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Liaison statement
Issues for a Control Plane for Wavelength Switched Optical Networks

State Posted
Submitted Date 2009-03-11
From Group ccamp
From Contact Adrian Farrel
To Group ITU-T-SG-15-Q6
To Contacts Greg Jones <>
Cc Peter Stassar <>
Ross Callon <>
Dave Ward <>
Scott Bradner <>
Pete Anslow <>
Francesco Montalti <>
Response Contact Adrian Farrel <>
Deborah Brungard <>
Technical Contact Adrian Farrel <>
Deborah Brungard <>
Purpose For information
Attachments (None)
Dear Peter,

CCAMP experts are looking forward to meeting with Q6/5 on March 20th to discuss
optical impairments and the control plane operation of wavelength switched
optical networks (WSONs). Many thanks for the invitation.

This liaison is to summarize the activity within CCAMP on this subject so far
and to set out our objectives for this work. We hope it will help provide a
stimulus for productive discussions.

As you will be aware, the GMPLS control plane is designed to provide a dynamic
control plane for a variety of switching technologies. Amongst these is the
"lambda switch capable" data plane where devices are OEOs, ROADMs, and photonic
cross-connects (PXCs). In fact, lambda switching was the technology that led to
the development of GMPLS from the packet switching MPLS control plane.

The IETF's CCAMP working group is the design authority for all extensions to
the GMPLS family of protocols.

The original work on lambda switching networks within CCAMP recognized that
there is a subset of optical networks in which it is possible to disregard
optical impairments and where the number of regeneration points is high. In
these environments, path computation can be performed on a reachability graph,
and lambda conversion can be performed as necessary within the network.

As PXCs were introduced into WSONs, it remained the case that optical
impairments could be disregarded by the control plane. Where necessary, optimal
impairment-aware paths could be computed off-line and supplied to the control
plane, leaving the control plane to handle establishment of connections and
recovery after failure. Failure recovery scenarios might lead to contention for
wavelengths or suboptimal optical paths, but these could be handled by
crankback within the signaling protocol.

More recent work on WSONs indicates that the proportion of pure optical devices
(ROADMs and PXCs) is increasing. This means that it is necessary to compute
paths that offer end-to-end lambda continuity. This problem (called the routing
and wavelength assignment (RWA) problem) must be solved, and may be compounded
by devices with limited cross-connect capabilities (for example, with
glass-through, a limited OEO matrix, or restricted port-to-port capabilities).
In approaching this problem it is convenient if there is a common
identification scheme for wavelengths across the whole network previously,
wavelength identification was a local matter between the nodes at the ends of
each link). To aid with this, the CCAMP working group has developed
that provides a protocol-independent encoding for wavelengths in a way that is
compliant with G.694. Further work on this problem space can be seen in the
following CCAMP documents:

"Framework for GMPLS and PCE Control of Wavelength Switched Optical Networks

"Routing and Wavelength Assignment Information Model for Wavelength Switched
Optical Networks"

"Routing and Wavelength Assignment Information Encoding for Wavelength Switched
Optical Networks"

CCAMP participants have further identified cases where they believe it would be
helpful to consider optical impairments for the control plane operation of a
WSON. The purpose of this work is to determine suitable end-to-end paths that
meet the service objectives in optical networks (i.e. to perform constrained
path computation).

This gives rise to four distinct deployment scenarios:

1. No concern for impairments or lambda continuity
   because there is sufficient margin in all impairments.
   (Original GMPLS)
2. No concern for impairments (again because there is
   sufficient margin), but lambda continuity is important.
   (The RWA problem)
3. Networks in which it is necessary to consider impairments,
   but there is sufficient margin such that approximate
   impairment estimation (using "simple" computation of the
   accumulation) could be used and still have a high
   probability that the optical path would be viable and would
   not perturb any existing paths.
4. Networks in which detailed impairment validation is
   necessary to perform a full computation of the accumulation
   of impairments including the impact on existing paths.

In focusing on the third of these categories, CCAMP intends to base its work on
G.680 and related Recommendations. We would appreciate Q6/15's view on the
following issues:

- What impairments are suitable for consideration in this
  type of network, and which Recommendations should we use
  as references?
- What rules should we use for these impairments to achieve
  a reasonable approximation of how they are accumulated
  along a path?
  That is, CCAMP is looking for the rules by which the end-
  to-end impairments of a path may be determined from a
  knowledge of parameters of the path and impairments on
  the path segments.
- What are the generic encodings and ranges of values for
  the impairment parameters?

The objective of CCAMP work is to distribute information about link impairments
to allow path computation and signaling of new paths.

It is not within the scope of CCAMP to determine how impairments are gathered.
They may be configured or reported by the equipment itself, and this will not
make a material difference to the protocols.

For reference, some early work on impairment-aware GMPLS is listed below. This
work is not yet adopted as CCAMP work, but will be considered by the working
group once we have discussed the way forward with Q6/15.

"A Framework for the Control of Wavelength Switched Optical Networks (WSON)
with Impairments"

"Information Model for Impaired Optical Path Validation"

Looking forward to a profitable meeting,
Deborah Brungard and Adrian Farrel
CCAMP Working Group Co-Chairs