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Network Working Group                                      G. Bernstein
Internet Draft                                        Grotto Networking
                                                                 Y. Lee
                                                                  D. Li
                                                                 Huawei
Intended status: Informational                         February 6, 2009
Expires: August 2009



    A Framework for the Control of Wavelength Switched Optical Networks
                          (WSON) with Impairments
               draft-bernstein-ccamp-wson-impairments-02.txt


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   carefully, as they describe your rights and restrictions with respect
   to this document.



Abstract

   The operation of optical networks can require a level of detail in
   the characterization of network elements, subsystems, devices, and
   cabling not typically encountered with other networking technologies.
   In addition, these physical characteristics may be important to
   consider during typical day-to-day operations such as optical path
   establishment, as well as during the network planning, installation,
   and turn-up phases. This document discusses how the definition and
   characterization of optical fiber, devices, subsystems, and network
   elements contained in various ITU-T recommendations can be combined
   with common control and measurement plane and path computation
   element technologies to support Impairment Aware Routing and
   Wavelength Assignment (IA-RWA) in optical networks.



Table of Contents


   1. Introduction...................................................3
   2. Impairment Aware Optical Path Computation......................4
      2.1. Optical Network Requirements and Constraints..............4
         2.1.1. Impairment Estimation Level of Detail................5
         2.1.2. Impairment Information Sharing Constraints...........6
      2.2. IA-RWA Computing and Control Plane Architectures..........7
         2.2.1. Combined Routing, WA, and IV.........................8
         2.2.2. Separate Routing, WA, or IV..........................8
         2.2.3. Distributed WA and/or IV.............................9
      2.3. Mapping Network Requirements to Architectures.............9
   3. Protocol Implications.........................................12
      3.1. Information Model for Impairments........................12
      3.2. Routing..................................................13
      3.3. Signaling................................................13
      3.4. PCE......................................................14
         3.4.1. Combined IV & RWA...................................14
         3.4.2. IV-Candidates + RWA.................................14
         3.4.3. Approximate IA-RWA + Separate Detailed IV...........16
   4. Security Considerations.......................................18
   5. IANA Considerations...........................................18
   6. Acknowledgments...............................................18
   APPENDIX A: Overview of Optical Layer ITU-T Recommendations......19


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      A.1. Fiber and Cables.........................................19
      A.2. Devices..................................................20
         A.2.1. Optical Amplifiers..................................20
         A.2.2. Dispersion Compensation.............................21
         A.2.3. Optical Transmitters................................22
         A.2.4. Optical Receivers...................................22
      A.3. Components and Subsystems................................23
      A.4. Network Elements.........................................24
   7. References....................................................26
      7.1. Normative References.....................................26
      7.2. Informative References...................................27
   Author's Addresses...............................................28
   Intellectual Property Statement..................................29
   Disclaimer of Validity...........................................30

1. Introduction

   As an optical signal progresses along its path it may be altered by
   the various physical processes in the optical fibers and devices it
   encounters. When such alterations result in signal degradation, we
   usually refer to these processes as "impairments". An overview of
   some critical optical impairments and their routing (path selection)
   implications can be found in [RFC4054]. Roughly speaking, optical
   impairments accumulate along the path (without 3R regeneration)
   traversed by the signal. They are influenced by the type of fiber
   used, the types and placement of various optical devices and the
   presence of other optical signals that may share a fiber segment
   along the signal's path. The degradation of the optical signals due
   to impairments can result in unacceptable bit error rates or even a
   complete failure to demodulate and/or detect the received signal.
   Therefore, path selection in any WSON requires consideration of
   optical impairments so that the signal will be propagated from the
   network ingress point to the egress point with acceptable amount of
   degradation.

   Some optical subnetworks are designed such that over any path the
   degradation to an optical signal due to impairments never exceeds
   prescribed bounds. This may be due to the limited geographic extent
   of the network, the network topology, and/or the quality of the
   fiber and devices employed. In such networks the path selection
   problem reduces to determining a continuous wavelength from source
   to destination (the Routing and Wavelength Assignment problem).
   These networks are discussed in [WSON-Frame]. In other optical
   networks, impairments are important and the path selection process
   must be impairment-aware.




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   Although [RFC4054] describes a number of key optical impairments, a
   more complete description of optical impairments and the processes
   that spawn them can be found in textbooks or reference books on
   optical communications [Agrawal02], [Agrawal07]. To be useful to
   consumers and producers of optical fiber, components, and subsystems,
   optical characteristics need to be precisely defined along with
   methods for their measurement, estimation and approximation. The ITU-
   T and other SDOs have assumed this responsibility and as optical
   technology has advanced these documents have been updated.  Appendix
   A of this document provides an overview of the extensive ITU-T
   documentation in this area.

   The benefits of operating networks of different technologies using an
   intelligent control plane have been described in many places, and the
   Generalized Multiprotocol Label Switching (GMPLS) control plane is
   described in [RFC3945]. The advantages of using a path computation
   element (PCE) to perform complex path computations are discussed in
   [RFC4655].

   Given the existing standards covering optical characteristics
   (impairments) and the knowledge of how the impact of impairments may
   be estimated along a path, this document provides a framework for
   impairment aware path computation and establishment utilizing GMPLS
   protocols and the PCE architecture. As in the impairment free case
   covered in [WSON-Frame] a number of different control plane
   architectural options are described.

2. Impairment Aware Optical Path Computation

   One of the most basic questions in communications is whether one can
   successfully transmit information from a transmitter to a receiver
   within a prescribed error tolerance, usually specified as a maximum
   permissible bit error ratio (BER). This generally depends on the
   nature of the signal transmitted between the sender and receiver and
   the nature of the communications channel between the sender and
   receiver. The optical path utilized (along with the wavelength)
   determines the communications channel.

   The optical impairments incurred by the signal along the fiber and at
   each optical network element along the path determine whether the BER
   performance or any other measure of signal quality can be met for
   this particular signal on this particular path.

