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Network Working Group                                            Y. Lee
Internet Draft                                                   Huawei
Intended status: Informational                             G. Bernstein
                                                      Grotto Networking
                                                      February 24, 2009
Expires: August 2009



          Information Model for Impaired Optical Path Validation
                draft-bernstein-wson-impairment-info-01.txt


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Abstract

   This document provides an information model for the optical
   impairment characteristics of optical network elements for use in
   path computation and optical path validation. This model is based on
   ITU-T defined optical network element characteristics as given in
   ITU-T recommendation G.680 and related specifications. This model is
   intentionally compatible with a previous impairment free optical
   information model used in optical path computations and wavelength
   assignment.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents


   1. Introduction...................................................3
   2. Optical Impairment Information Model...........................3
      2.1. Network Element Wide Parameters...........................3
      2.2. Per Port Parameters.......................................4
      2.3. Port to Port Parameters...................................4
      2.4. Frequency Dependent Parameters............................5
   3. Encoding Considerations........................................5
   4. Usage of Parameters in Optical Path Validation.................6
   5. Security Considerations........................................7
   6. IANA Considerations............................................7
   7. Conclusions....................................................7
   8. Acknowledgments................................................7
   APPENDIX A: Distributed Impairment Accumulation Model.............8
      A.1. Distributed Computation of OSNR...........................9
      A.2. Distributed Computation of Residual Dispersion...........10
      A.3. Distributed Computation of PMD...........................10
      A.4. Distributed Computation of PDL...........................11
   APPENDIX B: Optical Parameters...................................12
      B.1. Parameters for NEs without optical amplifiers............12
      B.2. Additional parameters for NEs with optical amplifiers....14
   References.......................................................16
      8.1. Normative References.....................................16
      8.2. Informative References...................................16
   Author's Addresses...............................................17
   Intellectual Property Statement..................................17
   Disclaimer of Validity...........................................17



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1. Introduction

   Impairments in optical networks can be accounted for in a number of
   ways as discussed in reference [Imp-Frame]. This document provides an
   information model for path validation in optical networks utilizing
   approximate computations. The definitions, characteristics and usage
   of the optical parameters that form this model are based on ITU-T
   recommendation G.680 [G.680]. This impairment related model is
   intentionally compatible with the impairment free model of reference
   [RWA-Info]. Although this document focuses on the optical impairment
   parameters from a control plane point of view, Appendix B provides a
   list of optical parameter definitions from ITU-T G.680 and related
   documents.

2. Optical Impairment Information Model

   The definitions of optical impairment parameters of network elements
   and examples of their use can be found in [G.680] and related
   documents (also see Appendix B). From an information modeling and
   control plane perspective, one basic aspect of a given parameter is
   the scope of its applicability within a network element. In
   particular we need to know which parameters will (a) apply to the
   network element as a whole, (b) can vary on a per port basis for a
   network element, and (c) can vary based on ingress to egress port
   pairs. A second orthogonal aspect of impairment parameters is whether
   a parameter exhibits a strong frequency variation over the optical
   frequencies supported by the subnetwork.

2.1. Network Element Wide Parameters

   Based on the definitions in [G.680] and related documents the
   following parameters apply to the network element as a whole. At most
   one of these parameters is required per network element.

   1. Channel frequency range (GHz, Max, Min)

   2. Channel insertion loss deviation (dB, Max)

   3. Ripple (dB, Max)

   4. Channel chromatic dispersion (ps/nm, Max, Min)

   5. Differential group delay (ps, Max)

   6. Polarization dependent loss (dB, Max)

   7. Reflectance (passive component) (dB, Max)


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   8. Reconfigure time/Switching time (ms, Max, Min)

   9. Channel uniformity (dB, Max)

   10.Channel addition/removal (steady-state) gain response (dB, Max,
      Min)

   11.Transient duration (ms, Max)

   12.Transient gain increase (dB, Max)

   13.Transient gain reduction (dB, Max)

   14.Multichannel gain-change difference (inter-channel gain-change
      difference) (dB, Max)

   15.Multichannel gain tilt (inter-channel gain-change ratio)(dB, Max)

2.2. Per Port Parameters

   The following optical parameters may exhibit per port dependence,
   hence may be specified at most once for each port of the network
   element.

   1. Total input power range (dBm, Max, Min)

   2. Channel input power range (dBm, Max, Min)

   3. Channel output power range (dBm, Max, Min)

   4. Input reflectance (dB, Max) (with amplifiers)

   5. Output reflectance (dB, Max) (with amplifiers)

   6. Maximum reflectance tolerable at input (dB, Min)

   7. Maximum reflectance tolerable at output (dB, Min)

   8. Maximum total output power (dBm, Max)

2.3. Port to Port Parameters

   The following optical parameters may exhibit a port-to-port
   dependence and hence may be specified at most once for each
   ingress/egress port pair of the network element.

