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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 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 • Cost optimization in the metro and wide area • Wavelength routing
Digital data transmission • All forms of information will soon be carried on an optical infrastructure Video MPEG II Optical Network Internet Images MP3 Voice
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
Receiver Bits Information Demodulator Decoding Medium Timing information
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
Modulation: FSK • FSK – Frequency shift keying. Different carrier frequencies represent different data symbols.
Modulation: PSK • PSK – Phase shift keying. Different phases of the carrier represent different data symbols.
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.
Examples of digital signals • 10/100 Ethernet • Gigabit Ethernet • FDDI • T1/DS3 • SONET/SDH • OC3 (STM1), OC12(STM4), OC48 (STM16), OC192 (STM64)
Phase diagrams • Phase diagrams show the phase and amplitude for different symbols PSK ASK
Modulation bandwidth For ASK modulated signals, bandwidth is usually more than twice the bandwidth. i.e. 10Gbps would occupy more than 20GHz
Optical Fiber • Single mode • Multimode • Attenuation characteristics • Definition of dB • Power in dBm • Loss vs. wavelength • Wavelength vs. frequency
Optical fiber buffer coating cladding core
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
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
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.
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
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)
Wavelength vs. frequency • In the neighborhood of 1550 nm, 0.8nm is 100 GHz, 0.4nm is 50 GHz, etc.
Wavelength plans • The ITU grid • Standard wavelength spaced 100 GHz apart. 40 channels currently specified. • WDM block diagram WDM Filters SONET NE Fiber Amp
Filter technologies • Thin-film • AWG • Bragg-gratings
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
Array waveguide l1, l2, l3, ... l1 l2 l3
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.
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
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
Equipment savings with Optical Add/Drop All traffic must be regenerated Dropped traffic After Pass-through traffic is all-optical Dropped traffic
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.
Amplifiers • Erbium doped fiber amplifiers (EDFAs) • Extended band amplifiers • Raman amplification
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
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
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
Fiber types • Dispersion • Chromatic dispersion • Polarization mode dispersion (PMD) • Dispersion management techniques • Lower bit rate • More frequent regeneration • Dispersion compensation • Advanced fiber types
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
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.
Dispersion for DS fibers Lucent TrueWave Corning LEAF +4 +2 Dispersion (ps/nm -km) 1530 1540 1550 1560 DSF - 2 Corning LS - 4
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
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.
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)
Switch technologies • Takes us to real optical networking • What are the obstacles? • Attenuation management • Dispersion management • Performance monitoring • Scalable switches • Wavelength conversion
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)
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
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
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)