Versatility of a colorless and directionless WSS based ROADM architecture

Abstract

In core/metro optical networks there are multi-degree nodes providing N path interconnections and alternate paths for protection. This paper presents the optimum implementation of a colorless and directionless add/drop switching feature in a mesh network, using the WSS based ROADM. The idea of scaling, viewed as an increase in route degree and number of wavelengths handled in each direction is then addressed. Subsequently, we suggest the best ways in which specific advantages like dynamically optimizing light paths by optical bridge-and-roll, avoiding O-E-O operations and blocking specific wavelengths can be leveraged. Finally, practical implementation issues like power loss, OSNR and cost considerations are discussed.

Full text

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Versatility of a colorless and directionless WSS based
ROADM architecture

Prasanna. G1, Sai Kishore. B2, Omprasad. G. K,
Sathyanarayana Raju. K, Gowrishankar. R,
Venkataramaniah. K
Department of Physics
Sri Sathya Sai University
Prasanthi Nilayam, India
1gprasanna.sssu@yahoo.co.in, 2mail2saikishore@gmail.com
Ron Johnson
Cisco Systems Inc.
San Jose
California, U.S.A
ronjohn@cisco.com
Pavan Voruganti
OpVista Inc.
Milpitas, California, U.S.A
pvoruganti@opvista.com


Abstract— In core/metro optical networks there are multi-degree
nodes providing N path interconnections and alternate paths for
protection. This paper presents the optimum implementation of a
colorless and directionless add/drop switching feature in a mesh
network, using the WSS based ROADM. The idea of scaling,
viewed as an increase in route degree and number of wavelengths
handled in each direction is then addressed. Subsequently, we
suggest the best ways in which specific advantages like
dynamically optimizing light paths by optical bridge-and-roll,
avoiding O-E-O operations and blocking specific wavelengths
can be leveraged. Finally, practical implementation issues like
power loss, OSNR and cost considerations are discussed.
Keywords- mesh optical networks, reconfigurable optical
add/drop multiplexer, wavelength selective switch, colorless and
directionless architecture, optical bridge-and-roll, O-E-O, MEMS.
I.
INTRODUCTION
The demands for higher bandwidths at lower costs are
increasing substantially in today’s communication networks.
The continued growth of video and other broadband services
are pushing the service providers and equipment vendors to
look for dynamic optical networks. Moreover, truck-rolls are
required to be sent in the field in order to re-assign the optical
paths and to launch new wavelengths, new equipment has to be
installed which increases both Capital- (CAPEX) and
Operational Expenditure (OPEX) for the service provider’s
network. Thus, it is very important for service providers to
attain flexibility in handling the churn and growth of the optical
network and also achieve complete control over every part of
the network. This has led to the development of
Reconfigurable All-Optical Networks (R-AONs) that can help
them to maximize network capacity, adapt to the surge of
traffic, reduce network complexity and provide transparent
wavelength services to customers, while also being cost
effective. The re-configurability mainly relies on the
integration of Reconfigurable Optical Add/Drop Multiplexer
(ROADM) modules in the present metro core optical networks.
This is achieved by the intelligent control plane that controls
optical networks dynamically in real time, providing a means
of both automating the services, and offering a range of
protection and restoration schemes, including sub-second
restoration, through enabling fully meshed optical networks.
This level of automation allows network operators to move
beyond simple ring configurations towards fully meshed
optical networks that not only allow the networks to survive
multiple failures, but also automates them for delivering new
services. The wavelength selective switch (WSS) based
ROADM supports colorless add/drop, per channel optical
power monitoring (OPM) and balancing, and multi-degree
interconnection. These valuable features make the WSS based
ROADM the choice for agile “all-optical” mesh networks.
Any agile optical mesh network calls for an architecture
which can dynamically route wavelengths in any desired
direction, terminate a certain wavelength and eliminate O-E-O
operations during ring-to-ring interconnections. In addition, the
architecture should be truly scalable to be future proof and
support the possibility of remote optimization of network
resources by optical bridge-and-roll during the evolution of the
mesh network. These needs can be fulfilled by a colorless and
directionless architecture [1]. We propose an optimum
implementation of the colorless and directionless WSS based
ROADM architecture. Subsequently, we address issues like
scalability (from the perspective of increase in number of
wavelengths and route degree), optical-bridge-roll and
elimination of O-E-O operations.
II.
DESIGN METHODOLOGY
Before presenting the details of the proposed architecture, it
is appropriate to begin with the description of a general design
methodology for an optical mesh network [2]. In transparent