2.1. Optical Network Requirements and Constraints

   This section examines the various optical network requirements and
   constraints that an impairment aware optical control plane may have


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   to operate under. These requirements and constraints motivate the IA-
   RWA architectural alternatives to be presented in the following
   section. We can break the different optical networks contexts up
   along two main criteria: (a) the level impairment detail and
   validation required, and (b) the constraints on sharing of impairment
   information.

      2.1.1. Impairment Estimation Level of Detail

   A. Ignored Impairments

   This situation is applicable to networks designed such that every
   possible path is valid for the signal types permitted on the network.
   In this case impairments are only taken into account during network
   design and after that, for example during optical path computation,
   they can be ignored. This is the case discussed in [WSON-Frame] where
   impairments could be ignored by the control plane.

   B. Approximated Impairment Estimation

   This situation is applicable to networks in which impairment effects
   can be estimated via approximation techniques such as link budgets
   and dispersion (rise time) budgets [Agrawal02],[G.680],[G.sup39]. As
   networks grow larger listing all useable paths for each signal type
   can encounter scaling issues. Instead the viability of most optical
   paths for a particular class of signals is performed using well
   defined approximation techniques [G.680], [G.sup39].

   Much work at ITU-T has gone into developing impairment models at this
   and more detailed levels. Impairment characterization of network
   elements could then made available via the control plane and then
   used to calculate which paths are conformant with a specified BER for
   a particular signal type. In such a case we have the potential, but
   not the requirement, to combine the IV process with RWA process to
   permit more optimal IA-RWA computations.

   C. Detailed Impairment Estimation

   This situation is applicable to networks in which impairment effects
   must be more accurately estimated. This includes evaluation of the
   impact to any existing paths prior to the addition of a new path.
   This is currently performed via methods that solve the partial
   differential equations describing signal propagation in fiber along
   with more detailed models for the other network elements
   [Agrawal02],[Agrawal07]. The estimation/simulation time required can
   be very situation dependent.



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   The implication is that a significant amount of time could be
   required to "qualify" a path. To increase the probability of passing
   this validation it is strongly recommended to use approximate
   impairment information prior to the detailed path validation via
   simulations. Since the detailed path validation utilizes calculations
   and techniques that are much different from that of RWA it could be
   useful to allow the IV process to run on a separate computation
   entity. Hence it can be useful to consider any necessary PCEP
   extensions required to allow this detailed IV computation to take
   place on an additional PCE.

      2.1.2. Impairment Information Sharing Constraints

   In GMPLS information used in for path computation is shared without
   constraints amongst the elements participating in the control plane.
   For optical impairments while this would typically be the case,
   however it may be the case that some of the information concerning
   impairments in an optical network is considered private and not to be
   shared with other vendors control systems. This might occur if
   proprietary impairment models are used to evaluate paths or a vendor
   chooses not to publish impairment information for a set of network
   elements. An example of this is document in "situation 1" of [G.680].
   In "situation 1" in [G.680] there is a single WDM line system vendor
   and multiple vendors of OADMs and PXCs and the impairment information
   for the line system can be considered "private", while the impairment
   information for any OADM or PXCs can be considered shared.

   To maintain the privacy of impairment information and provide for IA-
   RWA we have the following options:

   A. The vendor who wishes to keep impairment information private can
      furnish a list of all viable paths between all viable node pairs
      to a computational entity. This information would be particularly
      useful as an input to RWA optimization to be performed by another
      computation entity. The difficulty here is for larger networks
      such a list of paths along with any wavelength constraints could
      get unmanageably large.

   B. The vendor who wishes to keep impairment information private could
      furnish a PCE like entity that would furnish a list of viable
      paths/wavelengths between two requested nodes. This is useful as
      an input to RWA optimizations and can reduce the scaling issue
      previously mentioned. Such a PCE like entity would not need to
      perform a full RWA computation, i.e., it would not need to take
      into account current wavelength availability on links. Such an
      approach may require PCEP extensions for both the request and
      response information.


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   C. The vendor who wishes to keep impairment information private can
      furnish a PCE performs full IA-RWA services. The difficulty is
      this requires the one vendor to also become the sole source of all
      RWA optimization algorithms and such.

   In all the above cases it would be the responsibility of the vendor
   wishing to keep its information private to import the shared
   impairment information from the other NEs via the control plane or
   other means.

2.2. IA-RWA Computing and Control Plane Architectures

   From a control plane point of view optical impairments are additional
   constraints to the impairment-free RWA process described in [WSON-
   Frame]. In impairment aware routing and wavelength assignment (IA-
   RWA), there are conceptually three general classes of processes to be
   considered: Routing (R), Wavelength Assignment (WA), and Impairment
   Validation (IV).

   Impairment validation may come in many forms, and maybe invoked at
   different levels of detail in the IA-RWA process. From a process
   point of view we will consider the following three forms of
   impairment validation:

  o  IV-Candidates

   In this case an Impairment Validation (IV) process furnishes a set of
   paths between two nodes along with any wavelength restrictions such
   that the paths are valid with respect to optical impairments. These
   paths and wavelengths may not be actually available in the network
   due to its current usage state. This set of paths would be returned
   in response to a request for a set of at most K valid paths between
   two specified nodes. Note that such a process never directly
   discloses optical impairment information.

  o  IV-Detailed Verification

   In this case an IV process is given a particular path and wavelength
   through an optical network and is asked to verify whether the overall
   quality objectives for the signal over this path can be met. Note
   that such a process never directly discloses optical impairment
   information.

  o  IV-Distributed

   In this distributed IV process impairment approximate degradation
   measures such as OSNR, dispersion, DGD, etc. are accumulated along


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   the path via a signaling like protocol. When the accumulated measures
   reach the destination node a decision on the impairment validity of
   the path can be made. Note that such a process would entail revealing
   an individual network element's impairment information.

   The following subsections present three major classes of IA-RWA path
   computation architectures and their respective advantages and
   disadvantages.

