Network Working Group                                            Y. Lee
Internet Draft                                                   Huawei
                                                           G. Bernstein
                                                      Grotto Networking
                                                                  D. Li
                                                                 Huawei
                                                          G. Martinelli
                                                                  Cisco
Intended status: Informational                             July 9,                         October 21, 2010
Expires: November 2010 April 2011

    A Framework for the Control of Wavelength Switched Optical Networks
                          (WSON) with Impairments
                 draft-ietf-ccamp-wson-impairments-03.txt
                 draft-ietf-ccamp-wson-impairments-04.txt

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Abstract

   The operation of optical networks requires information on the
   physical characterization of optical network elements, subsystems,
   devices, and cabling. These physical characteristics may be important
   to consider when using a GMPLS control plane to support path setup
   and maintenance. 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 GMPLS control plane protocols and mechanisms to support
   Impairment Aware Routing and Wavelength Assignment (IA-RWA) in
   optical networks.

Table of Contents

   1. Introduction...................................................3 Introduction...................................................4
      1.1. Revision History..........................................4 History..........................................5
   2. Motivation.....................................................4 Motivation.....................................................5
   3. Impairment Aware Optical Path Computation......................5 Computation......................6
      3.1. Optical Network Requirements and Constraints..............6 Constraints..............7
         3.1.1. Impairment Aware Computation Scenarios ..............7 Scenarios...............7
         3.1.2. Impairment Computation and Information Sharing
         Constraints.................................................8
         3.1.3. Impairment Estimation Functional Blocks..............9 Process.......................10
      3.2. IA-RWA Computation and Control Plane Architectures.......11
         3.2.1. Combined Routing, WA, and IV........................12 IV........................13
         3.2.2. Separate Routing, WA, or IV.........................12 IV.........................13
         3.2.3. Distributed WA and/or IV............................13
      3.3. Mapping Network Requirements to Architectures............14

   4. Protocol Implications.........................................17
      4.1. Information Model for Impairments........................17
         4.1.1. Properties of an Impairment Information Model.......18
      4.2. Routing..................................................19 Routing..................................................18
      4.3. Signaling................................................19 Signaling................................................18
      4.4. PCE......................................................20 PCE......................................................19
         4.4.1. Combined IV & RWA...................................20 RWA...................................19
         4.4.2. IV-Candidates + RWA.................................20 RWA.................................19
         4.4.3. Approximate IA-RWA + Separate Detailed IV...........22 IV...........21
   5. Security Considerations.......................................24 Considerations.......................................23
   6. IANA Considerations...........................................24 Considerations...........................................23
   7. Acknowledgments...............................................24 Acknowledgments...............................................23
   8. References....................................................32 References....................................................31
      8.1. Normative References.....................................32 References.....................................31
      8.2. Informative References...................................34 References...................................33

   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 an acceptable signal quality.

      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.

      Although [RFC4054] describes a number of key optical impairments,
      a more complete description of optical impairments and processes
      can be found in the ITU-T Recommendations. Appendix A of this
      document provides an overview of the extensive ITU-T documentation
      in this area.

      The benefits of operating networks using 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].

      Based on the existing ITU-T 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.

   1.1. Revision History

      Changes from 00 to 01:

      Added discussion of regenerators to section 3.

      Added to discussion of interface parameters in section 3.1.3.

      Added to discussion of IV Candidates function in section 3.2.

      Changes from 01 to 02:

      Correct and refine use of "black link" concept based on liaison
      with ITU-T and WG feedback.

      Changes from 02 to 03:

      Insert additional information on use and considerations for
      regenerators in section 3.

   2. Motivation

      There are deployment scenarios for WSON networks where not all
      possible paths will yield suitable signal quality. There are
      multiple reasons behind this choice; here below is a non-exhaustive non-
      exhaustive list of examples:

     o WSON is evolving using multi-degree optical cross connects in a
        way that network topologies are changing from rings (and
        interconnected rings) to a full mesh. Adding network equipment
        such as amplifiers or regenerators, to make all paths feasible,
        leads to an over-provisioned network. Indeed, even with over
        provisioning, the network could still have some infeasible
        paths.

     o Within a given network, the optical physical interface may
        change over the network life, e.g., the optical interfaces might
        be upgraded to higher bit-rates. Such changes could result in
        paths being unsuitable for the optical signal. Although the same
        considerations may apply to other network equipment upgrades,
        the optical physical interfaces are a typical case because they
        are typically provisioned at various stages of the network's
        life span as needed by traffic demands.

     o There are cases where a network is upgraded by adding new
        optical cross connects to increase network flexibility. In such
        cases existing paths will have their feasibility modified while
        new paths will need to have their feasibility assessed.

     o With the recent bit rate increases from 10G to 40G and 100G over
        a single wavelength, WSON networks will likely be operated with
        a mix of wavelengths at different bit rates. This operational
        scenario will impose some impairment considerations due to
        different physical behavior of different bit rates and
        associated modulation formats.

