Optical Burst Switching (OBS): Issues in the Physical Layer

1
Optical Burst Switching (OBS) Issues in the
Physical Layer
A. E. Willner
University of Southern California Los Angeles, CA
2
Time Scale in OBS
Control Packet
O-E-O
Burst
Switch
Offset Time

• 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

3
Outline

• Degradations Due to Physical-Layer Impairments
• Fast Monitoring of a Burst
• Fiber-Loop Buffers for OBS Efficiency

4
Signal Degradation due to Chromatic Dispersion
Speed of Light in Vacuum
Photon Velocity (f)
Index of Refraction(f)
Information Bandwidth of Data
Vj
Vi
Vk
Fourier
0
1
0
0
1
1
fcarrier
time
freq.
Temporal Spreading ? f (distance, (bit rate)2)?
(ps/nm)/km
time
time
Fiber
5
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
t
Node
Payload
C.P.
t
t
Node
t
Node
Node
6
Offset Time Affected by Wavelength Skew
Uncompensated Systems (2.5 Gbit/s Payload?)
C.P.
t
400 km of Fiber (CD17 ps/(nm.km))
30 nm
Payload
t
Offset
C.P.
t
Skew
Payload
t
Offset
Offset time change 1 ?s
7
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.
8
Polarization-related Impairments in
High-Performance Systems
Degradation based on non-catastrophic events
Polarization state generally unknown and wanders
Polarization-mode-dispersion (PMD) Polarization
dependent loss (PDL)
Statistically varies with time
Bit-rate and wavelength dependent
Random polarization coupling
9
Polarization Mode Dispersion (PMD)
cross section
side view
Elliptical Fiber Core
1st-order PMD DGD
The 2 polarization modes propagate at different
speeds.

• PMD induces randomly changing degradations.
• Critical limitation at
• ? 10 Gbit/s payload data rates.

10
Time Rate of PMD Change
(b) Fast Fluctuation
(a) Slow Fluctuation
Frequency of occurrence induced by PMD fluctuation
PMD (ps)
Occurrence
48.8 km buried cable
52 km fiber lt?? gt2.8 ps
Temp. (?C)
0
400
800
Time Span (ms)
Time (min)

• PMD variations due to temperature
• changes hours to days

• Mechanical vibrations milliseconds
• to minutes

H. Bulow, et al., OFC 1999
J. Cameron, et al., OFC 1998
PMD temporal changes more rapidly with the fiber
length and average DGD
11
Fiber Nonlinearities
Refractive index depends on frequency and power

• Isolation of nonlinear effects is very difficult
• It is also difficult to monitor and compensate

n(?,P)
Chromatic Dispersion
Power
Chromatic dispersion changes the effects of
nonlinearity
4?10 Gb/s
Power Penalty (dB)
Dispersion Variation 4
50-ps RZ Pulses
Distance (km)
12
EDFA Gain


Deployed EDFA cross saturation causes gain
transientsdue to

• Channel turn-on
• Channel re-routing
• Network reconfiguration
• Link failures

EDFA
InputChannels
OutputChannels
EDFA
DroppedChannels
13
(No Transcript)
14
Time Response
1 dB power excursion for surviving channels
10
1.0
4 channels dropped
4 channels survive
7.5
0.75
s)
m
Reciprocal Time (
5.0
0.5
Time (
2.5
0.25
m
s
-1
)
0.0
0.0
0
2
4
6
8
10
12
of EDFAs
Zyskind, OFC96 PD-31
15
Outline

• Degradations Due to Physical-Layer Impairments
• Fast Monitoring of a Burst
• Fiber-Loop Buffers for OBS Efficiency

16
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
format
number of channels
power
nonlinearities
polarization effects
dispersion
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Monitoring in OBS Systems

• Monitoring includes
• - Power - Wavelength - Optical
  signal-to-noise ratio - Distortion CD, PMD,
  nonlinearities

• Monitoring time scale corresponds to that of OBS
  (?s ms)
• Dynamic monitoring covers the wide range of both
• multi-wavelength payloads and control packets

18
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

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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

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Vestigial Sideband Optical Filtering
Optical Carrier
?f
VSB-U
VSB-L
BW
fU
Frequency
fL
f0

• Filter BW (0.8 1.2) ?bit-rate (Rb)

• Filter detuning ?f (0.4 0.8) ? Rb

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Monitor Clock Phase

• Isolate CD from PMD effects
• Low cost

Entire channel
VSB-L
Filtered spectrum
40-Gb/sRZ Data
f
Dispersion
O/E
?t
VSB-U
f
Filtered spectrum

• Time delay ( ?t ) between two VSB signals is a
  function of CD
• Bits can be recovered from either part of the
  spectrum

Q. Yu, JLT, Dec., 2002
22
PMD Monitoring Techniques
B. RF spectrum analysis

• Simple
• Affected by other
• distortion sources
• Sensitivity and
• DGD range depends
• on monitored
• frequency

23
Outline

• Degradations Due to Physical-Layer Impairments
• Fast Monitoring of a Burst
• Fiber-Loop Buffers for OBS Efficiency

24
Research Goals (Generously Supported by Intel)
Control Unit
Control Line
Control Packet
Burst
(NM) x (NM)
Data Burst Lines
Switch
N
N M 8
Delay Lines
Optical Fiber Delay Lines
M

• 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

25
Optimal Number of Input Ports and Delay Lines
Buffered
(N,M) ? (N input data lines M
delay lines)
(4,4)
(5,3)
(7,1)
(4,0)
(6,2)
Buffer Size
of input ports 1st Buffer Kbytes 2nd Buffer Kbytes 3rd Buffer Kbytes 4th Buffer Kbytes
4 3 5.5 8 10
5 5.5 8 10 -
6 5.5 10 - -
7 10 - - -
Throughput Efficiency
(5,0)
(6,0)
(7,0)
Bufferless
Load

• (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 ?

26
Throughput Efficiency vs. Load for Different
Maximum Burst Sizes
Maximum 2 Kbytes burst size
Maximum 10 Kbytes burst size
Throughput Efficiency
Maximum 14 Kbytes burst size
Maximum 20 Kbytes burst size
Load

• The throughput efficiency decreases with increase
  in burst size.
• Buffer size max. burst size, 3 buffers for 5,3
  case.

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Effect of Adding Buffers on Throughput Efficiency
(4, 4) Switch
4 Buffers
3 Buffers
2 Buffers
Increase in Throughput Efficiency
1 Buffer
Bufferless
Load

• 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

28
Throughput Efficiency for Recirculation
(5, 3) Switch
Bufferless
1 Round Trip
Throughput Efficiency
2 Recirculations
3 Recirculations
5 Recirculations
10 Recirculations
Load

• With 3 recirculations the throughput efficiency
  of approximately 86 can be achieved.
• 5th recirculation increases the throughput by
  only 1.

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Increase in Throughput Efficiency with Buffers
and Recirculation
Bufferless
1 Buffer
Increase in Throughput Efficiency
2 Buffers
3 Buffers
3 Buffers with 2 recirculations
3 Buffers with 3 recirculations
Load

• 3 Buffers and 3 recirculations increase the
  throughput efficiency by 27
• Throughput efficiency does not increase linearly
  with number of delay lines

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Key Buffer Results for 8X8 Switch

• (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.

31
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.