      2.2.1. Combined Routing, WA, and IV

   From the point of view of optimality, the "best" IA-RWA solutions can
   be achieved if the path computation entity (PCE) can
   conceptually/algorithmically combine the processes of routing,
   wavelength assignment and impairment validation.

   Such a combination can take place if the PCE is given: (a) the
   impairment-free WSON network information as discussed in [WSON-Frame]
   and (b) impairment information to validate potential paths.

      2.2.2. Separate Routing, WA, or IV

   Separating the processes of routing, WA and/or IV can reduce the need
   for sharing of different types of information used in path
   computation. This was discussed for routing separate from WA in
   [WSON-Frame]. In addition, as will be discussed in the section on
   network contexts some impairment information may not be shared and
   this may lead to the need to separate IV from RWA.  In addition, as
   also discussed in the section on network contexts, if IV needs to be
   done at a high level of precision it may be advantageous to offload
   this computation to a specialized server.

   The following conceptual architectures belong in this general
   category:

  o  R+WA+IV -- separate routing, wavelength assignment, and impairment
     validation.

  o  R + (WA & IV) -- routing separate from a combined wavelength
     assignment and impairment validation process. Note that impairment
     validation is typically wavelength dependent hence combining WA
     with IV can lead to efficiencies.

  o  (RWA)+IV - combined routing and wavelength assignment with a
     separate impairment validation process.




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   Note that the IV process may come before or after the RWA processes.
   If RWA comes first then IV is just rendering a yes/no decision on the
   selected path and wavelength. If IV comes first it would need to
   furnish a list of possible (valid with respect to impairments) routes
   and wavelengths to the RWA processes.

      2.2.3. Distributed WA and/or IV

   In the non-impairment RWA situation [WSON-Frame] it was shown that a
   distributed wavelength assignment (WA) process carried out via
   signaling can eliminate the need to distribute wavelength
   availability information via an IGP. A similar approach can allow for
   the distributed computation of impairment effects and avoid the need
   to distribute impairment characteristics of network elements and
   links via route protocols or by other means. An example of such an
   approach is given in [Martinelli] and utilizes enhancements to RSVP
   signaling to carry accumulated impairment related information.

   A distributed impairment validation for a prescribed network path
   requires that the effects of impairments can be calculated by
   approximate models with cumulative quality measures such as those in
   [G.680].

   For such a system to be interoperable the various impairment measures
   to be accumulated would need to be agreed upon. Section 9 of [G.680]
   can be useful in deriving such cumulative measures but doesn't
   explicitly state how a distributed computation would take place. For
   example in the computation of the optical signal to noise ratio along
   a path (see equation 9-3 of [G.680]) one could accumulate the linear
   sum terms and convert to the optical signal to noise ratio (OSNR) in
   (dBs) at the destination or one could convert in and out of the OSNR
   in (dBs) at each intermediate point along the path.

   If distributed WA is being done at the same time as distributed IV
   then we may need to accumulate impairment related information for all
   wavelengths that could be used. This is somewhat winnowed down as
   potential wavelengths are discovered to be in use, but could be a
   significant burden for lightly loaded high channel count networks.

2.3. Mapping Network Requirements to Architectures

   In Figure 1 we show process flows for three main architectural
   alternatives to IA-RWA when approximate impairment validation
   suffices. In Figure 2 we show process flows for two main
   architectural alternatives when detailed impairment verification is
   required.



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                  +-----------------------------------+
                  |   +--+     +-------+     +--+     |
                  |   |IV|     |Routing|     |WA|     |
                  |   +--+     +-------+     +--+     |
                  |                                   |
                  |        Combined Processes         |
                  +-----------------------------------+
                                  (a)

           +--------------+      +----------------------+
           | +----------+ |      | +-------+    +--+    |
           | |    IV    | |      | |Routing|    |WA|    |
           | |candidates| |----->| +-------+    +--+    |
           | +----------+ |      |  Combined Processes  |
           +--------------+      +----------------------+
                                  (b)

            +-----------+        +----------------------+
            | +-------+ |        |    +--+    +--+      |
            | |Routing| |------->|    |WA|    |IV|      |
            | +-------+ |        |    +--+    +--+      |
            +-----------+        | Distributed Processes|
                                 +----------------------+
                                  (c)
     Figure 1 Process flows for the three main approximate impairment
                        architectural alternatives.

   The advantages, requirements and suitability of these options are as
   follows:

  o  Combined IV & RWA process

   This alternative combines RWA and IV within a single computation
   entity enabling highest potential optimality and efficiency in IA-
   RWA. This alternative requires that the computational entity knows
   impairment information as well as non-impairment RWA information.
   This alternative can be used when impairment information is required
   to be kept private, but would then need to be provided by the non-
   sharing vendor.

  o  IV-Candidates + RWA process

   This alternative allows separation of impairment information into two
   computational entities while still maintaining a high degree of
   potential optimality and efficiency in IA-RWA. The candidates IV
   process needs to know impairment information from all optical network


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   elements, while the RWA process needs to know non-impairment RWA
   information from the network elements. This alternative can be used
   when impairment information is required to be kept private, but the
   non-sharing vendor would need to provide the functionality of the IV-
   candidates process. Note that this is still very useful since the
   algorithmic areas of IV and RWA are very different and prone to
   specialization.

  o  Routing + Distributed WA and IV

   In this alternative a signaling protocol is extended and leveraged in
   the wavelength assignment and impairment validation processes.
   Although this doesn't enable as high a potential degree of optimality
   of optimality as (a) or (b), it does not require distribution of
   either link wavelength usage or link/node impairment information.
   Note that this is most likely not acceptable for keeping impairment
   information "private".



