wavelength division multiplexing
TRANSCRIPT
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Wavelength Division Multiplexing
www.swedishcr.weebly.com
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Multiplexing
• Multiplexing– a process where multiple analog message signals or digital data
streams are combined into one signal over a shared medium
• Types– Time division multiplexing
– Frequency division multiplexing
• Optically– Time division multiplexing
– Wavelength division multiplexing
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WDM wavelength-division multiplexing (WDM) is a
technology which multiplexes a number of optical carriers signals onto a single optical fiber by using different Wavelengths (i.e. colors) of laser light.
This technique enables bidirectional communications over one strand of fiber.
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Problem: Demand for massive increases in capacity
Immediate Solution: Dense Wavelength Division Multiplexing
Longer term Solution: Optical Fibre Networks
Problems and Solutions
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Multiple channels of information carried over the same fibre, each using an individual wavelength
A communicates with X and B with Y as if a dedicated fibre is used for each signal
Typically one channel utilises 1320 nm and the other 1550 nm
Broad channel spacing, several hundred nm
Recently WDM has become known as Coarse WDM or CWDM to distinguish it from DWDM
Wavelength Division
Multiplexer
Wavelength Division
Demultiplexer1
A
2B
1X
2Y1 2
Fibre
WDM Overview
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Multiple channels of information carried over the same fibre, each using an individual wavelength
Attractive multiplexing technique
High aggregate bit rate without high speed electronics or modulation
Low dispersion penalty for aggregate bit rate
Very useful for upgrades to installed fibres
Realisable using commercial components, unlike OTDM
Loss, crosstalk and non-linear effects are potential problems
Wavelength Division
Multiplexer
Wavelength Division
Demultiplexer1A2
3B
C
1 X2
3Y
Z1 2 + 3
Fibre
WDM Overview
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Multiple channels of information carried over the same fibre, each using an individual wavelength
Unlike CWDM channels are much closer together
Transmitter T1 communicates with Receiver R1 as if connected by a
dedicated fibre as does T2 and R2 and so on
Wavelength Division
Multiplexer
Wavelength Division
Demultiplexer1
T12
N
T2
TN
1R1
2
N
R2
RN
Source: Master 7_4
1 2 ... N
Fibre
Simple DWDM System
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Dense WDM is WDM utilising closely spaced channels
Channel spacing reduced to 1.6 nm and less
Cost effective way of increasing capacity without replacing fibre
Commercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fibre
Allows new optical network topologies, for example high speed metropolitian rings
Optical amplifiers are also a key component
www.swedishcr.weebly.com
DWDM
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DWDM Advantages and Disadvantages
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Greater fiber capacity
Easier network expansion
No new fibre needed
Just add a new wavelength
Incremental cost for a new channel is low
No need to replace many components such as optical amplifiers
DWDM Advantages
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DWDM Components
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Transmitters
DWDM Multiplexer
Power Amp
Line Amp
Line Amp
Receive Preamp
200 km
DWDM DeMultiplexer
Each wavelength behaves as if it has it own "virtual fibre"
Optical amplifiers needed to overcome losses in mux/demux and long fibre spans
Receivers
Optical fibre
DWDM System
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Transmitters
DWDM Multiplexer
Power Amp
Line Amp
Line Amp
Optical fibre
Receive Preamp
DWDM DeMultiplexer
Receivers
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Passive Components: Gain equalisation filter for fibre amplifiers Bragg gratings based demultiplexer Array Waveguide multiplexers/demultiplexers Add/Drop Coupler
Active Components/Subsystems: Transceivers and Transponders DFB lasers at ITU specified wavelengths DWDM flat Erbium Fibre amplifiers
DWDM: Typical Components
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Transmitters
DWDM Multiplexer
Power Amp
Line Amp
Line Amp
Receive Preamp
200 km
DWDM DeMultiplexer
Each wavelength behaves as if it has it own "virtual fibre"
Optical amplifiers needed to overcome losses in mux/demux and long fibre spans
Receivers
Optical fibre
DWDM System
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Transmitters
DWDM Multiplexer
Power Amp
Line Amp
Receive Preamp
200 km
DWDM DeMultiplexer
Each wavelength still behaves as if it has it own "virtual fibre"
Wavelengths can be added and dropped as required at some intermediate location
ReceiversAdd/Drop Mux/Demux
Optical fibre
DWDM System with Add-Drop
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Manufacturer&
System
Number of Channels
Channel Spacing
Channel Speeds
Maximum Bit RateTb/s
Nortel OPtera 1600 OLS
160 0.4 nm 2.5 or 10 Gb/s
1.6 Tbs/s
Lucent 40 2.5
Alcatel
MarconiPLT40/80/160
40/80/160 0.4, 0.8 nm 2.5 or 10 Gb/s
1.6 Tb/s
Typical DWDM Systems
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up to 600-700 km
L L RP
160-200 km
RP
700 + km
RL LP 3R Regen
Animation
Power/Booster Amp
Receive Preamp
Line Amp
P
R
L
Opt
ica
l A
mpl
ifier
s
DWDM System Spans
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Aggregate span capacities up to 320 Gbits/sec (160 Gbits/sec per direction) possible
Red band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm
Nortel S/DMS Transport System
Nortel DWDM
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8 wavelengths used (4 in each direction). 200 Ghz frequency spacing
Incorporates a Dispersion Compensation Module (DCM)
Expansion ports available to allow denser multiplexing
Red band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm
Nortel DWDM Coupler
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16 wavelengths used (8 in each direction). Remains 200 Ghz frequency spacing
Further expansion ports available to allow even denser multiplexing
Red band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm
Sixteen Channel Multiplexing
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32 wavelengths used (16 in each direction). 100 Ghz ITU frequency spacing
Per band dispersion compensation
Red band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm
32 Channel Multiplexing
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DWDM Transceivers and Transponders
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Transmission in both directions needed.