   1. Insertion loss (dB, Max, Min)


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   2. Isolation, adjacent channel (dB, Min)

   3. Isolation, non-adjacent channel (dB, Min)

   4. Channel extinction (dB, Min)

   5. Channel signal-spontaneous noise figure (dB, Max)

   6. Channel gain (dB, Max, Min)

2.4. Frequency Dependent Parameters

   Many of the previously mentioned parameters can exhibit a significant
   frequency dependence over the range of wavelength supported by a
   subnetwork. In reference [G.680] parameters denoted as related to
   "channel" could exhibit significant frequency variation that would
   need to be encoded efficiently. These parameters may include:

   1. Channel insertion loss deviation (dB, Max)

   2. Channel chromatic dispersion (ps/nm, Max, Min)

   3. Channel uniformity (dB, Max)

   4. Insertion loss (dB, Max, Min)

   5. Channel extinction (dB, Min)

   6. Channel signal-spontaneous noise figure (dB, Max)

   7. Channel gain (dB, Max, Min)

   Finalization of this list is TBD and will need liaison with ITU-T.



3. Encoding Considerations

   The units for the various parameters include GHz, dB, dBm, ms, ps,
   and ps/nm. These are typically expressed as floating point numbers.
   Due to the measurement limitations inherent in these parameters
   single precision floating point, e.g., 32 bit IEEE floating point,
   numbers should be sufficient.

   For realistic optical network elements per port and port-to-port
   parameters typically only assume a few values. For example, the
   channel gain of a ROADM is usually specified in terms of input to


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   drop, add to output, and input to output. This implies that many port
   and port-to-port parameters could be efficiently specified, stored
   and transported by making use of the Link Set Sub-TLV and
   Connectivity Matrix Sub-TLV of reference [Encode].

   For parameters that vary with frequency we have the following
   options:

   1. Explicit parameter list with associated frequencies: Here we would
      give the parameter and frequencies it applies to.  We would need
      as many of these parameter/frequency pairs as necessary to cover
      all the frequencies and parameters. This could get large for a
      high channel count system with strong frequency dependencies in
      some parameters.

   2. Provide "standardized" general interpolation formulas and
      parameters for use over an entire frequency range or sub-range.

   3. Use parameter specific interpolation formulas based on ITU-T and
      other standards. For example in reference [G.650.1]Annex A
      equations and fitting coefficients are given for chromatic
      dispersion interpolation. Such formulas may be valid over an
      entire frequency range or a sub-range.

4. Usage of Parameters in Optical Path Validation

   Given an optical path and the optical characteristics of each network
   element along the path we can then use these characteristics to
   validate the path. We envision that these parameters will be made
   available via some mechanism to the entity which validates optical
   paths. Refer to [Imp-Frame] for architectural options in which
   impairment validation for an optical path is defined.

   Sections 9 and 10 of G.680 give techniques and formulas for use in
   calculating the impact of a cascade of network elements such as
   occurs along an optical signal path. These range from relatively
   simple bounds on the sum of uncompensated chromatic dispersion
   (residual dispersion) to more elaborate formulas for overall optical
   signal to noise ration (OSNR) computations based on multiple
   parameters including noise factor.

   To further aid understanding and use of these optical parameters
   Appendix I of [G.680] provides example parameter values for different
   network element types and appendix II provides examples of
   computations involving the cascades of network elements along a path.




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5. Security Considerations

   This document defines an information model for impairments in optical
   networks. If such a model 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. Conclusions

   The state of standardization of optical device characteristics has
   matured from when initial IETF work concerning optical impairments
   was investigated in [RFC4054]. Relatively recent ITU-T
   recommendations provide a standardized based of optical
   characteristic definitions and parameters that control plane
   technologies such as GMPLS and PCE can make use of in performing
   optical path validation. The enclosed information model shows how
   readily such ITU-T optical work can be utilized within the control
   plane.

8. Acknowledgments

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






















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APPENDIX A: Distributed Impairment Accumulation Model

   In reference [Imp-Frame] an alternative impairment aware RWA control
   plane based on distributed impairment validation was discussed. In
   such a scheme the preceding impairment information model would not be
   distributed via a link state IGP, instead a set of impairment
   parameters would be computed along the proposed path and a final
   decision on whether the path is viable would be made based on these
   accumulated impairment parameters. It should be noted that these
   accumulated impairment parameters are estimated at each node along
   the path and not measured.

   When signaling a path we think of the "nodes" as being the switching
   nodes along the path. In the case of optical impairments the
   properties of the links (WDM line systems) are just as important as
   the properties of the nodes. In the following we will assume that the
   switching nodes (GMPLS nodes) will act on behalf of all the line
   systems corresponding to their egress ports. In particular this
   implies that some how these nodes will obtain the line system
   impairment information.