          +-----------------------------------+     +------------+
          | +-----------+  +-------+    +--+  |     | +--------+ |
          | |    IV     |  |Routing|    |WA|  |     | |  IV    | |
          | |approximate|  +-------+    +--+  |---->| |Detailed| |
          | +-----------+                     |     | +--------+ |
          |        Combined Processes         |     |            |
          +-----------------------------------+     +------------+
                                   (a)

    +--------------+      +----------------------+     +------------+
    | +----------+ |      | +-------+    +--+    |     | +--------+ |
    | |    IV    | |      | |Routing|    |WA|    |---->| |  IV    | |
    | |candidates| |----->| +-------+    +--+    |     | |Detailed| |
    | +----------+ |      |  Combined Processes  |     | +--------+ |
    +--------------+      +----------------------+     |            |
                                   (b)                 +------------+
        Figure 2 Process flows for the two main detailed impairment
                     validation architectural options.

   The advantages, requirements and suitability of these detailed
   validation options are as follows:

  o  Combined approximate IV & RWA + Detailed-IV

   This alternative combines RWA and approximate IV within a single
   computation entity enabling highest potential optimality and
   efficiency in IA-RWA; then has a separate entity performing detailed


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   impairment validation. In the case of non-shared impairment
   information the non-sharing vendor would need to provide all
   functionality.

  o  Candidates-IV + RWA + Detailed-IV

   This alternative allows separation of approximate impairment
   information into a computational entity while still maintaining a
   high degree of potential optimality and efficiency in IA-RWA; then a
   separate computation entity performs detailed impairment validation.
   In the case of non-shared impairment information the non-sharing
   vendor would need to provide both the candidates-IV and Detailed-IV
   processes, but not necessarily the RWA process.



3. Protocol Implications

   The previous IA-RWA architectural alternatives and process flows make
   differing demands on a GMPLS/PCE based control plane. In this section
   we discuss the use of (a) an impairment information model, (b) PCE as
   computational entity assuming the various process roles and
   consequences for PCEP, (c)any needed extensions to signaling, and (d)
   extensions to routing. The impacts to the control plane for IA-RWA
   are summarized in Figure 3.


        +-------------------+----+----+----------+--------+
        | IA-RWA Option     |PCE |Sig |Info Model| Routing|
        +-------------------+----+----+----------+--------+
        |          Combined |Yes | No |  Yes     |  Yes   |
        |          IV & RWA |    |    |          |        |
        +-------------------+----+----+----------+--------+-
        |     IV-Candidates |Yes | No |  Yes     |  Yes   |
        |         + RWA     |    |    |          |        |
        +-------------------+----+----+----------+--------+
        |    Routing +      |No  | Yes|  Yes     |  No    |
        |Distributed IV, RWA|    |    |          |        |
        +-------------------+----+----+----------+--------+
        |       Detailed IV |Yes | No |  Yes     |  Yes   |
        +-------------------+----+----+----------+--------+
     Figure 3 IA-RWA architectural options and control plane impacts.

3.1. Information Model for Impairments

   As previously discussed all IA-RWA scenarios to a greater or lesser
   extent rely on a common impairment information model. A number of


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   ITU-T recommendations cover detailed as well as approximate
   impairment characteristics of fibers and a variety of devices and
   subsystems. A well integrated impairment model for optical network
   elements is given in [G.680] and is used to form the basis for an
   optical impairment model in a companion document [Imp-Info].

   It should be noted that the current version of [G.680] is limited to
   the networks composed of a single WDM line system vendor combined
   with OADMs and/or PXCs from potentially multiple other vendors, this
   is known as situation 1 and is shown in Figure 1-1 of [G.680]. It is
   planed in the future that [G.680] will include networks incorporating
   line systems from multiple vendors as well as OADMs and/or PXCs from
   potentially multiple other vendors, this is known as situation 2 and
   is shown in Figure 1-2 of [G.680].

   The case of distributed impairment validation actually requires a bit
   more than an impairment information model. In particular, it needs a
   common impairment "computation" model. In the distributed IV case one
   needs to standardize the accumulated impairment measures that will be
   conveyed and updated at each node. Section 9 of [G.680] provides
   guidance in this area with specific formulas given for OSNR, residual
   dispersion, polarization mode dispersion/polarization dependent loss,
   effects of channel uniformity, etc... However, specifics of what
   intermediate results are kept and in what form would need to be
   standardized.

3.2. Routing

   Different approaches to path/wavelength impairment validation gives
   rise to different demands placed on GMPLS routing protocols. In the
   case where approximate impairment information is used to validate
   paths GMPLS routing may be used to distribute the impairment
   characteristics of the network elements and links based on the
   impairment information model previously discussed. In the case of
   distributed-IV no new demands would be placed on the routing
   protocol.

3.3. Signaling

   The largest impacts on signaling occur in the cases where distributed
   impairment validation is performed. In this we need to accumulate
   impairment information as previously discussed. In addition, since
   the characteristics of the signal itself, such as modulation type,
   can play a major role in the tolerance of impairments, this type of
   information will need to be implicitly or explicitly signaled so that
   an impairment validation decision can be made at the destination
   node.


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   It remains for further study if it may be beneficial to include
   additional information to a connection request such as desired egress
   signal quality (defined in some appropriate sense) in non-distributed
   IV scenarios.

3.4. PCE

   In section 2.3. we gave a number of computation architectural
   alternatives that could be used to meet the various requirements and
   constraints of section 2.1.  Here we look at how these alternatives
   could be implemented via either a single PCE or a set of two or more
   cooperating PCEs, and the impacts on the PCEP protocol.

      3.4.1. Combined IV & RWA

   In this situation, shown in Figure 1(a), a single PCE performs all
   the computations needed for IA-RWA.

  o  TE Database Requirements

     WSON Topology and switching capabilities, WSON WDM link wavelength
     utilization, and WSON impairment information

  o  PCC to PCE Request Information

     Signal characteristics/type, required quality, source node,
     destination node

  o  PCE to PCC Reply Information

     If the computations completed successfully then the PCE returns
     the path and its assigned wavelength. If the computations could
     not complete successfully it would be potentially useful to know
     the reason why. At a very crude level we'd like to know if this
     was due to lack of wavelength availability or impairment
     considerations or a bit of both. The information to be conveyed is
     for further study.