In practice each end has transmitters and receivers
Combination of transmitter and receiver for a particular wavelength is a "transceiver"
DWDM Multiplexer
Power Amp
Line Amp
Receive Preamp
DWDM DeMultiplexer
Re
ceiv
ers
Tra
nsm
itte
rs
DWDM Multiplexer
Power Amp
Line Amp
Receive Preamp
DWDM DeMultiplexer
Tra
ns
ce
ive
r
DWDM Transceivers
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In a "classic" system inputs/outputs to/from transceivers are electrical
In practice inputs/outputs are SDH, so they are optical, wavelength around 1550 nm
In effect we need wavelength convertors not transceivers
Such convertors are called transponders.
1550 nmSDH
1550 nmSDH
C band signal
C band signal
Transceivers .V. Transponders
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Full 3R transponders: (power, shape and time)
Regenerate data clock Bit rate specific More sensitive - longer range
2R transponders also available: (power, shape)
Bit rate flexible Less electronics Less sensitive - shorter range
Luminet DWDM Transponder
DWDM Transponders (II)
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Bidirectional Transmission using
WDM
Source: Master 7_4
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Source: Master 7_4
Local Transceiver Distant TransceiverFibres x2
Transmitter Receiver
Receiver Transmitter
Most common approach is "one fibre / one direction"
This is called "simplex" transmission
Linking two locations will involve two fibres and two transceivers
Conventional (Simplex) Transmission
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A
B
WDMMux/Demux
AA
B
Receiver
Transmitter
Local Transceiver
WDMMux/Demux
B
A
B
Receiver
Transmitter
Distant TransceiverFibre
Significant savings possible with so called bi-directional transmission using WDM
This is called "full-duplex" transmission
Individual wavelengths used for each direction
Linking two locations will involve only one fibres, two WDM mux/demuxs and two transceivers
Bi-directional using WDM
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Different wavelength bands are used for transmission in each direction
Typcially the bands are called: The "Red Band", upper half of the C-band to 1560 nmThe "Blue Band", lower half of the C-band from 1528 nm
Red Band
FibreBlue BandReceiver
1BReceiver2B
Receiver
nB
1RTransmitter
Transmitter
nRTransmitter
DWDMMux/Demux
2R
Receiver1R Receiver2R
Receiver
nR
1B Transmitter
Transmitter
nB Transmitter
DWDMMux/Demux
2B
Bi-directional DWDM
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To avoid interference red and blue bands must be separated
This separation is called a "guard band"
Guard band is typically about 5 nm
Guard band wastes spectral space, disadvantage of bi-directional DWDM
Blue channel
band
Red channel
band
1528 nm 1560 nm
Guard
Band
The need for a Guard Band
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DWDM Issues
Crosstalk between Channels
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With DWDM the aggregate optical power on a single fibre is high because:
Simultaneous transmission of multiple optical channels
Optical amplification is used
When the optical power level reaches a point where the fibre is non-linear spurious extra components are generated, causing interference, called "crosstalk"
Common non-linear effects:
Four wave mixing (FWM)
Stimulated Raman Scattering (SRS)
Non-linear effects are all dependent on optical power levels, channels spacing etc.
Non-linear Effects and Crosstalk
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Coarse Wavelength Division Multiplexing
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WDM with wider channel spacing (typical 20 nm)
More cost effective than DWDM
Driven by: Cost-conscious telecommunications environment
Need to better utilize existing infrastructure
Main deployment is foreseen on: Single mode fibres meeting ITU Rec. G.652.
Metro networks
Coarse Wavelength Division Multiplexing
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CWDM is a cheaper and simpler alternative to DWDM, estimates point to savings up to 30%
Why is CWDM more cost effective?
Less expensive uncooled lasers may be used - wide channel spacing.
Lasers used require less precise wavelength control,
Passive components, such as multiplexers, are lower-cost
CWDM components use less space on PCBs - lower cost
DFB laser, typical temperature drift 0.08 nm
per deg. CFor a 70 degree
temperature range drift is5.6 nm
Why CWDM?
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A clear migration route from CWDM to DWDM is essential
Migration will occur with serious upturn in demand for bandwidth along with a reduction in DWDM costs
Approach involves replacing CWDM single channel space with DWDM "band"
May render DWDM band specs such as S, C and L redundant?
CWDM Migration to DWDM