Chromatic & Polarization Mode Dispersion

8/8/2019 Chromatic & Polarization Mode Dispersion

1/31

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

University of Southern California

Los Angeles, CA

A. E. Willner


2/31

O-E-O

OffsetTime

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


3/31

Outline

1. Degradations Due to Physical-Layer

Impairments

2. Fast Monitoring of a Burst

3. Fiber-Loop Buffers for OBS Efficiency


4/31

Signal Degradation due to Chromatic Dispersion

0 1 0 01 1time fcarrier freq.

Vi

Vj

VkFourier

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)


5/31

Chromatic Dispersion Effects on Payload and

Control Packet

Control Packet (C.P.), not payload, is regeneratedat every node

C.P. has lower bit-rate (CD effect (bit-rate)2 )There is higher chance for payload to be degraded

Node

Node

Node

Node

t

tt

t

Payload C.P.


6/31

Offset Time Affected by Wavelength Skew:

Uncompensated Systems (2.5 Gbit/s Payload?)

t

t

30 nm400 km of Fiber

(CD=17 ps/(nm.km))

t

t

C.P.

Payload

Offset time change ~ 1 s

C.P.

Payload

Skew

Offset

Offset


7/31

Value of Tunable Dispersion Compensation

(40 Gbit/s Payload)

Distance (km)

0

1

2

3

4

5

0 20 40 60 80 100 120 140 160

OC-768

No Compensation

TunableCompensator(500-2100 ps/nm)

Fixed 80 km Compensator

Eye

closu

reP

ena

lty(d

B)

A tunable dispersion compensator allows for a wide

range of transmission distances at 40 Gbit/s.


8/31

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 timeBit-rate and

wavelength

dependent

Polarization state

generally unknown

and wanders


9/31

Polarization Mode Dispersion (PMD)

cross section

Elliptical Fiber Core

side view

0 10 20 30 40 50

0.111050

Probability of Exceeding a Specific DGD (%)

Differential Group Delay (ps)

Maxwellian

distribution

tail

0 10 20 30 40 500 10 20 30 40 50

0.111050

Probability of Exceeding a Specific DGD (%)

Differential Group Delay (ps)

Maxwellian

distribution

tail

PMD induces

randomly changingdegradations.

Critical limitation at

10 Gbit/s payload

data rates.

The 2 polarization modes propagate at different speeds.

1st-order PMD = DGD


10/31

Frequency of occurrence

induced by PMD

fluctuation

Time Span (ms)

Occurrence

52 km fiber

=2.8ps

(b) Fast Fluctuation

Time Rate of PMD Change

PMD(p

s)

1.5

2.0

2.5

10

14

18

Temp.(

C)

Time (min)

0 400 800

48.8 km buried cable

PMD temporal changes more rapidly with the fiber length and average DGD

(a) Slow Fluctuation

PMD variations due to temperaturechanges: hours to days

J. Cameron, et al., OFC 1998

Mechanical vibrations: millisecondsto minutes

H. Bulow, et al., OFC 1999


11/31

Fiber Nonlinearities

-35

-34

-33

-32

-31

-30

-29

-28

0 500 1000 1500 2000

wdm

50ps Pulse (+)50ps Pulse (0)

50ps Pulse (-)

15001000500 20000

6

5

4

3

2

1

0

50-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/sChromatic dispersion changes the effects of nonlinearity

Refractive index depends on frequency and power

n( ,P)Chromatic Dispersion Power

Power P

enalt

y(dB)

Isolation of nonlineareffects is very difficult It is also difficult to

monitor and compensate


12/31

EDFA GainDeployedEDFAcross saturation causesgain transients

due to:

Channel turn-on

Channel re-routing Network reconfiguration Link failures

Time scale of

gain saturation

and recovery is

~ s to ms

InputChannels

Dropped

Channels

EDFA

EDFA

OutputChannels


13/31

Power of the surviving channel

increases up to 14 dBPower Fluctuations

-5

0

5

10

15

0 200 400 600 800 1000

1

2

52010

15 Chs dropped

15 Chs added

Time (usec)

