #17 Before DWDM came into existence parallel systems were being deployed to meet the increased capacity requirements. By implementing a DWDM system the cost of using multiple fibers could be avoided. In addition, by virtue of it’s wide band of operation, EDFAs could replace a number of regenerators. This leads to substantial cost savings.
- #21 Wavelength Division Multiplexing is the term used we use 2 channels per fiber. This type of multiplexing can be used in access networks where traffic is less. with the help of WDM we can increase the distances between the central office and subscribers which is difficult to achieve with copper cable.
- #22 Coarse wavelength Division Multiplexing (CWDM) is the term use when we multiplex 4 or 8 channels in a fiber. This type of multiplexing is useful for metro type of networks, where number of access networks are connected. As number of wavelengths are less we can use low cost components, which are widely spaced and having more spectral width. These components doesnot required thermal cooling which is required when wavelengths are closely spaced.
- #23 Dense wavelength Division Multiplexing (DWDM) is the term used when we multiplex more number of channels in a fiber i.e. 16,32,64. DWDM is use for long haul and ultra long haul network which is use to connect metro network.
- #25 1550 nm frequency window is divided in two wavelength band known as C and L band.
- #28 We can divide C band further into several wavelength channels. As shown in figure we can divide into 40 channels 100 GHz apart or 80 channels 50 GHz apart.
- #30 Shows all wavelengths traveling in one direction
- #31 Communication is unidirectional when two different fibers are used for clockwise and anticlockwise direction
- #32 Booth side traffic in same fiber but in opposite direction
- #33 Bi directional communication means when clockwise and anticlockwise traffic travels in same fiber in opposite direction
- #36 Transponders convert incoming to one of the ITU s Transponders are also known as wavelength translators. Normally the input to the transponders are 1310nm which is the traditional short haul wavelength. In case the signal is coming from a long haul system, it could be 1550 nm. In case the signal is coming from a SONET node which has a DWDM interface, the transponder could be avoided and the signal can be directly fed into the DWDM Mux. If the incoming signal is an ITU grid wavelength (from other DWDM systems or line cards of a SONET mux), then transponders can be avoided.
- #37 Protocol specific and open interface transponders are the most commonly used ones. FEC transponders are used in systems with narrow channel spacing and bit rates equal to or above 10Gbps. FEC method used is normally vendor specific and this will restrict the interoperability with equipment of other vendors. But it will help in making the system more cost-effective by reducing the number of amplifiers, regenerators etc. used in the system. By using high dispersion tolerant transponders DCMs may be avoided. This can be cost effective in low capacity systems. In systems with large number of channels it may be cost effective to use DCMs instead of high dispersion tolerant transponders
- #42 In DWDM, the channel capacity can be increased by increasing the number of wavelengths in use. Normally the multiplexer architecture is such that 4-8 channels (wavelengths) can be added without any service disruption. Capacity can also be increased by increasing the input line rates. Thus the capacity can be increased in both ways, by increasing the line rate or the number of channels.
- #61 Due to dispersion the light pulses spread and tend to overlap with the adjoining pulse period. This leads to interference between adjacent bits and results in high BER. At high bit rates for the same pulse width, the BER (due to dispersion) will become excessive. DCM (Dispersion Compensation Modules) contain fiber rolls with negative slope of dispersion. Passing fiber through DCM will distort light pulses in a reverse manner thus countering the effects of distortion due to dispersion. DCM’s may be placed at transmitting terminal, receiving terminal, at OADM’s or at ILAs.
- #69 ILAs are used in high loss links Depending on the link loss the required number of ILAs are placed ILAs provide ‘1R’ Amplification The max number of ILAs that can be placed are normally limited by distance the signal can travel without ‘3R’ regeneration
- #70 Post amps are also known as Power amps. Post amps and pre amps are used only if the link budget demands it. In metro systems Post/ Pre amps may not be required.
