Overview on ROADM
Flexible Optical Network
• Functionality requirements of NG ROADMs
Technology building blocks of NG ROADMs
• Route and Select architecture
• Wavelength Switched Optical Network
• Benefits of NG ROADMs
ROADMs are evolving to support market demands for
increasing bandwidth and enhanced flexible
The trend towards dynamic capacity allocation
demands ROADMs to be-
Colorless, Directionless, Contentionless and Gridless.
ROADM evolution from 2-degree fixed wavelength
add-drop to present >= 8 degree CDC add-drop
Next-generation solution that offers true network
agility has put ROADM at the forefront of the race to
stay ahead of the traffic explosion.
Overview on ROADM
Fujitsu was the first vendor to develop and deploy ROADM technology
in 2003 and the first to incorporate a WSS-based optical switch fabric.
ROADM (Reconfigurable Optical Add Drop Multiplexer) is an optical
network element which is able to Add/Drop or Pass through any
ROADM along with optical amplifiers, multiplexer-demultiplexer,
transponder or/and muxponder cards constitutes a complete, flexible,
optical transport node.
All optical subsystem which enables remote configuration of
wavelengths at any time.
Technologies used- Wavelength blocking, Planar Lightwave Circuit
(PLC), Wavelength Selective Switch (WSS).
Per wavelength SW provisioning and
No re-engineering when capacity is
exceeded as in fixed OADM.
Network requires fewer manual
Software configuration reduces
erroneous cabling errors.
Flexible Optical Networks
WSS based ROADMs have been deployed widely in
networks through out the world –they are functionally
efficient but not flexible.
Flexibility is required to move towards next phase of optical
First generation ROADMs architectures are limited by:
-Fixed wavelength assignments to specific ports.
-Fixed direction assignments for multiplexers (i.e. North
only, South only…)
-Partitioned add/drop structures due to wavelength
Need for NEXT GENERATION ROADMs …
Functionality requirements of NG ROADMs
COLORLESS or COLOR INDEPENDENT functionality
DIRECTIONLESS or DIRECTION INDEPENDENT
CONTENTIONLESS or WAVELENGTH CONTENTION
GRIDLESS or FLEXIBLE ITU GRID functionality
High-Level NG ROADM Requirements
1 Multi-degree ROADM
N = 8-16 for Metro
N = 6-8 for Long Haul
9.6 Tb capacity per fiber
Flexible bandwidth allocation
Future proof express path
High level of integration
Supervision and Monitoring
Current Transponder Pools support
Directed V/s Directionless ROADM
•Fixed add/drop ports
for particular direction
•Change in direction
rewiring by technician
Figure : 3rd degree ROADM with directed architecture
NxN Switch FabricNxN Switch FabricNxN Switch Fabric
In directional ROADM recabling
of transponders needed.
In directionless ROADM ,
no recabling is needed
Colored V/s Colourless ROADM
Colored access ports imply that
physical access ports (add or drop)
are assigned to a specific wavelength.
Physically change the fiber
connectivity between the transceiver
and access ports to change
No dynamic wavelength change.
In colorless architecture, without
changing port the color of the
wavelength can be reconfigured.
No recabling needed.
Even with colorless and directionless functionality,
the ROADM network is still limited by the total
number of flexible CD add/drop ports available.
If two wavelengths of same color converge at same
add/drop port of WSS -
Leads to Wavelength blocking /wavelength contention!
Solution- Replace CD mux/demux units with new
set of add/drop units called Contentionless
add/drop units (n X m unit instead of 1 X n /n X 1)
What is Contentionless Add/Drop ?
Contention-less – In
the same Add/Drop
device you can add
and drop the same
frequency to multiple
Wavelength can be
routed from any
Add/Drop port to any
direction in software.
Colorless – ROADM
ports are not
(re-tuned laser does
not require fiber
Increased usage of video and online
applications through new devices such
as smart phones and tablets-
demands more BANDWIDTH
Need for faster network and higher
Pressure on optical networks!
1) Increase number of channels :80 96 100 150
More channels= more spectrum space.
But problem of physics!
2) Allocate bandwidth flexibly instead of 50GHz
Not a standard!
3) Signal shaping- DSP in transmitters of transponder
- Nyquist filtering
Gridless or Flexible GRID
Flexible Grid/Gridless implies a more granular version
of the spectrum usage, down to 12.5GHz (ITUT
Hence, ROADM nodes supporting a flexible grid could
operate at any speed that is based on increments of
This ensures optimum and wise usage of spectrum.
Moreover, it opens a gate for dynamic allocation and
usage of the ITU Grid.
Isn’t it AMAZING!
Technology building blocks of NG ROADMs
•Wavelength selective switch (WSS) is the
heart of current generation ROADM networks also for the
•CDC ROADM is nothing but N X M WSS capable of
switching wavelength from multiple input port to several
WSS Year of deployment
2 x 1 2003
9 x 1 2007
16 x 1 2012
Legacy ROADM – Broadcast & Select
• Ingress channels from each degree are
passively split (broadcast) to all other
degrees (plus the per-degree add/drop)
• Mux WSS blocks all channels not
intended for that degree (selects those
• Channel isolation becomes difficult as
the number of degrees increases (creates
large penalty for 16 QAM channels)
• directionless add/drop consumes an
available degree; Colorless requires
add/drop WSS ( using a larger split
ratio on the ingress not practical from a
network OSNR requirement)
“Broadcast and Select” Architecture
ROADM – Route & Select
• Demux WSS ‘routes’ any combination
of waves from COM-RX to any
output port (drop and other degrees)
• Mux WSS ‘selects’ any combination
of waves from its input ports (add and
other degrees) to COM-TX
• Channels are isolated by both the
ingress and egress WSS, improving
• By eliminating the splitter, insertion
loss is reduced, preserving channel
• Enables directionless and colorless at
“Route and Select” Architecture
WSON (wavelength swithed optical network) is a standards based GMPLS
control plane which imparts intelligence to the optical layer.
Two types of control plane are prominent : Peer type and overlay type.
WSON is an overlay based control plane.
Control plane can be centralised or distributed.
Looking ahead for – Agile Optical Layer!
Wavelength Switched Optical Network
Complete Control in Software, No Physical
Benefits of NG ROADMs
Wavelength and Route flexibility (with CDC
Automatic wavelength restoration
Load balancing as network and traffic evolves
Support for datarates beyond 100Gb/s
Fully automated link provisioning
Next-generation ROADM devices prepares service
providers to meet today’s traffic needs and the needs of
“Gridless” or flex spectrum enables mixed channel
plans adjustable in software to accommodate a mix of
today’s modulation as well as future modulations.
Firstly, we would like to thank Prof. Dr. Chen for giving
us an opportunity to share a gist of our knowledge in
next generation optical networks.
Also , we thank our friends and colleagues for their
immense support and co-operation for making this
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