      Not having an impairment aware control plane for such networks
      will require a more complex network design phase that has to also take that, since the
      beginning, takes into account evolving network status in term of
      equipments and traffic. Moreover, This could result in over-engineering the
      DWDM network with additional regenerators nodes and optical
      amplifiers. Optical impairment awareness allows for the concept of
      photonic switching where possible and provides regeneration when
      it is a must. In addition, network operations such as path
      establishment, will require significant pre-design via non-control
      plane processes resulting in significantly slower network
      provisioning.

   3. Impairment Aware Optical Path Computation

      The basic criteria for path selection is whether one can
      successfully transmit the signal 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 a signal on a particular end-to-end path. This could
      include parameters such as the Q factor to correlate both linear
      and non-linear parameters into one value.

      The impairment-aware path calculation needs also to take into
      account when regeneration happens along the path. [WSON-Frame]
      introduces the concept of Optical translucent network that
      contains transparent elements and electro-optical elements such as
      OEO regenerations. In such networks a generic lightpath light path can go
      through a certain number of regeneration points.

      Regeneration points could happen for two reasons:

     (i)              wavelength conversion to assist the RWA process to avoid
        wavelength blocking. This is the impairment free case covered
        by[WSON-Frame].

     (ii)  the optical signal is too degraded. This is the case when
        the RWA take into consideration impairment estimation covered by
        this document.

   In the latter case a lightpath light path can be seen as a set of transparent
   segments. The optical impairments calculation needs to be reset at
   each regeneration point so each transparent segment will have its own
   impairment evaluation.

      +---+    +----+   +----+     +---+     +----+    +---+
      | I |----| N1 |---| N2 |-----| R |-----| N3 |----| E |
      +--+     +----+   +----+     +---+     +----+    +---+

      |.--------------------------.|.------------------.|
              Segment 1                      Segment 2

             Figure 1 Lightpath Light path as a set of transparent segments

   For example, Figure 1 represents a lightpath Light path from node I to node E
   with a regeneration point R in between. The lightpath It is feasible from an
   impairment validation perspective if each segment both segments (I, N1, N2, R) and
   (R, N3, E) is are feasible.

   3.1. Optical Network Requirements and Constraints

      This section examines the various optical network requirements and
      constraints that an impairment aware optical control plane may
      have to operate under. These requirements and constraints motivate
      the IA-
   RWA 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 accuracy required in
      the estimation of impairment effects, and (b) the constraints on
      the impairment estimation computation and/or sharing of impairment
      information.

   3.1.1. Impairment Aware Computation Scenarios

      A. No concern for impairments or Wavelength Continuity Constraints

      This situation is covered by existing GMPLS with local wavelength
      (label) assignment.

      B. No concern for impairments but Wavelength Continuity
        Constraints

      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 may be ignored by the control
      plane and only optical parameters related to signal compatibility
      are considered.. considered.

      C. Approximated Impairment Estimation

      This situation is applicable to networks in which impairment
      effects need to be considered but there is sufficient margin such
      that they can be estimated via approximation techniques such as
      link budgets and dispersion[G.680],[G.sup39]. The viability of
      optical paths for a particular class of signals can be estimated
      using well defined approximation techniques [G.680], [G.sup39]. Note that currently
      This is the generally known as linear case where only linear impairments
      effects are considered. Also, adding taken into account. Adding or removing an optical
      signal on the path will not render any of the existing signals in
      the network as non-viable.  For example, one form of non-
   viability non-viability
      is the occurrence of transients in existing links of sufficient
      magnitude to impact the BER of those existing signals.