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optical networks built using intelligent switches like ROADMs
the design is too complex to be accomplished by a single
holistic computer design tool. Therefore, the entire design
process is usually partitioned into different sub-modules with
effective heuristics. The sub-modules include routing and
ROADM choice, span engineering and power dynamics
simulation – each accomplished by a single class of design
tools [3].
The traffic pattern and the fiber network are the inputs to
the routing and ROADM choice module. The output is a set of
configured ROADMs that create the light paths required to
satisfy the routing demands. The second module, span
engineering requires the design resulting from the first phase
and accordingly chooses the optical amplifiers (OAs) and
dispersion compensating modules (DCMs) to correctly
mitigate all optical impairments, at minimal cost. The detailed
configuration produced by the first two modules is fed as the
input to the power dynamics simulation module. During the
addition of new services or failures (like fiber cuts) or transient
scenarios the channel power levels and amplifier working
change considerably. The power dynamics simulation module
simulates such scenarios to determine suitable counter
measures to restore stability to the network. In the following
section we present an optimum implementation of the colorless
and directionless WSS based ROADM architecture at the
optical layer. Designing a network based on this architecture
calls for a design methodology such as the one just described.
III. COLORLESS AND DIRECTIONLESS ARCHITECTURE
The colorless and directionless feature is defined with
regard to the functionality of the local add/drop feature at a

















Fig. 2 Colorless and directionless architecture for a 4 degree node
certain node in a mesh network. While colorless means the
ability to drop or add any wavelength at any port, directionless
refers to the ability to connect to all the directions or routes
from the local transponders. Consider the following sample
mesh network in Fig. 1.


Fig. 1 A mesh optical network

As shown, A is a 4-degree node. A colorless and
directionless architecture is realised at node A by using WSS
based ROADMs and multi-port splitter combiners as shown in
Fig. 2. The WSS shown are 1x9 devices.


















A
F
G
E
D
C
B

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Consider the connection along the west direction at node A.
Wavelengths coming from C encounter a splitter that provides
a 1:8 power-spilt. Of the eight ports in the splitter, three are
connected to the WSSs along the other three directions, two are
connected to the two WSSs that are local to node A, and the
rest are for scaling in the future. Therefore, the wavelengths
entering A from C can be routed in any direction, while
making all of them available at the local drop of node A too.
Similarly, the WSSs along the east, north, south directions and
the two local add combiners are connected to the S/C along the
west direction, providing a path to C from all directions. Thus
every direction is connected to every other direction creating
the possibility of routing a certain wavelength in any given
direction. The colorless drop feature is realised as follows. At
the local drop site, the WSS that is connected to the splitters in
different directions consolidates
Subsequently, another WSS can provide a colorless drop to its
ports. Since the number of wavelengths to be received could
exceed the number of ports available on the WSS, an
expansion facility is provided using a splitter prior to this WSS,
which can have multiple WSSs connected to it. The colorless
add feature is realised by using a WSS to consolidate all the
wavelengths that originate at A. Since any wavelength can be
added at a certain port of the WSS it becomes a colorless add.
Next a combiner is present to provide expansion much like that
addressed in the drop side. After this a splitter splits it 8 ways
providing connections to all the directions. It is to be noted that
the protection switch card (PSC) shown in Fig. 2, acts as a
splitter on the add side making the same wavelength available
at both the mux groups and also acts as a switch on the drop
side choosing the same wavelength from one of the demux
groups.
the wavelengths.
Along each direction the wavelengths encounter a WSS,
which either blocks or allows a certain wavelength along that
direction. Thus a wavelength that originates at A can
potentially be sent along any chosen direction, realising a
directionless feature. The idea of using the WSS instead of a
combiner at the add side is to make available the intelligent
functionalities offered by the WSS for all the add wavelengths.
In the event of a transient wavelength or power drift in an add
channel, the co-propagating channels face adverse effects like
severe cross talk or even complete attenuation. Because of the
channel specific blocking feature in a WSS, the drifting
channel alone can be blocked, whereas the combiner being a
passive device cannot provide such functionality.
IV. ADVANTAGES OF THE COLORLESS AND DIRECTIONLESS
ARCHITECTURE
Apart from the valuable features offered by a ROADM
based network, we suggest ways to leverage special advantages
like dynamic optimization of lightpaths by optical bridge-and-
roll, elimination of O-E-O operations and providing alternate
paths for protection by having a colorless and directionless
architecture in a mesh optical network.
A. Optical bridge-and-roll
Bridge-and-roll of wavelengths is a valuable management
tool in a mesh optical network. Using this feature, it is possible
for the network operators to perform a hitless reroute of an in-
service wavelength to allow for optimization of network
resources. When the network scales, newer nodes and thus
routes come into existence. Therefore the network operators
aspire to move a certain wavelength to its optimal path. To
perform the bridge-and-roll of a primary wavelength path, a
secondary bridged path is established via the ROADMs
between the connection end points. Once the secondary path is
established, the roll command is issued, and the connection is
switched from the primary to the secondary path. The
requirement for switching speeds is specific to the kind of
service offered. The colorless and directionless architecture is
very valuable in this regard as it permits a dynamic
optimization of lightpaths that can be remotely performed in a
hitless fashion.

