High performance optical networks
EITP10
Lecture 6: WDM networking
STEFAN HÖST
WDM
WDM (Wavelength division multiplexing)
• The optical layer (lower part of physical layer)
• Utilize the physical network infrastructure to build several optical networks
• Use different wavelengths to carry different traffic types over one physical fiber
• Circuit switched network on optical level
• Similar idea of network elements as SONET/SDH
Mainly ring topology
Stefan Höst EITP10 Lecture 6 1
WDM network example of lightpaths
Stefan Höst EITP10 Lecture 6 2
WDM elements
A WDM network is built with the following elements / functionallity
• OLT, Optical line terminal
End points for the optical paths
• OLA, Optical line amplifier
For long lines, every 80-120 km
• OADM, Optical add/drop module
Drops or adds one or more optical paths from main path
• ROADM, Reconfigurable OADM
Remotely recontrolled OADM, via software settings
• OXC, Optical crossconnect
Optical switch (remotely configureable)
• WC, Wavelength conversion
Change wavelength during optical path (normally before or after OXC switch)
Stefan Höst EITP10 Lecture 6 3
OLT (Optical line terminal)
• Transponder (wavelength convereter)
• Optical multiplexer/demultiplexer
• Optical amplifier
• Supervision channel
Stefan Höst EITP10 Lecture 6 4
Transponder
• Typically O/E/O conversion
• Convert from non ITU λ to ITU λ (ITU G.692)
• May add FEC
• End-to-end monitoring (e.g. BER)
• Typically expensive and
adds to power consumption
Stefan Höst EITP10 Lecture 6 5
ITU Frequency grid, G.692
• Defines a wavelength grid between 1528.77–1560.61 nm
• Spacing 50 GHz, 100 GHz or 200 GHz
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· · · · · ·
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Stefan Höst EITP10 Lecture 6 6
OADM, Optical add/drop multiplexers
• Add or drop one or several wavelength paths without disrupting the others
• Construction with
– MUX/deMUX (parallel)
based on dielecctric thin-film filters or arrayed waveguid grating (AWG)
– Single wavelength add/wavelength (serial)
based on Bragg grating or filtering
Stefan Höst EITP10 Lecture 6 7
OADM, parallel setup
Extract from multiplexer
• Few drops: high cost
• Many drops: low cost
Multiplex set of wavelengths
• Few drops: moderate cost
• Many drops: moderate cost
Stefan Höst EITP10 Lecture 6 8
Add/drop one wavelength
Construction based on Bragg grating and circulators
Bragg grating
[wikipedia]
Stefan Höst EITP10 Lecture 6 9
OADM, serial setup
Cascade several single wavelength OADM
• Few drops: low cost
• Many drops: high cost
Stefan Höst EITP10 Lecture 6 10
ROADM, Reconfigurable OADM
• Remotely reconfigurable settings
• Reconfigure on predefined set of wavelengths
Fix set of transponders
• Reconfigure on arbitrary wavelengths
Tubnable transponders, i.e. tunable lasers
Stefan Höst EITP10 Lecture 6 11
ROADM, fix wavelength
• Choose from multiplexed
wavelengths with (2× 2) optical
switch
• Must have switches for all
wavelengths
• Choose from predefined set of
single wavelength OADM
• Must have one OADM for each
wavelengths
Stefan Höst EITP10 Lecture 6 12
ROADM
From silicon photonics:
Stefan Höst EITP10 Lecture 6 13
ROADM, arbitrary wavelength
• Can add/drop any wavelength
• Requires tunable transponder (i.e.
tunable laser)
• More expensive but saves in
number of devices
• Use an optical switch
• Very close to OXC
Stefan Höst EITP10 Lecture 6 14
Optical switch
Optical
switch...
· · ·
• Remotely reconfigurable switching in area
• Constructions:
– Turnable micro mirrors (large areas)
– Opto-electrical conversion and electrical
switching
– Mach-Zehnder interferometers (MZI)
(Silicon photonics)
Stefan Höst EITP10 Lecture 6 15
Optical switch
Example of 4× 4 switch with 2× 2 controllable MZI elements
Stefan Höst EITP10 Lecture 6 16
OXC, Optical crossconnect
For use in large central office (CO), e.g. connections between metro rings
Stefan Höst EITP10 Lecture 6 17
Wavelength conversion
WLC typically done in electrical domain
WLC
WLC
WLC
WLC
WLC
WLC
WLC
WLC
Stefan Höst EITP10 Lecture 6 18
OXC, Optical crossconnect
• Remote reconfiguaration of large number of connections
• Performance monitoring and fault localisation
Connection of test equipment
• Protection against failures, Automatic rerouting
• Data rate transparency
• Wavelength conversion, before or after switching
Stefan Höst EITP10 Lecture 6 19
Lightpath topology design (Ex)
Example (10.1)
Consider a network with three IP nodes labeled A, B and C connected in series. At each
node there are routers with with 10 Gbps ports. Between each pair of nodes (A− B,
A− C and B − C) the estimed max load is 50 Gbps, i.e. five wavelengths between each
node pair. Construct a lightpath topology.
Stefan Höst EITP10 Lecture 6 20
Lightpath topology design (Ex)
Gives
Total number of router ports:
10 + 20 + 10 = 40
2 networks
Stefan Höst EITP10 Lecture 6 21
Lightpath topology design (Ex)
Gives
Total number of router ports:
10 + 10 + 10 = 30
3 networks
Stefan Höst EITP10 Lecture 6 22
Ring structure
Often WDM netwiorks are built on the physical infrastructure of a ring
• Flexible and can build most other network structures
• Built in redundancy (that can be extended in a natural way)
OLT 1
OLT 2 OLT 3
OLT 4
IP 1
IP 2 IP 3
IP 4
Stefan Höst EITP10 Lecture 6 23
Ring structure, Ex
Two connections between every neighbour.
Stefan Höst EITP10 Lecture 6 24
Ring structure, Ex
Star network.
Stefan Höst EITP10 Lecture 6 25
Ring structure, Ex
All to all connections
Stefan Höst EITP10 Lecture 6 26
Wavelength assignment
An algorithm for simple wavelength to light-path assignment (no cycles and no
wavelength conversion)
• Let L be the maximum number of parallel light-paths
• Number wavelengths from 1 to L
• Assign (one of) the left-most light-paths to λ1
• Go to next ligh-path and assing the lowest available wavelength to it.
Repeat until all light-paths assigned
Stefan Höst EITP10 Lecture 6 27
Wavelength assignment, Figure 10.18
Stefan Höst EITP10 Lecture 6 28
Wavelength assignment in ring, Figure 10.19
• Cut the ring open at node with fewest
light-paths crossing
• Assign non-cut light-paths
• Assign cut light-paths
(Proof of Th 10.3: new wavelengths)
Stefan Höst EITP10 Lecture 6 29

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