      3.4.2. IV-Candidates + RWA

   In this situation, shown in Figure 1(b), we have two separate
   processes involved in the IA-RWA computation. This requires at least
   two cooperating PCEs: one for the Candidates-IV process and another
   for the RWA process. In addition, the overall process needs to be
   coordinated. This could be done with yet another PCE or we can add
   this functionality to one of previously defined PCEs. We choose this
   later option and require the RWA PCE to also act as the overall


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   process coordinator. The roles, responsibilities and information
   requirements for these two PCEs are given below.

   RWA and Coordinator PCE (RWA-Coord-PCE):

   Responsible for interacting with PCC and for utilizing Candidates-PCE
   as needed during RWA computations. In particular it needs to know to
   use the Candidates-PCE to obtain potential set of routes and
   wavelengths.

  o  TE Database Requirements

     WSON Topology and switching capabilities and WSON WDM link
     wavelength utilization (no impairment information).

  o  PCC to RWA-PCE request: same as in the combined case.

  o  RWA-PCE to PCC reply: same as in the combined case.

  o  RWA-PCE to IV-Candidates-PCE request

     The RWA-PCE asks for a set of at most K routes along with
     acceptable wavelengths between nodes specified in the original PCC
     request.

  o  IV-Candidates-PCE reply to RWA-PCE

     The Candidates-PCE returns a set of at most K routes along with
     acceptable wavelengths between nodes specified in the RWA-PCE
     request.

  IV-Candidates-PCE:

     The IV-Candidates-PCE is responsible for impairment aware path
     computation. It needs not take into account current link
     wavelength utilization, but this is not prohibited. The
     Candidates-PCE is only required to interact with the RWA-PCE as
     indicated above and not the PCC.

  o  TE Database Requirements

     WSON Topology and switching capabilities and WSON impairment
     information (no information link wavelength utilization required).

   In Figure 4 we show a sequence diagram for the interactions between
   the PCC, RWA-PCE and IV-Candidates-PCE.



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     +---+                +-------------+          +-----------------+
     |PCC|                |RWA-Coord-PCE|          |IV-Candidates-PCE|
     +-+-+                +------+------+          +---------+-------+
       ...___     (a)            |                           |
       |     ````---...____      |                           |
       |                   ```-->|                           |
       |                         |                           |
       |                         |--..___    (b)             |
       |                         |       ```---...___        |
       |                         |                   ```---->|
       |                         |                           |
       |                         |                           |
       |                         |           (c)       ___...|
       |                         |       ___....---''''      |
       |                         |<--''''                    |
       |                         |                           |
       |                         |                           |
       |          (d)      ___...|                           |
       |      ___....---'''      |                           |
       |<--'''                   |                           |
       |                         |                           |
       |                         |                           |

     Figure 4 Sequence diagram for the interactions between PCC, RWA-
                Coordinating-PCE and the IV-Candidates-PCE.

   In step (a) the PCC requests a path meeting specified quality
   constraints between two nodes (A and Z) for a given signal
   represented either by a specific type or a general class with
   associated parameters. In step (b) the RWA-Coordinating-PCE requests
   up to K candidate paths between nodes A and Z and associated
   acceptable wavelengths. In step (c) The IV-Candidates-PCE returns
   this list to the RWA-Coordinating PCE which then uses this set of
   paths and wavelengths as input (e.g. a constraint) to its RWA
   computation. In step (d) the RWA-Coordinating-PCE returns the overall
   IA-RWA computation results to the PCC.

      3.4.3. Approximate IA-RWA + Separate Detailed IV

   In Figure 2 we showed two cases where a separate detailed impairment
   validation process could be utilized. We can place the detailed
   validation process into a separate PCE. Assuming that a different PCE
   assumes a coordinating role and interacts with the PCC we can keep
   the interactions with this separate IV-Detailed-PCE very simple.

   IV-Detailed-PCE:


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  o  TE Database Requirements

     The IV-Detailed-PCE will need optical impairment information, WSON
     topology, and possibly WDM link wavelength usage information. This
     document puts no restrictions on the type of information that may
     be used in these computations.

  o  Coordinating-PCE to IV-Detailed-PCE request

     The coordinating-PCE will furnish signal characteristics, quality
     requirements, path and wavelength to the IV-Detailed-PCE.

  o  IV-Detailed-PCE to Coordinating-PCE reply

     The reply is essential an yes/no decision as to whether the
     requirements could actually be met. In the case where the
     impairment validation fails it would be helpful to convey
     information related to cause or quantify the failure, e.g., so a
     judgment can be made whether to try a different signal or adjust
     signal parameters.

   In Figure 5 we show a sequence diagram for the interactions for the
   process shown in Figure 2(b). This involves interactions between the
   PCC, RWA-PCE (acting as coordinator), IV-Candidates-PCE and the IV-
   Detailed-PCE.

   In step (a) the PCC requests a path meeting specified quality
   constraints between two nodes (A and Z) for a given signal
   represented either by a specific type or a general class with
   associated parameters. In step (b) the RWA-Coordinating-PCE requests
   up to K candidate paths between nodes A and Z and associated
   acceptable wavelengths. In step (c) The IV-Candidates-PCE returns
   this list to the RWA-Coordinating PCE which then uses this set of
   paths and wavelengths as input (e.g. a constraint) to its RWA
   computation. In step (d) the RWA-Coordinating-PCE request a detailed
   verification of the path and wavelength that it has computed. In step
   (e) the IV-Detailed-PCE returns the results of the validation to the
   RWA-Coordinating-PCE. Finally in step (f)IA-RWA-Coordinating PCE
   returns the final results (either a path and wavelength or cause for
   the failure to compute a path and wavelength) to the PCC.









