Optical Burst Switching (OBS): Issues in the Physical Layer University of Southern California Los Angeles, CA A. E. Willner

O-E-O Offset Time Switch Time Scale in OBS Control Packet Burst Generally, …. Offset time between control packet & burst is 1-5 microsecs Burst ranges in time from 1 microsec to 100 millisecs Control packet has a lower bit rate than the data payload

Outline 1.Degradations Due to Physical-Layer Impairments 2.Fast Monitoring of a Burst 3.Fiber-Loop Buffers for OBS Efficiency

Signal Degradation due to Chromatic Dispersion time f carrier freq. ViVi VjVj VkVk Fourier Information Bandwidth of Data Temporal Spreading  f (distance, (bit rate) 2 )  (ps/nm)/km time Fiber time Photon Velocity (f) = Speed of Light in Vacuum Index of Refraction(f)

Chromatic Dispersion Effects on Payload and Control Packet Control Packet (C.P.), not payload, is regenerated at every node C.P. has lower bit-rate (CD effect  (bit-rate) 2 ) There is higher chance for payload to be degraded Node t t t t PayloadC.P.

Offset Time Affected by Wavelength Skew: Uncompensated Systems (2.5 Gbit/s Payload?) t t 30 nm 400 km of Fiber (CD=17 ps/(nm.km)) t t C.P. Payload Offset time change ~ 1  s C.P. Payload Skew Offset

Value of Tunable Dispersion Compensation (40 Gbit/s Payload) A tunable dispersion compensator allows for a wide range of transmission distances at 40 Gbit/s.

Polarization-related Impairments in High- Performance Systems Polarization-mode-dispersion (PMD) Polarization dependent loss (PDL) Degradation based on non-catastrophic events Random polarization coupling Statistically varies with time Bit-rate and wavelength dependent Polarization state generally unknown and wanders

Polarization Mode Dispersion (PMD) cross section Elliptical Fiber Core side view PMD induces randomly changing degradations. Critical limitation at  10 Gbit/s payload data rates. The 2 polarization modes propagate at different speeds. 1st-order PMD = DGD

Frequency of occurrence induced by PMD fluctuation Time Span (ms) Occurrence 52 km fiber  >=2.8 ps (b) Fast Fluctuation Time Rate of PMD Change PMD (ps) Temp. (  C) Time (min) km buried cable PMD temporal changes more rapidly with the fiber length and average DGD (a) Slow Fluctuation PMD variations due to temperature changes: hours to days J. Cameron, et al., OFC 1998 Mechanical vibrations: milliseconds to minutes H. Bulow, et al., OFC 1999

Fiber Nonlinearities ps RZ Pulses 0.4 ps/nm/km -0.2 ps/nm/km 0.08 ps/nm/km Link Dispersion Dispersion Variation ~ 4% Distance (km) 4  10 Gb/s Chromatic dispersion changes the effects of nonlinearity Refractive index depends on frequency and power n(,P)n(,P) Chromatic Dispersion Power Power Penalty (dB) Isolation of nonlinear effects is very difficult It is also difficult to monitor and compensate

EDFA Gain Deployed EDFA cross saturation causes gain transients due to: Channel turn-on Channel re-routing Network reconfiguration Link failures Time scale of gain saturation and recovery is ~ µs to ms Input Channels Dropped Channels EDFA Output Channels

# of EDFAs Time (  s) Reciprocal Time (  s ) dB power excursion for surviving channels 4 channels dropped 4 channels survive Time Response Zyskind, OFC’96 PD-31

Outline 1.Degradations Due to Physical-Layer Impairments 2.Fast Monitoring of a Burst 3.Fiber-Loop Buffers for OBS Efficiency

Window of Operability in OBS Window of operability is shrinking as systems become more complex Ensuring a long-term stable and healthy network is tricky bit rate power nonlinearities dispersion number of channels polarization effects format

Monitoring in OBS Systems Monitoring time scale corresponds to that of OBS (  s ~ ms) Dynamic monitoring covers the wide range of both multi-wavelength payloads and control packets Monitoring includes; - Power - Wavelength - Optical signal-to-noise ratio - Distortion: CD, PMD, nonlinearities

Impact of Monitoring on OBS Systems Need to find the non-catastrophic problems in OBS systems - Enable the functionality of error-free assembly nodes combined with tunable compensator - Maintain the accurate offset time - Locate and measure the distortion of payload and control packets - Support protocol-independent WDM transport - Isolate different degrading effects

