Transcript Optical Burst Switching (OBS): Issues in the Physical Layer A. E. Willner

Optical Burst Switching (OBS):
Issues in the Physical Layer
A. E. Willner
University of Southern California
Los Angeles, CA
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
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
Speed of Light in Vacuum
Photon Velocity (f) =
Index of Refraction(f)
• Information Bandwidth of Data
0 1 1 0 1 0
Fourier
Vi
Vj
Vk
fcarrier
time
freq.
• Temporal Spreading  f (distance, (bit rate)2) (ps/nm)/km
time
Fiber
time
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
Payload C.P.
t
Node
t
Node
Node
Node
t
Offset Time Affected by Wavelength Skew:
Uncompensated Systems (2.5 Gbit/s Payload?)
C.P.
t
30 nm
Payload
t
400 km of Fiber
(CD=17 ps/(nm.km))
Offset
C.P.
t
Skew
Payload
t
Offset
Offset time change ~ 1 s
Eye closure Penalty (dB)
Value of Tunable Dispersion Compensation
(40 Gbit/s Payload)
No Compensation
5
Fixed 80 km Compensator
OC-768
4
3
Tunable
Compensator
(500-2100 ps/nm)
2
1
0
0
20 40 60 80 100 120 140 160
Distance (km)
A tunable dispersion compensator allows for a wide
range of transmission distances at 40 Gbit/s.
Polarization-related Impairments in HighPerformance 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
Random polarization
coupling
Bit-rate and
wavelength
dependent
Polarization Mode Dispersion (PMD)
cross section
side view
Elliptical Fiber Core
1st-order PMD = DGD
The 2 polarization modes propagate at different speeds.
Probability of Exceeding a Specific DGD (%)
10
0.1
1
Probability
Distribution
50
Maxwellian
distribution
tail
0
10
20
30
40
Differential Group Delay (ps)
50
• PMD induces
randomly changing
degradations.
• Critical limitation at
 10 Gbit/s payload
data rates.
Time Rate of PMD Change
2.5
2.0
48.8 km buried cable
Temp. (C)
(b) Fast Fluctuation
1.5
Occurrence
PMD (ps)
(a) Slow Fluctuation
Frequency of occurrence
induced by PMD
fluctuation
52 km fiber
< >=2.8 ps
18
14
10
0
400
Time (min)
800
• PMD variations due to temperature
changes: hours to days
J. Cameron, et al., OFC 1998
Time Span (ms)
• Mechanical vibrations: milliseconds
to minutes
H. Bulow, et al., OFC 1999
PMD temporal changes more rapidly with the fiber length and average DGD
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
50ps Pulse (+)
50ps Pulse (0)
50ps Pulse (-)
Power Penalty (dB)
Chromatic dispersion changes the effects of nonlinearity
wdm
-28
6
410 Gb/s
-29
5
Link Dispersion
0.4 ps/nm/km
-30
4
0.08 ps/nm/km
-0.2 ps/nm/km
-31
3
-32
2
-33
50-ps RZ Pulses
1
-34
0
-35
00
500
500
1000
1000
1500
1500
Distance (km)
2000
2000
Dispersion
Variation
~ 4%
EDFA Gain
Deployed EDFA cross saturation causes gain transients
due to:
Time scale of
• Channel turn-on
gain saturation
• Channel re-routing
and recovery is
• Network reconfiguration
• Link failures
~ µs to ms
EDFA
Input
Channels
Output
Channels
EDFA
Dropped
Channels
Fiber Nonlinearity Penalties
Power Fluctuations
Single Mode Fiber
16 ch System
15 Chs
dropped
15 Chs added
15 Chs dropped
Power of the surviving channel
increases up to 14 dB
15 Chs
added
Large penalties in surviving
channel due to SPM
10 Gb/s Simulation Results
Hayee, OFC’99 ThU2
Time Response
1 dB power excursion for surviving channels
10
1.0
Time (s)
7.5
0.75
5.0
0.5
2.5
0.25
0.0
0.0
0
2
4
6
8
# of EDFAs
10
12
Reciprocal Time (s-1)
4 channels dropped
4 channels survive
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
format
number of
channels
bit rate
power
nonlinearities
polarization
effects
dispersion
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
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
Optical Carrier
f
VSB-U
VSB-L
BW
fU f0
fL
Frequency
• Filter BW = (0.8 ~ 1.2) bit-rate (Rb)
• Filter detuning f = (0.4 ~ 0.8)  Rb
Monitor Clock Phase
• Isolate CD from PMD effects
• Low cost
Entire
channel
VSB-L
40-Gb/s
RZ Data
Intensity
Filtered
spectrum
1.5
0.5
0.0
f
O/E
VSB-U
f
Filtered
spectrum
Intensity
Dispersion
1.0
0
50
100
150
Time (ps)
t
1.5
1.0
0.5
0.0
0
50
100
150
Time (ps)
• 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
PMD Monitoring Techniques
A.
Eye opening
measurement
B.
RF spectrum
analysis
– Requires high- speed
devices (demonstrated
for 160 Gb/s RZ signal)
– Affected by other
distortion sources
+ Can be integrated
with electronic
equalization
+ Simple
– Affected by other
distortion sources
– Sensitivity and
DGD range depends
on monitored
frequency
C.
Degree of
polarization (DOP)
measurement
+ No high speed electronics
+ Depends only on PMD
+ Bit-rate independent
+ Unaffected by other
distortion sources
– Pulse-width dependent
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)
Control Line
Control Unit
Control Packet
Burst
(N+M) x (N+M)
Data Burst
Lines
N
Delay Lines
M
•
•
•
•
Switch
N+M=8
Optical Fiber
Delay Lines
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
Optimal Number of Input Ports and
Delay Lines
Throughput Efficiency
Buffered
(5,3)
(4,0)
(N,M)  (N input data lines
(4,4)
M delay lines)
(7,1)
(6,2)
(5,0)
(6,0)
(7,0)
Bufferless
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
-
-
-
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 ?
Throughput Efficiency vs. Load for
Different Maximum Burst Sizes
Maximum = 2 Kbytes
Throughput Efficiency
burst size
Maximum = 10 Kbytes
burst size
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.
Effect of Adding Buffers on
Throughput Efficiency
Increase in
Throughput Efficiency
(4, 4) Switch
4 Buffers
3 Buffers
2 Buffers
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
Throughput Efficiency
Throughput Efficiency for Recirculation
(5, 3) Switch
Bufferless
1 Round Trip
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%.
Increase in Throughput
Efficiency
Increase in Throughput Efficiency
with Buffers and Recirculation
Bufferless
1 Buffer
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
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.
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.