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Fundamentals of Optical Networking Mark E. Allen, Ph.D. [email protected] 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 SONET and Optical Communications Digital data transmission • All forms of information will soon be carried on an optical infrastructure MPEG II Optical Network Internet Images MP3 Voice Transmitter Carrier: RF, Laser, etc. Communications Medium Bits Information Coding •Voice •Video •Data Voice over IP MPEG II Ethernet ATM Packet over SONET SONET Modulator Timing source Copper Coax cable Fiber optics Free space Receiver Bits Demodulator Medium Timing information Information Decoding Representing bits: NRZ vs. RZ Return to Zero (RZ) Pulse stream 1 0 1 1 1 Non Return to Zero (NRZ) Pulse stream 1 0 1 1 1 • RZ pulse have better timing information and dispersion tolerance, but are more complicated to process Modulation: FSK • FSK – Frequency shift keying. Different carrier frequencies represent different data symbols. "ONE" "ZERO" "ONE" "ONE" "ZERO" Modulation: PSK • PSK – Phase shift keying. Different phases of the carrier represent different data symbols. "ONE" "ZERO" "ONE" "ONE" "ZERO" 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. "ONE" "ZERO" "ONE" "ONE" "ZERO" 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 90 90 O 180 270 ASK O 180 270 PSK Modulation bandwidth Power Unmodulated carrier carrier Freq Power For ASK modulated signals, Modulated carrier width = 2X bit rate bandwidth is usually more than twice the bandwidth. i.e. 10Gbps would occupy more than 20GHz carrier Freq Optical Fiber • Single mode • Multimode • Attenuation characteristics – Definition of dB – Power in dBm – Loss vs. wavelength – Wavelength vs. frequency Optical fiber cladding core buffer coating 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 Attenuation (dB/km) 0.4 0.3 1550 window 1310 nm 0.2 1550 nm 0.1 1100 1300 1500 Wavelength (l) 1700 Wavelength vs. frequency • In the neighborhood of 1550 nm, 0.8nm is 100 GHz, 0.4nm is 50 GHz, etc. l l f 2 c Wavelength plans • The ITU grid – Standard wavelength spaced 100 GHz apart. 40 channels currently specified. • WDM block diagram SONET NE WDM Filters 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 Thin film operation l3 l2 l3 Array waveguide l1, l2, l3, ... l1 l2 l3 Bragg grating Port 2 l1 Bragg grating Port 1 Port 3 l1, l2, l3, ... l2, l3 optical circulator l1 passes through the Bragg grating, but l2 and l3 are reflected by it. Wavelength Division Multiplexing (WDM) Economics of long-haul WDM: Amplifiers replace regenerators Terminal Conventional Networks 1310nm 37 km Terminal 1550 nm Terminal Terminal 100 km Number of spans Loss per span Optically Amplified 4 x 25dB WDM equipment savings The Optical to electronic compromise •WDM + TDM •3 amplifiers •1 fiber pair •TDM only •80 regens •8 fiber pairs •Reduce regeneration costs •Reduce fiber costs •Quicker turn-up time for new bandwidth Equipment savings with Optical Add/Drop All traffic must be regenerated Dropped traffic After Dropped traffic Pass-through traffic is all-optical Optical Spectrum Analyzer (OSA) output 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) Weak signal Amplified signal Pump source Doped fiber • 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 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 (THz) l (nm) 1505 1510 1530 S-Band OA Flat Gain Region 1535 1540 1545 1550 C-Band OA Flat Gain Region 1555 1560 1565 1570 190.0 186.0 1610 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? 1 0 1 1 1 1 0 1 1 1 • Dispersion causes pulses to be smeared together as they travel through the fiber. 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. ` Single mode fiber (SMF) dispersion D ps/(nm*km) Dispersion coefficient vs. wavelength 22 20 18 16 14 12 10 1500 1520 1540 1560 wavelength (nm) 1580 1600 Dispersion for DS fibers Lucent TrueWave Dispersion (ps/nm -km) Corning LEAF +4 +2 -2 -4 DSF 1530 Corning LS 1540 1550 1560 Characteristics for common fibers Fiber type l0, (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 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 Distance (km) Switch technologies • Takes us to real optical networking • What are the obstacles? – Attenuation management – Dispersion management – Performance monitoring – Scalable switches – Wavelength conversion Design considerations Data traffic is driving network growth 100 Percent 80 Data demand 60 40 Voice demand 20 0 1997 Time 1998 