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                +----------+      +--------------+      +------------+
    +---+       |RWA-Coord |      |IV-Candidates |      |IV-Detailed |
    |PCC|       |   PCE    |      |     PCE      |      |    PCE     |
    +-+-+       +----+-----+      +------+-------+      +-----+------+
      |.._   (a)     |                   |                    |
      |   ``--.__    |                   |                    |
      |          `-->|                   |                    |
      |              |        (b)        |                    |
      |              |--....____         |                    |
      |              |          ````---.>|                    |
      |              |                   |                    |
      |              |         (c)  __..-|                    |
      |              |     __..---''     |                    |
      |              |<--''              |                    |
      |              |                                        |
      |              |...._____          (d)                  |
      |              |         `````-----....._____           |
      |              |                             `````----->|
      |              |                                        |
      |              |                 (e)          _____.....+
      |              |          _____.....-----'''''          |
      |              |<----'''''                              |
      |     (f)   __.|                                        |
      |    __.--''   |
      |<-''          |
      |              |
     Figure 5 Sequence diagram for the interactions between PCC, RWA-
         Coordinating-PCE, IV-Candidates-PCE and IV-Detailed-PCE.



4. Security Considerations

   This document discusses a number of control plane architectures that
   incorporate knowledge of impairments in optical networks. If such
   architecture is put into use within a network it will by its nature
   contain details of the physical characteristics of an optical
   network. Such information would need to be protected from intentional
   or unintentional disclosure.

5. IANA Considerations

   This draft does not currently require any consideration from IANA.

6. Acknowledgments

   This document was prepared using 2-Word-v2.0.template.dot.


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APPENDIX A: Overview of Optical Layer ITU-T Recommendations

   For optical fiber, devices, subsystems and network elements the ITU-T
   has a variety of recommendations that include definitions,
   characterization parameters and test methods. In the following we
   take a bottom up survey to emphasize the breadth and depth of the
   existing recommendations.  We focus on digital communications over
   single mode optical fiber.

A.1. Fiber and Cables

   Fibers and cables form a key component of what from the control plane
   perspective could be termed an optical link. Due to the wide range of
   uses of optical networks a fairly wide range of fiber types are used
   in practice. The ITU-T has three main recommendations covering the
   definition of attributes and test methods for single mode fiber:

  o  Definitions and test methods for linear, deterministic attributes
     of single-mode fibre and cable  [G.650.1]

  o  Definitions and test methods for statistical and non-linear
     related attributes of single-mode fibre and cable [G.650.2]

  o  Test methods for installed single-mode fibre cable sections
     [G.650.3]

   General Definitions[G.650.1]: Mechanical Characteristics (numerous),
   Mode field characteristics(mode field, mode field diameter, mode
   field centre, mode field concentricity error, mode field non-
   circularity), Glass geometry characteristics, Chromatic dispersion
   definitions (chromatic dispersion, group delay, chromatic dispersion
   coefficient, chromatic dispersion slope, zero-dispersion wavelength,
   zero-dispersion slope), cut-off wavelength, attenuation. Definition
   of equations and fitting coefficients for chromatic dispersion (Annex
   A). [G.650.2] polarization mode dispersion (PMD) - phenomenon of PMD,
   principal states of polarization (PSP), differential group delay
   (DGD), PMD value, PMD coefficient, random mode coupling, negligible
   mode coupling, mathematical definitions in terms of Stokes or Jones
   vectors. Nonlinear attributes: Effective area, correction factor k,
   non-linear coefficient (refractive index dependent on intensity),
   Stimulated Billouin scattering.

   Tests defined [G.650.1]: Mode field diameter, cladding diameter, core
   concentricity error, cut-off wavelength, attenuation, chromatic
   dispersion. [G.650.2]: test methods for polarization mode dispersion.
   [G.650.3] Test methods for characteristics of fibre cable sections
   following installation: attenuation, splice loss, splice location,


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   fibre uniformity and length of cable sections (these are OTDR based),
   PMD, Chromatic dispersion.



   With these definitions a variety of single mode fiber types are
   defined as shown in the table below:

      ITU-T Standard |  Common Name
      ------------------------------------------------------------
      G.652 [G.652]  |  Standard SMF                              |
      G.653 [G.653]  |  Dispersion shifted SMF                    |
      G.654 [G.654]  |  Cut-off shifted SMF                       |
      G.655 [G.655]  |  Non-zero dispersion shifted SMF           |
      G.656 [G.656]  |  Wideband non-zero dispersion shifted SMF  |
      ------------------------------------------------------------

A.2. Devices

A.2.1. Optical Amplifiers

   Optical amplifiers greatly extend the transmission distance of
   optical signals in both single channel and multi channel (WDM)
   subsystems. The ITU-T has the following recommendations:

  o  Definition and test methods for the relevant generic parameters of
     optical amplifier devices and subsystems [G.661]

  o  Generic characteristics of optical amplifier devices and
     subsystems [G.662]

  o  Application related aspects of optical amplifier devices and
     subsystems [G.663]

  o  Generic characteristics of Raman amplifiers and Raman amplified
     subsystems [G.665]

   Reference [G.661] starts with general classifications of optical
   amplifiers based on technology and usage, and include a near
   exhaustive list of over 60 definitions for optical amplifier device
   attributes and parameters. In references [G.662] and [G.665] we have
   characterization of specific devices, e.g., semiconductor optical
   amplifier, used in a particular setting, e.g., line amplifier. For
   example reference[G.662] gives the following minimum list of relevant
   parameters for the specification of an optical amplifier device used
   as line amplifier in a multichannel application:



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   a) Channel allocation.

   b) Total input power range.

   c) Channel input power range.

   d) Channel output power range.

   e) Channel signal-spontaneous noise figure.

   f) Input reflectance.

   g) Output reflectance.

   h) Maximum reflectance tolerable at input.

   i) Maximum reflectance tolerable at output.

   j) Maximum total output power.

   k) Channel addition/removal (steady-state) gain response.

   l) Channel addition/removal (transient) gain response.

   m) Channel gain.

   n) Multichannel gain variation (inter-channel gain difference).

   o) Multichannel gain-change difference (inter-channel gain-change
   difference).

   p) Multichannel gain tilt (inter-channel gain-change ratio).

   q) Polarization Mode Dispersion (PMD).