Impairment- & Security-Aware Routing Present network : very few variables (i.e. # of hops) are used to determine the routing table although there are several variables on the physical state Future networks: –Monitor the channel quality and link security and update the routing look-up tables continually –In the routing decisions ensure that: Channels achieve acceptable BER Network achieves sufficient transmission and protection capacity Highest priority data is transmitted on the strongest and most secure links

Vestigial Sideband Optical Filtering Filter BW = (0.8 ~ 1.2)  bit-rate (R b ) Filter detuning  f = (0.4 ~ 0.8)  R b Frequency BW ff VSB-U VSB-L Optical Carrier fUfU f0f0 fLfL

40-Gb/s RZ Data VSB-L VSB-U f Dispersion f O/E tt Monitor Clock Phase Isolate CD from PMD effects Low cost Q. Yu, JLT, Dec., 2002 Filtered spectrum Entire channel Filtered spectrum Time delay (  t ) between two VSB signals is a function of CD Bits can be recovered from either part of the spectrum

PMD Monitoring Techniques – Requires high- speed devices (demonstrated for 160 Gb/s RZ signal) – Affected by other distortion sources + Can be integrated with electronic equalization A. Eye opening measurement B. RF spectrum analysis + No high speed electronics + Depends only on PMD + Bit-rate independent + Unaffected by other distortion sources – Pulse-width dependent C. Degree of polarization (DOP) measurement + Simple – Affected by other distortion sources – Sensitivity and DGD range depends on monitored frequency

Outline 1.Degradations Due to Physical-Layer Impairments 2.Fast Monitoring of a Burst 3.Fiber-Loop Buffers for OBS Efficiency

Research Goals (Generously Supported by Intel) Simulate an 8 X 8 switch with feedback buffering Determine the optimal number of input/output ports and delay lines Simulate delay lines having recirculation capability Investigate the effect of random burst size Control Unit N M N + M = 8 Switch Delay Lines Data Burst Lines Control Line Burst (N+M) x (N+M) Control Packet Optical Fiber Delay Lines

Optimal Number of Input Ports and Delay Lines Throughput Efficiency (5,3) setup gives a higher throughput than a (4,4) and (6,2) setup Is this scalable to a switch with more number to ports ? Load (4,4) (5,3) (6,2) Buffered Bufferless (5,0) (6,0) (4,0) (N,M)  (N input data lines M delay lines) (7,1) (7,0) # of input ports 1 st Buffer Kbytes 2 nd Buffer Kbytes 3 rd Buffer Kbytes 4 th Buffer Kbytes Buffer Size

Throughput Efficiency vs. Load for Different Maximum Burst Sizes Load Throughput Efficiency The throughput efficiency decreases with increase in burst size. Buffer size = max. burst size, 3 buffers for 5,3 case. Maximum = 14 Kbytes burst size Maximum = 10 Kbytes burst size Maximum = 2 Kbytes burst size Maximum = 20 Kbytes burst size

Effect of Adding Buffers on Throughput Efficiency Throughput efficiency does not increases with the number of delay lines For an 8 x 8 switch, it is beneficial to have 2 or 3 delay lines Increase in Throughput Efficiency 1 Buffer 2 Buffers 3 Buffers Bufferless 4 Buffers (4, 4) Switch Load

Throughput Efficiency Load Throughput Efficiency for Recirculation With 3 recirculations the throughput efficiency of approximately 86% can be achieved. 5th recirculation increases the throughput by only ~1%. 1 Round Trip 2 Recirculations 3 Recirculations 5 Recirculations 10 Recirculations Bufferless (5, 3) Switch

Load 1 Buffer 2 Buffers 3 Buffers 3 Buffers with 2 recirculations 3 Buffers with 3 recirculations Bufferless Increase in Throughput Efficiency Increase in Throughput Efficiency with Buffers and Recirculation 3 Buffers and 3 recirculations increase the throughput efficiency by 27 % Throughput efficiency does not increase linearly with number of delay lines

(5,3) configuration provides higher throughput than other configurations. ~25% increase in throughput efficiency is obtained with 3 buffers and recirculations. Number of delay lines should be limited to 2 or 3, as the throughput does not increase much with an increase in number of delay lines. BUT, …, the fiber delay line has loss, …, optical amplifiers add noise, and, … recirculations can degrade the payload. Key Buffer Results for 8X8 Switch

Summary Degradation effects including CD, PMD, nonlinearities should be addressed in OBS. Fast monitoring can help the long-term stability and robustness of a OBS network. Optical buffers enable enhanced OBS functionality.