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

Submission date 2009-03-11
From IETF CCAMP WG (Adrian Farrel)
To ITU-T Study Group 15 Question 6 (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

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

"Routing and Wavelength Assignment Information Model for Wavelength
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
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