A.2.2. Dispersion Compensation

   In optical systems two forms of dispersion are commonly encountered
   [RFC4054] chromatic dispersion and polarization mode dispersion
   (PMD). There are a number of techniques and devices used for
   compensating for these effects. The following ITU-T recommendations
   characterize such devices:

  o  Characteristics of PMD compensators and PMD compensating receivers
     [G.666]


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  o  Characteristics of Adaptive Chromatic Dispersion Compensators
     [G.667]

   The above furnish definitions as well as parameters and
   characteristics. For example in [G.667] adaptive chromatic dispersion
   compensators are classified as being receiver, transmitter or line
   based, while in [G.666] PMD compensators are only defined for line
   and receiver configurations. Parameters that are common to both PMD
   and chromatic dispersion compensators include: line fiber type,
   maximum and minimum input power, maximum and minimum bit rate, and
   modulation type. In addition there are a great many parameters that
   apply to each type of device and configuration.

A.2.3.  Optical Transmitters

   The definitions of the characteristics of optical transmitters can be
   found in references [G.957], [G.691], [G.692] and [G.959.1]. In
   addition references [G.957], [G.691], and [G.959.1] define specific
   parameter values or parameter ranges for these characteristics for
   interfaces for use in particular situations.

   We generally have the following types of parameters

   Wavelength related: Central frequency, Channel spacing, Central
   frequency deviation[G.692].

   Spectral characteristics of the transmitter: Nominal source type
   (LED, MLM lasers, SLM lasers) [G.957], Maximum spectral width, Chirp
   parameter, Side mode suppression ratio, Maximum spectral power
   density [G.691].

   Power related: Mean launched power, Extinction ration, Eye pattern
   mask [G.691], Maximum and minimum mean channel output power
   [G.959.1].

A.2.4. Optical Receivers

   References [G.959.1], [G.691], [G.692] and [G.957], define optical
   receiver characteristics and [G.959.1], [G.691] and [G.957]give
   specific values of these parameters for particular interface types
   and network contexts.

   The receiver parameters include:

   Receiver sensitivity: minimum value of average received power to
   achieve a 1x10-10 BER [G.957] or 1x10-12 BER [G.691]. See [G.957] and
   [G.691] for assumptions on signal condition.


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   Receiver overload: Receiver overload is the maximum acceptable value
   of the received average power for a 1x10.10 BER [G.957] or a 1x10-12
   BER [G.691].

   Receiver reflectance: "Reflections from the receiver back to the
   cable plant are specified by the maximum permissible reflectance of
   the receiver measured at reference point R."

   Optical path power penalty: "The receiver is required to tolerate an
   optical path penalty not exceeding X dB to account for total
   degradations due to reflections, intersymbol interference, mode
   partition noise, and laser chirp."

   When dealing with multi-channel systems or systems with optical
   amplifiers we may also need:

   Optical signal-to-noise ratio: "The minimum value of optical SNR
   required to obtain a 1x10-12 BER."[G.692]

   Receiver wavelength range: "The receiver wavelength range is defined
   as the acceptable range of wavelengths at point Rn. This range must
   be wide enough to cover the entire range of central frequencies over
   the OA passband." [G.692]

   Minimum equivalent sensitivity: "This is the minimum sensitivity that
   would be required of a receiver placed at MPI-RM in multichannel
   applications to achieve the specified maximum BER of the application
   code if all except one of the channels were to be removed (with an
   ideal loss-less filter) at point MPI-RM." [G.959.1]

A.3. Components and Subsystems

   Reference [G.671] "Transmission characteristics of optical components
   and subsystems" covers the following components:

  o  optical add drop multiplexer (OADM) subsystem;

  o  asymmetric branching component;

  o  optical attenuator;

  o  optical branching component (wavelength non-selective);

  o  optical connector;

  o  dynamic channel equalizer (DCE);



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  o  optical filter;

  o  optical isolator;

  o  passive dispersion compensator;

  o  optical splice;

  o  optical switch;

  o  optical termination;

  o  tuneable filter;

  o  optical wavelength multiplexer (MUX)/demultiplexer (DMUX);

       - coarse WDM device;

       - dense WDM device;

       - wide WDM device.

   Reference [G.671] then specifies applicable parameters for these
   components. For example an OADM subsystem will have parameters such
   as: insertion loss (input to output, input to drop, add to output),
   number of add, drop and through channels, polarization dependent
   loss, adjacent channel isolation, allowable input power, polarization
   mode dispersion, etc...

A.4. Network Elements

   The previously cited ITU-T recommendations provide a plethora of
   definitions and characterizations of optical fiber, devices,
   components and subsystems. Reference [G.Sup39] "Optical system design
   and engineering considerations" provides useful guidance on the use
   of such parameters.

   In many situations the previous models while good don't encompass the
   higher level network structures that one typically deals with in the
   control plane, i.e, "links" and "nodes". In addition such models
   include the full range of network applications from planning,
   installation, and possibly day to day network operations, while with
   the control plane we are generally concerned with a subset of the
   later. In particular for many control plane applications we are
   interested in formulating the total degradation to an optical signal
   as it travels through multiple optical subsystems, devices and fiber
   segments.


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   In reference [G.680] "Physical transfer functions of optical networks
   elements", a degradation function is currently defined for the
   following optical network elements: (a) DWDM Line segment, (b)
   Optical Add/Drop Multiplexers (OADM), and (c) Photonic cross-connect
   (PXC). The scope of [G.680] is currently for optical networks
   consisting of one vendors DWDM line systems along with another
   vendors OADMs or PXCs.