      Much work at ITU-T has gone into developing impairment models at
      this and more detailed levels. Impairment characterization of
      network elements could then may be used to calculate which paths
      are conformant with a specified BER for a particular signal type.
      In such a case, we can combine the impairment aware (IA) path
      computation with the RWA process to permit more optimal IA-RWA
      computations. Note, the IA path computation may also take place in
      a separate entity, i.e., a PCE.

      D. Detailed Impairment Computation

      This situation is applicable to networks in which impairment
      effects must be more accurately computed. For these networks, a
      full computation and evaluation of the impact to any existing
      paths needs to be performed prior to the addition of a new path.
      Currently no impairment models are available from ITU-T and this
      scenario is outside the scope of this document.

   3.1.2. Impairment Computation and Information Sharing Constraints

      In GMPLS, information used for path computation is standardized
      for distribution amongst the elements participating in the control
      plane and any appropriately equipped PCE can perform path
      computation. For optical systems this may not be possible. This is
      typically due to only portions of an optical system being subject
      to standardization. In ITU-T recommendations [G.698.1] and
      [G.698.2] which specify single channel interfaces to multi-channel
      DWDM systems only the single channel interfaces (transmit and
      receive) are specified while the multi-channel links are not
      standardized. These DWDM links are referred to as "black links"
      since their details are not generally available. Note however the
      overall impact of a black link at the single channel interface
      points is limited by [G.698.1] and [G.698.2].

      Typically a vendor might use proprietary impairment models for
      DWDM spans and to estimate the validity of optical paths. For
      example, models of optical nonlinearities are not currently
      standardized. Vendors may also choose not to publish impairment
      details for links or a set of network elements in order not to
      divulge their optical system designs.

      In general, the impairment estimation/validation of an optical
      path for optical networks with "black links" (path) could not be
      performed by a general purpose impairment aware (IA) computation
      entity since it would not have access to or understand the "black
      link" impairment parameters. However, impairment estimation
      (optical path validation) could be performed by a vendor specific
      impairment aware computation entity. Such a vendor specific IA
      computation, could utilize standardized impairment information
      imported from other network elements in these proprietary
      computations.

      In the following we will use the term "black links" to describe
      these computation and information sharing constraints in optical
      networks. From the control plane perspective we have the following
      options:

      A. The authority in control of the "black links" 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 authority in control of the "black links" could provide 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.

      C. The authority in control of the "black links" can provide a PCE
        that performs full IA-RWA services. The difficulty is this
        requires the one authority 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
      authority in control of the "black links" to import the shared
      impairment information from the other NEs via the control plane or
      other means as necessary.

   3.1.3. Impairment Estimation Functional Blocks Process

      The Impairment Estimation process Process can be modeled by through the
      following functional blocks. These blocks are independent of from any
      Control Plane architecture, that is, they can be implemented by
      the same or by different control plane functional blocks. functions as detailed in
      following sections.

                                                 +-----------------+
          +------------+        +-----------+    |  +------------+ |
          |            |        |           |    |  |            | |
          | Optical    |        | Optical   |    |  | Optical    | |
          | Interface  |------->| Path      |--->| Impairment|--->|  | Channel    | |
          | (Transmit/ |        | Path      |    |  | Estimation | |
          |  Receive)  |        |           |    |  |            | |
          +------------+        +-----------+    |  +------------+ |
                                                 |        ||       |
                                                 |        ||       |
                                                 |    Estimation   |
                                                 |        ||       |
                                                 |        \/       |
                                                 |  +------------+ |
                                                 |  |  BER /     | |
                                                 |  |  Q Factor  | |
                                                 |  +------------+ |
                                                 +-----------------+

      Starting from functional block on the left the Optical Interface
      represents where the optical signal is transmitted or received and
      defines the properties at the end points path. For WSON even Even the no-
      impairment case
   with no IA has like scenario B in section 3.1.1 needs to consider
      a minimum set of interface characteristics. As an example, the document [G.698.1] reports the
   full set of those parameters for certain interfaces. In this function such case only a significant subset of those parameters would be considered. In
   addition transmit and receive interface might consider a different
   subset of properties. In term of GMPLS, [WSON-Comp] provides a
   minimum set of few
      parameters to characterize assess the interface. During an
   impairment estimation process signal compatibility will be taken into
      account (see [WSON-Frame]). For the impairment-awareness case
      signal compatibility these parameters may be sufficient or not
      depending on the accepted level of approximation (Section 3.1.1). (scenarios C and
      D). This functional block highlights the need to consider a set of
      interface parameters during an Impairment Validation Process.