16 ch System15 Chs added15 Chs dropped Hayee, OFC99 ThULarge penalties in survivingchannel due to SPM

Single Mode Fiber

0

5

10

15

20

25

30

0 200 400 600 800 100

Time (usec)

15 Chsdropped

15 Chsadded

1 EDFA

10

20

15 Chs

dropped

15 Chs

added

Fiber Nonlinearity Penalties10 Gb/s Simulation Results


14/31

0 2 4 6 8 10 12

# of EDFAs

Time (

s)

Recip

rocal

Time(

s -1)

10

7.5

5.0

2.5

0.0

1.0

0.75

0.5

0.25

0.0

1 dB power excursion for surviving channels

4 channels dropped4 channels survive

Time Response

Zyskind, OFC96 PD-31


15/31

Outline


Impairments




16/31

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


17/31

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


18/31

Impact of Monitoring on OBS Systems

Need to find the non-catastrophic problemsin 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


19/31

Impairment- & Security-Aware Routing

Present network : very few variables (i.e. # of hops)

are used to determine the routing table although thereare several variables on the physical state Future networks:

Monitor the channel quality and link security

and update the routing look-up tablescontinually

In the routing decisions ensure that: Channels achieve acceptable BER

Network achieves sufficient transmission andprotection capacity

Highest priority data is transmitted on the strongestand most secure links


20/31


21/31

40-Gb/s

RZ Data

VSB-L

VSB-U

f

Dispersion

f

O/E

0 50 100 1500.0

0.5

1.0

1.5

Time (ps)

0 50 100 1500.0

0.5

1.0

1.5

Time (ps)

t

Monitor Clock Phase

Isolate CD from PMD effects

Low cost

Q. Yu, JLT, Dec., 2002

Filteredspectrum

Entirechannel

Filteredspectrum

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


22/31

PMD Monitoring Techniques

Requires high- speed

devices (demonstratedfor 160 Gb/s RZ signal)

Affected by other

distortion sources

+ Can be integratedwith electronic

equalization

A.

Eye openingmeasurement

B.

RF spectrumanalysis

+ No high speed electronics

+ Depends only on PMD

+ Bit-rate independent

+ Unaffected by other

distortion sourcesPulse-width dependent

C.

Degree ofpolarization (DOP)

measurement

+ Simple

Affected by other

distortion sources

Sensitivity and

DGD range depends

on monitored

frequency


23/31

Outline


Impairments



R h G l


24/31

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


25/31

Optimal Number of Input Ports and

Delay Lines

Throug

hp

utE

fficiency

(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 linesM delay lines)(7,1)

(7,0)

# ofinputports

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 - - -

Buffer Size

Th h t Effi i L d f


26/31

Throughput Efficiency vs. Load for

Different Maximum Burst Sizes

Load

ThroughputE

fficien

cy

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

Eff t f Addi B ff


27/31

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

Increa

sein

Throug

hputEfficien

cy

1 Buffer

2 Buffers

3 Buffers

Bufferless

4 Buffers

(4, 4) Switch

Load


28/31

Throug h

putEfficien

cy

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

Increase in Throughput Efficiency


29/31

Load

1 Buffer

2 Buffers

3 Buffers3 Buffers with 2

recirculations

3 Buffers with 3

recirculations

Bufferless

Incr

easein

Throu

ghp

utEffic

iency

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 ofdela lines


30/31

(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

S


31/31

Summary

Degradation effects including CD, PMD,

nonlinearities should be addressed in OBS.

Fast monitoring can help the long-term stabilityand robustness of a OBS network.

Optical buffers enable enhanced OBSfunctionality.

Documents

Chromatic & Polarization Mode Dispersion