- #71 WDM can be deployed in unidirectional and bidirectional applications Unidirectional applications use two fibers to carry “send” and “receive” traffic separately in opposite directions Bidirectional applications, use a single fiber to carry traffic in both directions Different set of wavelengths are used for each direction of transmission Bidirectional transmission supports fewer wavelengths than unidirectional transmission. Bidirectional system is a good solution only if there are fiber constraints. At terminals couplers are used for separating or combining the wavelengths At ILA’s and OADM’s these couplers are used for separating the wavelengths before amplification or add/drop and are later used once again for combining traffic in both directions.
- #81 The spectral response as viewed on a spectrum analyzer is shown here. Wavelengths are sometimes referred to by the corresponding frequency in THZ Wavelength to Frequency Relationship is given below C = F x i.e. F = C / C = 3 E +08 m/sec Let = 1552.52 nm Then F = 3 E +08 / 1552.52 E -09 = 193.1 E +12 = 193.1 THz
- #94 OADM modules may be added to ILA’s to convert an ILA into an OADM OADM’s normally add or drop a few of the s
- #102 DWDM networks could be in linear configuration or ring configuration Typically in metro networks due to the short link lengths DCMs and regenerators may not be used. Some cases amplifiers are also not used.
- #103 ILAs provide 1R regeneration as against 3R regeneration of SDH regenerators Due to ASE (Amplifier Spontaneous Emission) signal becomes increasingly noisy as it is amplified by many EDFAs As per ITU-T G. 692, DWDM systems should be capable of transmitting signals without regeneration for 8 spans of 22dB If Regeneration is required in all the signals, two back to back terminals may be used or else an OADM may be used
- #104 Optical Cross Connect technologies are still not mature enough for large scale integration into DWDM networks. Use of Optical Cross Connects with wavelength conversion will lead to the realization of all optical networks. Presently, OXCs are realized by interconnecting multiple Terminals. Also, OXCs currently working with O-E-O conversions are available. These OXCs are in service upgradeable to O-O-O OXCs.
- #105 Virtual SONET rings are overlaid on physical DWDM rings. Any SONET topologies such as UPSR or BLSR could be implemented over the physical DWDM ring. The protection will be implemented in the SONET nodes.
- #117 There are two categories of nonlinear effects: Kerr effects and scattering effects. Kerr effects consists of three phenomena, SPM, XPM and FWM. In an optical fiber the core in which the optical signals travel has a specific refractive index that determines how light travels through it. However, depending upon the intensity of light traveling in the core, this refractive index can change. This intensity-dependence of refractive index is called the Kerr effect. There are two scattering effects,SRS and SBS
- #118 Due to optical nonlinear effects, nice crisp signals can be smeared and mixed, making it difficult to distinguish them at the end of the system.
- #122 Multiple wavelengths are generated leading to in-band and out-of-band cross talk occurs when multiple wavelengths on the same fiber are near the Zero-dispersion wavelength. They interact to create additional wavelengths that can interfere with other wavelengths on the fiber Enhanced by phase-matching of the signal with ASE noise frequencies in the zero dispersion region Predominant in DSF (ITU-T G.653) Limits multi-channel transmission Solutions Uneven channel spacing Careful monitoring of input power levels into the EDFAs Use of non-zero dispersion shifted fiber (NZDSF)
- #123 “Stimulated Raman scattering” involves light losing energy to molecules in the fiber and being re-emitted at a longer wavelength (due to the loss of energy). Also known as Forward Scattering A short wavelength source can excite atoms to a higher energy level Excited atoms are triggered by other photons and ‘drop’ to an intermediate energy level . The non linear properties of the fiber cause this. While dropping to an intermediate energy level atoms release energy as emission of a longer wavelength. Eventually all atoms will fall back to the lower level.
- #126 With the demand for more bandwidth and the rise in the number of OC-48 and OC-192 channels in a practical DWDM system, the output power of the EDFAs can be extremely high. This can cause SBS problems. In “Stimulated Brillouin scattering” light in the fiber can create lower wavelengths, which then scatter light to different wavelengths. .
- #127 In practice, SPM can be a significant consideration in designing systems at 10Gbps and higher, and leads to a restriction that the maximum power per channel should not exceed a few dBm. Kerr effect can cause “self-phase modulation” of a signal, whereby a wavelength can spread out onto adjacent wavelengths.