Fig. 3 Phase 1 of a mesh optical network
Consider the network shown in Fig. 3. It is in the first phase
of its operation. The working and protection paths between
nodes E and B are E-D-C-B and E-A-C-B respectively. Note



Fig. 4 Phase 2 of a mesh optical network
A
F
E
D
C
B
G
Working path

Protection path
A
F
E
D
C
B
Working path

Protection path

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that both the paths are not entirely diversified as they share the
common arm C-B. The second phase of the network is shown
in Fig. 4 in which a new node G has come up and the presence
of new route G-B calls for the optimization of the link between
E and B. The colorless and directionless architecture at A
enables a dynamic bridge-and-roll to switch the protection path
from E-A-C-B to E-A-G-B. In Phase 1, as shown in Fig. 5(a)
the wavelength taking the path E-A-C-B, on entering node A
was split and sent to C and F. At A the WSS along F was
configured to block it, while the WSS towards C passed it
through. On the other hand, when the path is E-A-G-B in Phase
2, the WSSs along F and C are configured to block it, while it
is passed through by the WSS facing G, as shown in Fig. 5(b).
This diversifies the working and protection paths completely.
An alternate protection path can also be E-F-G-B. It is thus
evident that a colorless and directionless architecture
implemented not just at A but also at E and B enables such a
bridge–and-roll. Therefore, it is necessary to have a colorless
and directionless architecture at all the nodes in a mesh
network to carry out efficient bridge-and-roll operations. A
bridge-and-roll also becomes significant when shorter paths for
a certain link are available. For example if a route between A
and B appears during a later phase of operation, then the
communication link E-A-G-B could be rolled over to the
shorter path E-A-B.




Fig. 5 (a) Phase 1, (b) Phase 2 - operation at node A
B. Elimination of O-E-O
A wavelength conversion is performed in fixed architecture
based mesh optical networks when a wavelength contention is
to be resolved during ring interconnections. This typically
involves an optical-electrical-optical (O-E-O) operation
specific for that wavelength. Another situation where an O-E-O
operation is carried out is when two optical network domains
are to be interconnected. Evidently, this means the cost of the
nodal architecture increases due to additional transponders
involved in the O-E-O process for such wavelengths [4]. A
colorless and directionless nodal architecture enables the
routing of any wavelength in any given direction from that
node. The necessary power equalization is provided by the
WSS. Such features are not present in a fixed or a direction
dependent architecture where there is a limitation to the
direction in which a certain wavelength can be sent.
Consequently, for a given situation the fixed architecture
would incur more O-E-O operations compared to the colorless
and directionless WSS based ROADM architecture that can
transfer the wavelengths all-optically [5], resolving contentions
and performing power equalization.

