1. IP over DWDM
NANOG May 24, 1999
Larry McAdams

2. Outline
Optical Transmission Fundamentals
DWDM Systems
IP over DWDM

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

4. 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

5. 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)

6. 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

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

8. 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

9. Outline
Optical Transmission Fundamentals
DWDM Systems
IP over DWDM

10. 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

11. 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

12. 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

13. 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!

14. Outline
Optical Transmission Fundamentals
DWDM Systems
IP over DWDM

15. 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

16. 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

17. 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

18. 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

19. 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