      The block "Optical Impairment Path" represents all kinds of
      impairments affecting a wavelength as it traverses the networks
      through links and nodes. In the case where the control plane has
      no IA IV this block will not be present. Otherwise, this function
      must be implemented in some way via the control plane. Options for
      this will be given in the next section on control plane architectural alternatives.
      This block implementation (e.g. through routing, signaling or PCE)
      may influence the way the control plane distributes impairment
      information within the network.

      The last block implements the decision function for path
      feasibility. Depending on the IA level of approximation this
      function can be more or less complex. For example in case of no IA
      only the signal class compatibility will be verified.

3.2. IA-RWA Computation and Control Plane Architectures

   From a control plane In addition
      to feasible/not-feasible result, it may be worth for decision
      functions to consider the case in which paths can be likely-to-be-
      feasible within some degree of confidence. The optical impairments
      are usually not fixed values as they may vary within ranges of
      values according to the approach taken in the physical modeling
      (worst-case, statistical or based on typical values). For example,
      the utilization of the worst-case value for each parameter within
      impairment validation process may lead to marking some paths as
      not-feasible while they are very likely to be feasible in reality.

   3.2. IA-RWA Computation 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]. [WSON-Frame]. In impairment aware routing and
      wavelength assignment (IA-
   RWA), (IA-RWA), there are conceptually three
      general classes of processes to be considered: Routing (R),
      Wavelength Assignment (WA), and Impairment Validation (estimation)
      (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. Note that that
      this case includes any paths between source and destination that
      may have been "pre-validated".

      In this case the control plane simply makes use of candidate paths
      but does not know any optical impairment information. Another
      option is when the path validity is assessed within the control
      plane. The following cases highlight this situation.

     o IV-Approximate Verification

      Here approximation methods are used to estimate the impairments
      experienced by a signal. Impairments are typically approximated by
      linear and/or statistical characteristics of individual or
      combined components and fibers along the signal path.

     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.

      The next two cases refer to the way an impairment validation
      computation can be performed.

     o IV-Centralized

      In this case impairments to a path are computed at a single
      entity. The information concerning impairments may still be
      gathered from network elements however. Depending how information
      are gathered this may put requirements on routing protocols. This
      will be detailed in following sections.

     o IV-Distributed

      In the distributed IV process, impairment approximate degradation
      measures such as OSNR, dispersion, DGD, etc. are accumulated along
      the path via a signaling like protocol. Each node on the path may
      already perform some part of the impairment computation (i.e.
      distributed). 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. information but it does
      not generally require spreading optical parameters at network
      level.

      The Control Plane however must not preclude the possibility to operate any one
      or all the above cases concurrently in the same network. For
      example there could be cases where a certain number of paths are
      already pre-validates (IV-Candidates) so the control plane may
      setup one of those path without requesting any impairment
      validation procedure. On the same network however the control
      plane may compute a path outside the set of IV-Candidates for
      which an impairment evaluation can be necessary.

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

   3.2.1. 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] [WSON-
      Frame] and (b) impairment information to validate potential paths.

   3.2.2. 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.

      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.

   3.2.3. 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. So the following conceptual options belong to
      this category:

     o RWA+D(IV) - Combined routing and wavelength assignment and
        distributed impairment validation.

     o R + D(WA & IV) -- routing separate from a distributed wavelength
        assignment and impairment validation process.

      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
      according to [G.680].

      If distributed computation would take place. For
   example in the computation of WA is being done at 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 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.

   3.3. Mapping Network Requirements to Architectures

      In Figure 2 we show process flows for three main architectural
      alternatives to IA-RWA when approximate impairment validation
      suffices. In Figure 3 we show process flows for two main
      architectural alternatives when detailed impairment verification
      is required.

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

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

               +-----------+        +----------------------+
               | +-------+ |        |    +--+    +--+      |
               | |Routing| |------->|    |WA|    |IV|      |
               | +-------+ |        |    +--+    +--+      |
               +-----------+        | Distributed Processes|
                                    +----------------------+
                                     (c)
       Figure 2 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 with "black links", but would then
      need to be provided by the authority controlling the "black
      links".