1999 2000 2001 2002 2003 2004 Assumptions - 10% growth in voice traffic per year - Sidgemore’s law for data growth (data demand doubles every 6 months) Characteristics of data traffic Voice traffic Number of calls miles IP Traffic • Voice – Slow steady growth – Predictable growth pattern – Low bandwidth consumption – Most calls terminate within the local area • Data Number of flows miles – Rapid, unpredictable growth – Huge bandwidth consumption – Distance insensitive Ring inefficiencies ADM ADM Bottlenecks due to low drop capacity ADM ADM ADM ADM ADM ADM ADM Wasted protection capacity ADM ADM ADM ADM ADM ADM ADM ADM Interconnections with Switch w p ADM w p Switch w p ADM Local drop traffic Switch w p 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) Span design • Signal loss of ~.25dB per km – ~100 km is limit without amplification • Noise accumulates degrading SNR – Eventually, 3R regeneration required to clean the signal. • Dispersion accumulates – Dispersion compensation and 3R to correct. Non linear effects in fiber • These non-linear effect result because the signals traveling through the fiber slightly change the index of refraction of the fiber – Four-wave mixing (FWM) – When three frequencies in the fiber: w1, w2 and w3 interact to create for example w4 = w1+w3-w2. W4 might interfere with a desirable wavelength. – Cross phase modulation (XPM) – When the intensity variations of a signal modulate the phase of other signals in the fiber. – Self phase modulation (SPM) – When the intensity variations of a signal modulate the phase of the signal. Other non-linear effects • SBS – Stimulated Brillouin Scattering is produced by acoustic waves in the fiber. – Backscattered light depletes power from the forward traveling lightwaves. – This can be minimized by reducing the signal power and dithering the wavelengths • SRS – Stimulated Raman is an interaction between light waves and silica molecules. Power is transferred to wavelengths several nm away. – This can be used for amplifiers IP over SONET • Well-known technology • Provides for ~50ms restoration • Useful but expensive and not needed when building an IP network. Provisioned Connection Protection SONET ADM Working SONET Ring SONET ADM SONET ADM SONET ADM Packet over SONET • Packet over SONET is the serial transmission of data using SONET framing. • RFC 1619, “PPP over SONET/SDH” • RFC 1662, “PPP in HDLC-like Framing” • ITU-T G.703 / ANSI T1X1 CRC 16/32 1 flag IP packets flag PPP header 4 address (1) control (1) protocol (2) 2/4 data 1 Reducing the number of boxes B-ISDN IP over ATM IP over SONET/SDH IP over Optical Multiplexing, Protection, and Management at Every Layer IP ATM IP IP SONET/SDH ATM SONET/SDH IP Optical Optical Optical Optical Lower Cost, Complexity, and Overhead Options when building backbone transport • High speed OC-192 (10G) backbone – Wavelength specific optics on SONET ADMs – Cutting edge • OC48 (2.5G) backbone with “Open Interfaces” – – – – – ADMs use short-reach optics Transponders have wavelength specific lasers “Tried-and-true” technology Commodity components Access to full protect bandwidth Advantages of each • OC192 – Fewer wavelengths to manage – Lower cost for 3R regeneration – Filter technologies match spectrum of signal • Wavelength drift not issue • Filter drift not an issue – 100 GHz thin-film are passive – Arguably maximum capacity method today • OC48 – Dispersion less of an issue – PMD not an issue – Fewer 3Rs required – Off-the-shelf technology – Common rate of ATM and IP switches – Open interfaces – Less expensive electronics – Runs anywhere OC192 economics cost Lower average cost per bps for fully-loaded systems Higher up front cost when lighting fiber 10 20 30 40 50 60 Gbps OC48 economics Next fibers must be lit sooner Lower up cost front cost when lighting fiber Higher average cost per bps for high capacity routes 10 20 30 40 50 60 Gbps Repeater spacing • Increasing the repeater spacing – reduces the construction costs – reduces the electronics costs for the first few systems • But .. WDM system performance will suffer – WDM adds additional losses in the system – Total power must be divided over the number of waves – Non-linearities are a function of the launch power The impact of limited launch power Number of waves (with limited peak power) Number of waves 200 150 100 50 0 50 60 70 80 90 100 110 Distance (km) 12 km reduction in spacing allows twice the number of waves if launch power is the limiting factor!!