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Fundamentals of Optical Networking

Fundamentals of Optical Networking. Mark E. Allen, Ph.D. mark.allen@ieee.org. Agenda. Part I: Component overview Wavelength division multiplexing Filter technologies Amplifiers Fiber and switch technologies Part II: Design considerations Span design Restorability

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Fundamentals of Optical Networking

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  1. Fundamentals of Optical Networking Mark E. Allen, Ph.D. mark.allen@ieee.org

  2. Agenda • Part I: Component overview • Wavelength division multiplexing • Filter technologies • Amplifiers • Fiber and switch technologies • Part II: Design considerations • Span design • Restorability • Cost optimization in the metro and wide area • Wavelength routing

  3. SONET and Optical Communications

  4. Digital data transmission • All forms of information will soon be carried on an optical infrastructure Video MPEG II Optical Network Internet Images MP3 Voice

  5. Transmitter Carrier: RF, Laser, etc. Communications Medium Coding Modulator Bits Information Copper Coax cable Fiber optics Free space • Voice • Video • Data Voice over IP MPEG II Ethernet ATM Packet over SONET SONET Timing source

  6. Receiver Bits Information Demodulator Decoding Medium Timing information

  7. Representing bits: NRZ vs. RZ Return to Zero (RZ) Pulse stream • RZ pulse have better timing information and dispersion tolerance, but are more complicated to process 1 0 1 1 1 Non Return to Zero (NRZ) Pulse stream 1 0 1 1 1

  8. Modulation: FSK • FSK – Frequency shift keying. Different carrier frequencies represent different data symbols.

  9. Modulation: PSK • PSK – Phase shift keying. Different phases of the carrier represent different data symbols.

  10. Modulation: ASK • ASK – Amplitude shift keying. Different amplitudes of the carrier represent different data symbols. This is the most common technique for modulating a laser source.

  11. Examples of digital signals • 10/100 Ethernet • Gigabit Ethernet • FDDI • T1/DS3 • SONET/SDH • OC3 (STM1), OC12(STM4), OC48 (STM16), OC192 (STM64)

  12. Phase diagrams • Phase diagrams show the phase and amplitude for different symbols PSK ASK

  13. Modulation bandwidth For ASK modulated signals, bandwidth is usually more than twice the bandwidth. i.e. 10Gbps would occupy more than 20GHz

  14. Optical Fiber • Single mode • Multimode • Attenuation characteristics • Definition of dB • Power in dBm • Loss vs. wavelength • Wavelength vs. frequency

  15. Optical fiber buffer coating cladding core

  16. Optical source • Typical low cost optical transmitter • 850nm or 1310 nm • Modest power –5 to -10dBm (how many milliwatts is this?) • Uses a laser diode • The current level is modulated to create ASK “on-off” light signal for 1’s and 0’s

  17. Higher quality source(more $) • May use 1550nm wavelength or “ITU” optics (15XX where exact wavelength is specified) • ITU optics makes it WDM capable • High power ~ 0dBm for 100km + reach • Laser diode with external modulator for cleaner pulses (faster speeds) • 10Gbps bit rate capable • $10K or more for transmitter

  18. Detector • Detectors are typically semiconductor based photodiodes • Generate current based on detecting photons • Low-cost :: PIN Diodes • Higher cost : Avalanche Photodiodes (APD) • Include some amplification within the detector based on the Avalanche process • Cost, reach and speed are all considerations in receiver designs.

  19. Single mode vs. multi-mode • Multimode fiber allows light many possible paths down the fiber. Different paths have different distances. • Single mode fiber has a small core and allows only one ‘mode.’ Varying delays in the path length can result in dispersion when the fiber is long and high bit rates are transmitted

  20. Low-loss regions of fiber 0.5 0.4 0.3 1550 window Attenuation (dB/km) 1310 nm 0.2 1550 nm 0.1 1100 1300 1500 1700 Wavelength (l)

  21. Wavelength vs. frequency • In the neighborhood of 1550 nm, 0.8nm is 100 GHz, 0.4nm is 50 GHz, etc.

  22. Wavelength plans • The ITU grid • Standard wavelength spaced 100 GHz apart. 40 channels currently specified. • WDM block diagram WDM Filters SONET NE Fiber Amp

  23. Filter technologies • Thin-film • AWG • Bragg-gratings

  24. WDM Operation • Current technologies allow 50GHz (.4nm) spacing • Dielectric thin-film • Array wave guide (AWG) • Bragg grating l1, l2, l3, ... l1 l2,l3 l2 l3 l3 Thin film operation

  25. Array waveguide l1, l2, l3, ... l1 l2 l3

  26. Bragg grating Port 2 l1 Bragg grating Port 3 Port 1 l1, l2, l3, ... l2, l3 optical circulator l1 passes through the Bragg grating, but l2 and l3 are reflected by it.