Fig. 6 Phase 3 of a mesh optical network
Consider the 3rd phase of operation of the mesh optical
network shown in Fig. 6. Node H has been added, resulting in
an extra route between A and H. Consequently, node A has
become a 5 degree node. A wavelength contention occurs
when a service at a wavelength λ is to be set up between H and
G via A, while a service at the same wavelength already exists
between E and G via A.

Fig. 7 Routing at node A
A
F
G
E
D
C
B
H
Pass through

Blocked

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In a fixed architecture, a wavelength conversion would be
required at node A for the service from H to G in order to
resolve the contention. This is because of the limitation in the
fixed architecture that a certain wavelength can be sent only in
a particular direction fixed at the time of designing the
network. In contrast, a colorless and directionless architecture
can route the service from H to G as H-A-F-G, thus eliminating
the need for an O-E-O operation. At node A this service is
blocked in all directions except F. This is shown in Fig. 7.
Travelling further, at node F it is passed on to G and blocked
along E by the respective WSSs in those directions. It is also
important to note that these two services at the same
wavelength are to be resolved at node G. But since there are
two sets of WSS drop modules present at the local site of G,
the two services could be received independently. This is
shown in Fig. 8.

Fig. 8 Drop side at node G
C. Alternate paths for protection
Extensive protection mechanisms are available in the
SONET/SDH and IP layers [6]. Of the many optical protection
schemes 1+1 protection scheme and the MPLS fast re-route are
the most common. In a colorless and directionless architecture
a wavelength originating at certain node can be sent in any
desired direction and can be received from any direction at the
destination node. Consider a wavelength originating at A and
destined for G in the network shown in Fig. 6. The protection
path can be chosen as A-F-G while the working path remains
as A-G. At A the wavelength is allowed to pass through in
directions along G and F and blocked along C and E by the
respective WSSs. At G the wavelength is received from both
directions along A and F. It is to be noted that the protection
and working paths are not necessarily static and can be
changed dynamically as required in the event of any link
failure.
V.
SCALABILITY
Scalability refers to carrying increasing amounts of data
and service flexibly while minimizing the operational
complexity. The metro core backbone is becoming more
diverse geographically and it needs to support a large volume
of bandwidth. Many of the new services in residential,
industrial and scientific sectors like Video on demand (VoD),
IPTV, voice over IP (VoIP), distance learning, e-commerce, e-
health, IP virtual private networks (VPN) etc. are very difficult
to forecast in terms of bandwidth requirements and traffic
routing patterns. As a result, traffic patterns are becoming
diversified and unpredictable, thus creating a new operational
challenge for service providers to effectively forecast
bandwidth requirements at the numerous sites spread across the
network. Alternatively, providing high bandwidth connectivity
(e.g. a wavelength) to every single site requires a major capital
investment and results in a significant increase in network
complexity. Therefore, service providers are looking for a
flexible and cost-effective solution to dynamically provide ‘on
the fly’ high bandwidth connectivity to any network site
without interrupting the existing services or re-engineering the
network. In other words it means scaling the network as and
when required. A colorless and directionless network
architecture is truly scalable in this regard. In this paper, the
issue of scalability is addressed with respect to increase in the
number of wavelengths and route degree.
A. Scalability as the number of wavelengths increases
As shown in Fig. 2, the local site of node A has 1x9 WSSs
that provide a colorless add/drop for 9 wavelengths at the most.
As the mesh network scales, the number of wavelengths
handled by A may increase considerably. This issue is
addressed by having a 1:8 splitter-combiner before the drop
WSS and after the add WSS giving a flexibility to handle a
maximum of 72 wavelengths on the drop and add sides. While
a 1x9 WSS handles 72 wavelengths, a 1x5 only 40, and with
the emergence of a 1x20 WSS 160 wavelengths can be
handled.

Fig. 9 WSS count for a 4 degree node
As shown in Fig. 9, lesser number of higher port count
WSSs are required compared to lower port count WSSs as the
number of wavelengths per direction increases. Therefore, a
higher port count WSS can address scalability more efficiently.
B. Scalability as the route degree increases
In the third phase of operation of the mesh optical network
shown in Fig. 6, a new route between A and H comes up
making A a 5 degree node. The architecture shown in Fig. 2
can seamlessly scale to that in Fig. 10 without disrupting the