     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 elements, while the RWA process needs to know non-impairment non-
      impairment RWA information from the network elements. This
      alternative can be used with "black links", but the authority in
      control of the "black links" 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 suitable
      for "black links".

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

       +--------------+      +----------------------+     +------------+
       | +----------+ |      | +-------+    +--+    |     | +--------+ |
       | |    IV    | |      | |Routing|    |WA|    |---->| |  IV    | |
       | |candidates| |----->| +-------+    +--+    |     | |Detailed| |
       | +----------+ |      |  Combined Processes  |     | +--------+ |
       +--------------+      +----------------------+     |            |
                                      (b)                 +------------+
         Figure 3 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 impairment validation. In the case of "black links" the
      authority controlling the "black links" 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. Note that detailed impairment estimation is not
      standardized.

   4. 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 4.

           +-------------------+----+----+----------+--------+
           | 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 4 IA-RWA architectural options and control plane impacts.

   4.1. 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 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.

4.1.1. Properties of an Impairment Information Model

   In term of information model there are a set of property that needs
   to be defined for each optical parameters that need to be in some way
   considered within an impairment aware control plane.

   The properties will help to determine how the control plane can deal
   with it depending also on the above control plane architectural
   options. In some case properties value will help to indentify the
   level of approximation supported by the IV process.

  o  Time Dependency. This will identify how the impairment may vary
     along the time. There could be cases where there's no time
     dependency, while in other cases there is need of an impairment
     re-evaluation after a certain time. In some cases a level of
     approximation will consider an impairment that has time dependency
     as constant.

  o  Wavelength Dependency. This property will identify if an
     impairment value can be considered as constant over all the
     wavelength spectrum of interest or if it has different values.
     Also in this case a detailed impairment evaluation might lead to
     consider the exact value while an approximation IV might take a
     constant value for all wavelengths.

  o  Linearity. As impairments are representation of physical effects
     there are some that have a linear behavior while other are non
     linear. Linear impairments are in general easy to consider while a
     non linear will require the knowledge of the full path to be
     evaluated. An approximation level could only consider linear
     effects or approximate non-linear impairments in linear ones.

  o  Multi-Channel. There are cases where than an impairments take different
     values depending on the aside wavelengths already in place. impairment information model. In
     this case particular, it
      needs a dependency among different LSP is introduced. An
     approximation level can neglect or not common impairment "computation" model. In the effects on neighbor
     LSPs.

  o  Value range. An distributed
      IV case one needs to standardize the accumulated impairment
      measures that has to be considered by a
     computational element will needs a representation in bits. So
     depending on the impairments different types can be considered
     form integer to real numbers as well as a fixed set conveyed and updated at each node. Section 9
      of values.
     This information is important [G.680] provides guidance in term this area with specific formulas
      given for OSNR, residual dispersion, polarization mode
      dispersion/polarization dependent loss, effects of protocol definition and
     level channel
      uniformity, etc... However, specifics of approximation introduced by the number representation. what intermediate results
      are kept and in what form would need to be standardized.

   4.2. 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.

      Depending on the computational alternative the routing protocol
      may need to advertise information necessary to impairment
      validation process. This can potentially cause scalability issues
      due to the high amount of data that need to be advertised. Such
      issue can be addressed separating data that need to be advertised
      rarely and data that need to be advertised more frequently or
      adopting other form of awareness solutions described in previous
      sections (e.g. centralized and/or external IV entity).

      In term of approximated scenario (see Section 3.1.1. ) the model
      defined by [G.680] will apply and routing protocol will need to
      gather information required for such computation.

      In the case of distributed-IV no new demands would be placed on
      the routing protocol.

   4.3. 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.

      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 non-
      distributed IV scenarios.

   4.4. PCE

      In section 3.3. we gave a number of computation architectural
      alternatives that could be used to meet the various requirements
      and constraints of section 3.1.  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.

   4.4.1. Combined IV & RWA

      In this situation, shown in Figure 2(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.

   4.4.2. IV-Candidates + RWA

      In this situation, shown in Figure 2(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 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 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 5 we show a sequence diagram for the interactions
      between the PCC, RWA-PCE and IV-Candidates-PCE.

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

       Figure 5 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 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.

   4.4.3. Approximate IA-RWA + Separate Detailed IV

      In Figure 3 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 IV-Detailed-
      PCE very simple.

