IP over DWDM NANOG May 24, 1999 Larry McAdams lmcadams@cisco.com

Outline Optical Transmission Fundamentals DWDM Systems IP over DWDM

Its Analog Transmission Attenuation Dispersion Nonlinearity Reflectance Transmitted data waveform Waveform after 1000 km PEAKING CROSS-OVER POINTLOWERED ZERO LEVEL BROADENED

Fiber Attenuation Telecommunications industry uses two windows: 1310 & 1550 1550 window is preferred for long-haul applications Less attenuation Wider window Optical amplifiers 1550 window 1310 window l

Fiber Dispersion l Normal fiber Non-dispersion shifted fiber (NDSF) >95% of deployed plant 18 Wavelength l Dispersion ps/nm-km 1310 nm 1550nm Reduced dispersion fibers Dispersion shifted fiber (DSF) Non-zero dispersion shifted fibers (NZDSF)

Dispersion Interference Dispersion causes the pulse to spread as it travels along the fiber Chromatic dispersion is important for singlemode fiber Depends on fiber type and laser used Degradation scales as (data-rate)2 Modal dispersion limits use of multimode fiber to short distances

Polarization Mode Dispersion Most severe in older fiber Caused by several sources Core shape External stress Material properties Becomes an issue at OC-192

Four-Wave Mixing (FWM) Creates in-band crosstalk that can not be filtered Problem increases geometrically with Number of ls Spacing between ls Optical power level Chromatic dispersion minimizes FWM fF = fp + fq - fr where fp , fq , and fr are the original light frequencies that combine to create fF. Notice that three wavelengths combine to make a fourth, hence the term four-wave mixing. In multi-channel systems, an intensity modulation at the beat frequency modulates the fiber’s refractive index that produces a phase modulation at the difference frequency. This phase modulation generates new wavelengths, as shown in Figure 3 below. FWM impairs transmission by transferring amplifier signal power from the original wavelengths to the created wavelengths. Also, in DWDM systems, the newly created wavelength often falls right on top of an existing channel. Because the number of FWM products increases geometrically with the number of original wavelengths, and because the electric fields of the new wavelengths can interfere both constructively and destructively with original wavelengths, FWM can severely affect the quality of transmission. The key is that as long as there is a 25 dB difference between the power of the in-band crosstalk and the power of the transmitted frequencies, no significant BER degradation will occur. Chromatic dispersion, discussed earlier, plays a crucial role in the build-up of FWM. Since chromatic dispersion will affect each WDM channel differently, it minimizes or eliminates phase matching between the interacting wavelengths. Thus, the higher the chromatic dispersion, the lower the FWM. The greatest problem, however, is in the initial portions of a span just after optical amplification. At this point, if the chromatic dispersion is low, the per channel optical power is high enough to cause non-linear effects

Outline Optical Transmission Fundamentals DWDM Systems IP over DWDM

EDFAs Enable DWDM 40-80 km Terminal Regenerator - 3R (Reamplify, Reshape and Retime) 120 km Terminal EDFA - 1R (Reamplify) Terminal Terminal Terminal Terminal Terminal Terminal EDFA amplifies all ls

EDFA Schematic ... ... EDF EDF WDM Coupler WDM Coupler Optical Filter Optical Isolator Optical Isolator DCF 980 Pump Laser 1480 Pump Laser EDFAs amplify all ls in 1550 window simultaneously Key performance parameters include Saturation output power, noise figure, gain flatness/passband

DWDM System Design 1 1 Optical Combiner 2 DWDM Filter 2 3 3 4 4 5 5 1550 1 1551 1 2 1552 2 3 1553 Optical Combiner DWDM Filter 3 4 1554 4 5 1555 5 Amplify 6 1556 6 7 1557 7 1310 nm Rx External Modulator Laser 15xx nm 1310 nm Reamplify Reshape Retime Rx Tx 15xx nm

DWDM State-of-the-Art Point-to-point systems 40l x OC-48 deployed 16l x OC-192 deployed 160l x OC-192 announced Configurable OADMs Metro rings Data Rate 1-10 Tbps per fiber is just around the corner!

Outline Optical Transmission Fundamentals DWDM Systems IP over DWDM

Synchronization for IP over DWDM FIBER Point-to-point application Synchronization driven from router Router interface internal timed ~ WDM ~ REGEN OC-48c OC-48c DS1 Ethernet Ethernet T1 Gig-Ethernet OC-12c OC-3c SONET NETWORK OC-48c OC-48c PRS SONET network application Synchronization driven from network Router interface timed to PRS via Rx OC-48c

Protection for IP over DWDM Optical Cloud Optical protection is not sufficient Only protects transmission infrastructure Layer 3 must provide path restoration Opportunity for differentiation at the service level

Ciena 40l DWDM GSR 12000 SR OC-48 PoS RC TX 500 km 100 km 25 dB RC TX TX RC TX RC GSR 12000 SR OC-48 PoS TX RC Error-free transmission over 20,000 kms without SONET regeneration

Nortel 16l DWDM and OC-192 Ring PRS Working Protect SONET alarm reporting between the ADM and the router OC-192 interoperability: demonstrate OC-48c IP connectivity through a 3 node Nortel OC-192 ring/WDM system. All the POS interfaces were alarm/error free and we were able to ping successfully between the two MFRs. Both the OC-48 line cards were loop timed off the Nortel equipment. We did not see any synchronization problems we tried some OC-192 ring switches on the Nortel gear while pinging between the MFRs. There were no ping failures as a result of the protection switch. We also monitored the SONET overhead on the POS links during the switch. As expected there were no section(B1) and line(B2) errors and only a few path(B3) errors (approx 10/20) Lack of transponders prohibits direct connections at OC-192

Conclusion - IP over DWDM Transmission is an analog problem Proprietary solutions abound DWDM provides 100s Gbps of capacity Transponders are required for an open architecture Large scale deployments have been achieved IP directly over DWDM is a reality