    Mux           PXC                                   PXC        Demux
    |\            +--+        ROADM        ROADM        +--+         /|
   -|| BA  LA  LA |  | LA  LA +---+ LA  LA +---+ LA  LA |  | LA  LA | |-
   -||_|\__|\__|\_|  |_|\__|\_|   |_|\__|\_|   |_|\__|\_|  |_|\__|\_| |-
   -|| |/  |/  |/ |  | |/  |/ |   | |/  |/ |   | |/  |/ |  | |/  |/ | |-
   -||           -|  |-       +---+        +---+        |  |-       | |-
    |/            +--+         | |          | |         +--+         \|

   <---- NE1 ----><--- NE2 --><--- NE3 ---><--- NE4 ---><--- NE5 -->
    Figure 1 A path through an optical network with line systems, PXCs,
                         ROADMs, and multiplexers.

   In Figure 1 we show an example system from appendix II of [G.680].
   This diagram shows the DWDM line systems including amplifiers, BA =
   booster amplifier, LA = line amplifier. For distributed impairment
   validation we would group the line systems with their preceding nodes
   as shown for computational purposes.


   Section 9 of ITU-T G.680 [G.680] shows how various impairment
   parameters accumulate and this suggests that the following parameters
   or subset thereof could be used in distributed impairment estimation:

   o  Optical Signal to Noise Ratio (OSNR)

   o  Residual Dispersion (chromatic)


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   o  Polarization Mode Dispersion (PMD)

   o  Polarization Dependent Loss (PDL)

   o  Ripple

   o  Channel Uniformity

   For each of the above the units and accumulation procedure needs to
   be defined. In the following we suggest units and procedures for the
   above for which computation of cascaded elements are suitably defined
   in [G.680]. Note: ONE = Optical Network Element.

A.1. Distributed Computation of OSNR

   Section 9.1 of ITU-T G.680 gives several equivalent formulas for the
   estimation of OSNR. For distributed impairment validation the
   following formula from [G.680] is convenient:

   OSNR_out = -10*log(Term1 + Term2)

   Where

   Term1 = 10^-(ONSR_in/10), and

   Term2 = 10^-((P_in-NF-10*log(h*v*vr))/10)

   and we have the following additional definitions:

   OSNR_out is the output optical signal to noise ratio in dB of the ONE

   OSNR_in is the input optical signal to noise ratio in dB of the ONE

   P_in is the channel power (dBm) at the input port of the ONE

   NF is the noise figure (dB) of the relevant path through the ONE

   h is Planck's constant (in mJ*s to be consistent with P_in in dBm)

   v is the optical frequency in Hz

   vr is the reference bandwidth in Hz (usually the frequency equivalent
   of 0.1nm)

   From the previous formula, a distributed computation of OSNR requires
   knowing the OSNR_in and the P_in based on computations from the
   previous node along the path. The noise figure, F, is something that


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   the current node performing the computation would know along with the
   frequency, v, and the reference bandwidth vr (TBD: confirm with ITU-
   T).

   The control plane will need to distribute the following information
   from node to node along the path:

   o  OSNR_in (this is the accumulated OSNR along the path)(dB)

   o  P_in (this is the estimated power into the next node)(dBm)

   The input power would be calculated by the previous node by taking
   into account gain and attenuation on the link between the nodes.

A.2. Distributed Computation of Residual Dispersion

   The residual dispersion for a path is required to be bounded, in
   particular from [G.680] equation 9-4:

   Min RD < Residual Dispersion < Max RD

   Where Min RD and Max RD are the minimum and maximum tolerable
   residual dispersion for a particular transmitter/receiver
   combination.

   The residual dispersion for a cascade of network elements can be
   computed by [G.680] equation 9-5:

   Residual dispersion = sum(fiber dispersion) + sum(DCM dispersion)
                         + sum(ONE dispersion)


   Where DCM dispersion is from Dispersion Compensation Modules (DCM),
   and ONE dispersion is due to optical network elements.

   Although the residual dispersion formula is a relatively simple
   linear formula [G.680] indicates two possible methods for its
   evaluation (a) Worst-case upper and lower bounds, or (b)Statistical
   approach. In case (a) two parameters would need to be accumulated
   along the path a worst case upper and lower bound. In case (b) some
   type of statistical information would be needed in [G.680] mean and
   standard deviation are used under a Gaussian assumption.