      IV-Detailed-PCE:

     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 6 we show a sequence diagram for the interactions for
      the process shown in Figure 3(b). This involves interactions
      between the PCC, RWA-PCE (acting as coordinator), IV-Candidates-PCE IV-Candidates-
      PCE and the IV-
   Detailed-PCE. 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 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 (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.

                   +----------+      +--------------+      +------------+      +------------
      +
       +---+       |RWA-Coord |      |IV-Candidates |      |IV-Detailed
      |
       |PCC|       |   PCE    |      |     PCE      |      |    PCE
      |
       +-+-+       +----+-----+      +------+-------+      +-----+------+      +-----+------
      +
         |.._   (a)     |                   |                    |
         |   ``--.__    |                   |                    |
         |          `-->|                   |                    |
         |              |        (b)        |                    |
         |              |--....____         |                    |
         |              |          ````---.>|                    |
         |              |                   |                    |
         |              |         (c)  __..-|                    |
         |              |     __..---''     |                    |
         |              |<--''              |                    |
         |              |                                        |
         |              |...._____          (d)                  |
         |              |         `````-----....._____           |
         |              |                             `````----->|
         |              |                                        |
         |              |                 (e)          _____.....+
         |              |          _____.....-----'''''          |
         |              |<----'''''                              |
         |     (f)   __.|                                        |
         |    __.--''   |
         |<-''          |
         |              |
       Figure 6 Sequence diagram for the interactions between PCC, RWA-
           Coordinating-PCE, IV-Candidates-PCE and IV-Detailed-PCE.

   5. 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.

   6. IANA Considerations

      This draft does not currently require any consideration from IANA.

   7. Acknowledgments

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

   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), 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, 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:

      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]

     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.

      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);

     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.

      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 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

   8. References

   8.1. Normative References

      [G.650.1] ITU-T Recommendation G.650.1, Definitions and test
               methods for linear, deterministic attributes of single-mode 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 single-
               mode optical fibre and cable, June 2005.

      [G.653] ITU-T Recommendation G.653, Characteristics of a dispersion-
             shifted
               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.

      [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.698.1] ITU-T Recommendation G.698.1, Multichannel DWDM
               applications with Single-Channel optical interface,
               December 2006.

      [G.698.2] ITU-T Recommendation G.698.2, Amplified multichannel
               DWDM applications with Single-Channel optical interface,
               July 2007.

      [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-02.txt, draft-ietf-ccamp-
               wavelength-switched-framework-02.txt, March 2009.

   8.2. Informative References

      [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.
               draft-bernstein-wson-impairment-info.

      [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-02.txt, draft-martinelli-
               ccamp-optical-imp-signaling-02.txt, February 2008.

      [WSON-Comp] G. Bernstein, Y. Lee, Ben Mack-Crane, "WSON Signal
               Characteristics and Network Element Compatibility
               Constraints for GMPLS", work in progress: draft-bernstein-
             ccamp-wson-signal. draft-
               bernstein-ccamp-wson-signal.

   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
      Giovanni Martinelli
      Cisco
      Via Philips 12
      20052 Monza, Italy

      Phone: +39 039 2092044
      Email: giomarti@cisco.com

   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

      Gabriele Galimberti
      Cisco
      Via Philips 12,
      20052 Monza, Italy

     Phone: +39 039 2091462
      Email: ggalimbe@cisco.com

      Alberto Tanzi
      Cisco
      Via Philips 12,
      20052 Monza, Italy

     Phone: +39 039 2091469
      Email: altanzi@cisco.com
      David Bianchi
      Cisco
      Via Philips 12,
      20052 Monza, Italy

      Email: davbianc@cisco.com

      Moustafa Kattan
      Cisco
      Dubai  500321
      United Arab Emirates

      Email: mkattan@cisco.com

      Dirk Schroetter
      Cisco

      Email: dschroet@cisco.com

      Daniele Ceccarelli
      Ericsson
      Via A. Negrone 1/A
      Genova - Sestri Ponente
      Italy

      Email: daniele.ceccarelli@ericsson.com

      Elisa Bellagamba
      Ericsson
      Farogatan 6,
      Kista   164 40
      Sweeden

      Email: elisa.bellagamba@ericcson.com

      Diego Caviglia
      Ericsson
      Via A. negrone 1/A
      Genova - Sestri Ponente
      Italy

      Email: diego.caviglia@ericcson.com

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