  27. Wavelength Division Multiplexing (WDM)

  28. Economics of long-haul WDM: Amplifiers replace regenerators Terminal Conventional Networks Terminal 1310nm 37 km Terminal Terminal 1550 nm 100 km Loss per span Number of spans Optically Amplified 4 x 25dB

  29. WDM + TDM • 3 amplifiers • 1 fiber pair WDM equipment savingsThe Optical to electronic compromise • Reduce regeneration costs • Reduce fiber costs • Quicker turn-up time for new bandwidth • TDM only • 80 regens • 8 fiber pairs

  30. Equipment savings with Optical Add/Drop All traffic must be regenerated Dropped traffic After Pass-through traffic is all-optical Dropped traffic

  31. Optical Spectrum Analyzer (OSA) output

  32. How much bandwidth in a fiber? • The 1550 nm window has more than 10 THz of bandwidth. • Current systems exploit less than 1% of this bandwidth.

  33. Amplifiers • Erbium doped fiber amplifiers (EDFAs) • Extended band amplifiers • Raman amplification

  34. Erbium Doped Fiber Amplifier (EDFA) • Pump source operates at 980 nm or 1480 nm • These wavelength are matched to characteristics of erbium • Stimulated emission occurs around 1530 nm • New photons at the same wavelength are created Weak signal Amplified signal Pump source Doped fiber

  35. 1555 1505 1510 1530 1540 1560 1565 1570 1535 1545 1550 Extended band amplification ITU Channel 60 ITU Channel 20 ITU Grid Reference Point (193.1THz) 199.0 196.0 195.0 194.0 193.0 192.0 191.0 190.0 186.0 ¦(THz) l (nm) 1610 C-Band OA Flat Gain Region S-Band OA Flat Gain Region L-Band OA Flat Gain Region

  36. Raman amplification • Raman is a phenomenon where a fiber pumped at a certain wavelength exhibits gain 100 nm away. • Doesn’t require specially doped fiber • Raman amplifiers can be made by pumping the fiber in the ground • Acts as a distributed amplifier compensating for loss along the fiber • Normal EDFA is a lump source amplifier • Effective noise figure for Raman can be lower than EDFAs

  37. Fiber types • Dispersion • Chromatic dispersion • Polarization mode dispersion (PMD) • Dispersion management techniques • Lower bit rate • More frequent regeneration • Dispersion compensation • Advanced fiber types

  38. What is dispersion? • Dispersion causes pulses to be smeared together as they travel through the fiber. 1 0 1 1 1 1 0 1 1 1

  39. Eye patterns and SNR • Overlay plotting a 3 symbol sequence (randomly either 000,001, 010,… or 111) yields an ‘eye’ pattern. • The eye pattern can be used to measure signal quality in terms of dispersion and SNR. Two examples of eye patterns. The lower Figure has more dispersion and noise.

  40. Single mode fiber (SMF) dispersion

  41. Dispersion for DS fibers Lucent TrueWave Corning LEAF +4 +2 Dispersion (ps/nm -km) 1530 1540 1550 1560 DSF - 2 Corning LS - 4

  42. Fiber type 0, (nm) S0 (ps/nm2*km) D (ps/nm*km) Comments Corning SMF-28 1312 0.09 17 @ 1550 nm Standard single mode fiber. Corning SMF/DF 1535-1565 0.075 <=2.7 Dispersion shifted or dispersion compensated fiber. Corning SMF/LS >=1560 0.08 -0.1>=D>=3.5 Lambda-shifted Non Zero Dispersion Shifted Fiber (NZDSF) Lucent TrueWave 1518 0.08 1<D<5.5 NZDSF Characteristics for common fibers

  43. Polarization mode dispersion (PMD) • PMD is caused when different polarizations of the signal experience different amount of dispersion. • PMD is most prominent when using older fiber that is not perfectly round. • PMD is most common at 10 Gbps and above. • New PMD compensators are being developed.

  44. Optical time domain reflectometer (OTDR) • OTDR plot shows where reflections occur • Location and loss of splices • Location of Fiber cuts • Overall span loss Splice 1 Splice 2 Cable end Loss (dB) Distance (km)

  45. Switch technologies • Takes us to real optical networking • What are the obstacles? • Attenuation management • Dispersion management • Performance monitoring • Scalable switches • Wavelength conversion

  46. Design considerations

  47. Data traffic is driving network growth Data demand Voice demand Assumptions - 10% growth in voice traffic per year - Sidgemore’s law for data growth (data demand doubles every 6 months)

  48. IP Traffic Voice traffic Number of flows Number of calls miles miles Characteristics of data traffic • Voice • Slow steady growth • Predictable growth pattern • Low bandwidth consumption • Most calls terminate within the local area • Data • Rapid, unpredictable growth • Huge bandwidth consumption • Distance insensitive

  49. ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM ADM Ring inefficiencies Bottlenecks due to low drop capacity Wasted protection capacity

  50. w w ADM p p Switch Local drop traffic ADM w w p p ADM Interconnections with Switch Switch Local drop traffic Top view Multi-ring scenario Interconnecting 8 OC192 rings requires about 640 Gbps switch capacity 320 Gbps (line) + 320 Gbps (local and drop)

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