   The DWDM line system of [G.680] consists of the optical fiber, line
   amplifiers and any embedded dispersion compensators. Similarly the
   OADM/PXC network element may consist of the basic OADM component and
   optionally included optical amplifiers. The parameters for these
   optical network elements (ONE) are given under the following
   circumstances:

  o  General ONE without optical amplifiers

  o  General ONE with optical amplifiers

  o  OADM without optical amplifiers

  o  OADM with optical amplifiers

  o  Reconfigurable OADM (ROADM) without optical amplifiers

  o  ROADM with optical amplifiers

  o  PXC without optical amplifiers

  o  PXC with optical amplifiers



















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7. References

7.1. Normative References

   [G.650.1] ITU-T Recommendation G.650.1, Definitions and test methods
             for linear, deterministic attributes of single-mode fibre
             and cable, June 2004.

   [650.2]  ITU-T Recommendation G.650.2, Definitions and test methods
             for statistical and non-linear related attributes of
             single-mode fibre and cable, July 2007.

   [650.3]  ITU-T Recommendation G.650.3

   [G.652] ITU-T Recommendation G.652, Characteristics of a single-mode
             optical fibre and cable, June 2005.

   [G.653] ITU-T Recommendation G.653, Characteristics of a dispersion-
             shifted single-mode optical fibre and cable, December 2006.

   [G.654] ITU-T Recommendation G.654, Characteristics of a cut-off
             shifted single-mode optical fibre and cable, December 2006.

   [G.655] ITU-T Recommendation G.655, Characteristics of a non-zero
             dispersion-shifted single-mode optical fibre and cable,
             March 2006.

   [G.656] ITU-T Recommendation G.656, Characteristics of a fibre and
             cable with non-zero dispersion for wideband optical
             transport, December 2006.

   [G.661]  ITU-T Recommendation G.661, Definition and test methods for
             the relevant generic parameters of optical amplifier
             devices and subsystems, March 2006.

   [G.662]  ITU-T Recommendation G.662, Generic characteristics of
             optical amplifier devices and subsystems, July 2005.

   [G.671]  ITU-T Recommendation G.671, Transmission characteristics of
             optical components and subsystems, January 2005.

   [G.680]  ITU-T Recommendation G.680, Physical transfer functions of
             optical network elements, July 2007.

   [G.691]  ITU-T Recommendation G.691, Optical interfaces for
             multichannel systems with optical amplifiers, November
             1998.


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   [G.692]  ITU-T Recommendation G.692, Optical interfaces for single
             channel STM-64 and other SDH systems with optical
             amplifiers, March 2006.

   [G.872]  ITU-T Recommendation G.872, Architecture of optical
             transport networks, November 2001.

   [G.957]  ITU-T Recommendation G.957, Optical interfaces for
             equipments and systems relating to the synchronous digital
             hierarchy, March 2006.

   [G.959.1] ITU-T Recommendation G.959.1, Optical Transport Network
             Physical Layer Interfaces, March 2006.

   [G.694.1] ITU-T Recommendation G.694.1, Spectral grids for WDM
             applications: DWDM frequency grid, June 2002.

   [G.694.2] ITU-T Recommendation G.694.2, Spectral grids for WDM
             applications: CWDM wavelength grid, December 2003.

   [G.Sup39] ITU-T Series G Supplement 39, Optical system design and
             engineering considerations, February 2006.

   [RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
             Switching (GMPLS) Architecture", RFC 3945, October 2004.

   [RFC4054] Strand, J., Ed., and A. Chiu, Ed., "Impairments and Other
             Constraints on Optical Layer Routing", RFC 4054, May 2005.

   [RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation
             Element (PCE)-Based Architecture", RFC 4655, August 2006.

   [WSON-Frame] G. Bernstein, Y. Lee, W. Imajuku, "Framework for GMPLS
             and PCE Control of Wavelength Switched Optical Networks",
             work in progress: draft-ietf-ccamp-wavelength-switched-
             framework-01.txt, November 2008.



7.2. Informative References

   [Agrawal02] Govind P. Agrawal, Fiber-Optic Communications Systems -
             Third Edition, Wiley-Interscience, 2002.

   [Agrawal07] Govind P. Agrawal, Nonlinear Fiber Optics - Fourth
             Edition, Academic Press, 2007.



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   [Imp-Info]  G. Bernstein, Y. Lee, D. Li, "A Framework for the Control
             and Measurement of Wavelength Switched Optical Networks
             (WSON) with Impairments", work in progress: draft-
             bernstein-wson-impairment-info-00.txt, October 2008.

   [Martinelli]   G. Martinelli (ed.) and A. Zanardi (ed.), "GMPLS
             Signaling Extensions for Optical Impairment Aware Lightpath
             Setup", Work in Progress: draft-martinelli-ccamp-optical-
             imp-signaling-01.txt, February 2008.

Author's Addresses

   Greg M. Bernstein (ed.)
   Grotto Networking
   Fremont California, USA

   Phone: (510) 573-2237
   Email: gregb@grotto-networking.com


   Young Lee (ed.)
   Huawei Technologies
   1700 Alma Drive, Suite 100
   Plano, TX 75075
   USA

   Phone: (972) 509-5599 (x2240)
   Email: ylee@huawei.com


   Dan Li
   Huawei Technologies Co., Ltd.
   F3-5-B R&D Center, Huawei Base,
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

   Phone: +86-755-28973237
   Email: danli@huawei.com











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Contributor's Addresses

   Ming Chen
   Huawei Technologies Co., Ltd.
   F3-5-B R&D Center, Huawei Base,
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

   Phone: +86-755-28973237
   Email: mchen@huawei.com


   Rebecca Han
   Huawei Technologies Co., Ltd.
   F3-5-B R&D Center, Huawei Base,
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

   Phone: +86-755-28973237
   Email: hanjianrui@huawei.com


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Disclaimer of Validity

   All IETF Documents and the information contained therein are provided
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   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
   IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
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Acknowledgment

   We thank Chen Ming of DICONNET Project who provided valuable
   information relevant to this document.


































Bernstein & Lee         Expires August 6, 2009                [Page 30]


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