A.3. Distributed Computation of PMD

   The accumulated impact of line system and ONE polarization mode
   dispersion can be estimated via the formula [G.680] equation (9-6):


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   DGDmax_link = {DGDmaxf^2 + S^2*sum_i(PMDc_i^2)}^(1/2)

   where

      DGDmax_link is the max link DGD (ps)

      DGDmxf      is the max concatenated optical fiber cable DGD (ps)

      S           is the Maxwell adjustment factor(Table 9-2 of [G.680])

      PMDc_i      is the PMD value for the ith component (ps)

   Under a distributed computation approach the above could be computed
   by keeping track of DGDmaxf and the running sum of PMDc_i^2. The
   Maxwell adjustment factor and final square root can be applied at the
   final node in the path. [Question for Q6: does DGDMaxf^2 need to be
   accumulated over the different link segments?]

A.4. Distributed Computation of PDL

   See section 9.3.2 of [G.680]




























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APPENDIX B: Optical Parameters

   The following provides an annotated list of optical characteristics
   from ITU-T recommendation G.680 [G.680] for use in optical path
   impairment computations. For each parameter we specify the units to
   be used, whether minimum or maximum values are used, and whether the
   parameters applies to the optical network element as a whole, on a
   per port basis or on a port-to-port pair basis.

   Not all these parameters will apply to all devices. The main
   differentiation in G.680 comes from those network elements that
   include or do not include optical amplifiers.

B.1. Parameters for NEs without optical amplifiers

   Channel frequency range (GHz, Max, Min): [G.671] The frequency range
   within which a DWDM device is required to operate with a specified
   performance. For a particular nominal channel central frequency,
   fnomi, this frequency range is from fimin = (fnomi - dfmax) to fimax
   = (fnomi + dfmax), where dfmax is the maximum channel central
   frequency deviation. Nominal channel central frequency and maximum
   channel central frequency deviation are defined in ITU-T Rec. G.692.

   Insertion loss (dB, Port-Port, Max, Min):[G.671] It is the reduction
   in optical power between an input and output port of a WDM device in
   decibels (dB).

   Channel insertion loss deviation (dB, Max):[G.671] This is the
   maximum variation of insertion loss at any frequency within the
   channel frequency range (DWDM devices) or channel wavelength range
   (CWDM and WWDM devices).

   Ripple (dB, Max): [G.671] For WDM devices and tuneable filters, the
   peak-to-peak difference in insertion loss within a channel frequency
   (or wavelength) range.

   Channel chromatic dispersion (ps/nm, Max, Min): [G.650.1] Change of
   the group delay of a light pulse for a unit fibre length caused by a
   unit wavelength change.

   Differential group delay (ps, Max): [G.671] Polarization Mode
   Dispersion (PMD) is usually described in terms of a Differential
   Group Delay (DGD), which is the time difference between the principal
   States of Polarization (SOPs) of an optical signal at a particular
   wavelength and time.




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   Polarization dependent loss (dB, Max): [G.671] Maximum variation of
   insertion loss due to a variation of the state of polarization (SOP)
   over all SOPs.

   Reflectance (dB, Max): [G.671] The ratio of reflected power Pr to
   incident power, Pi at a given port of a passive component, for given
   conditions of spectral composition, polarization and geometrical
   distribution.

   Isolation, adjacent channel (dB, Min, Port-Port): [G.671] The
   adjacent channel isolation (of a WDM device) is defined to be equal
   to the unidirectional (far-end) isolation of that device with the
   restriction that x, the isolation wavelength number, is restricted to
   the channels immediately adjacent to the (channel) wavelength number
   associated with port o.

   Isolation, non-adjacent channel (dB, Min, Port-Port): [G.671] The
   non-adjacent channel isolation (of a WDM device) is defined to be
   equal to the unidirectional (far-end) isolation of that device with
   the restriction that x, the isolation wavelength number, is
   restricted to each of the channels not immediately adjacent to the
   (channel) wavelength number associated with port o.

   Note: [G.671] In a WDM device able to separate k wavelengths (w1, w2,
   ... , wk) radiation coming from one input port into k output ports,
   each one nominally passing radiation at one specific wavelength only.
   The unidirectional (far-end) isolation is a measure of the part of
   the optical power at each wavelength exiting from the port at
   wavelengths different from the nominal wavelength relative to the
   power at the nominal wavelength.

   Channel extinction (dB, Min, Port-Port): [G.671] Within the operating
   wavelength range, the difference (in dB) between the maximum
   insertion loss for the non-extinguished (non-blocked) channels and
   the minimum insertion loss for the extinguished (blocked) channels.

   Reconfigure time (ms, Max, Min): [G.680] The reconfigure time (of an
   ROADM) is the elapsed time measured from the earliest point that the
   actuation energy is applied to reconfigure the ONE to the time when
   the channel insertion loss for all wanted channels has settled to
   within 0.5 dB of its final steady state value and all other
   parameters of the device (e.g., isolation and channel extinction)are
   within the allowed limits.

   Switching time (for PXC) (ms, Max, Min): [G.671] The elapsed time it
   takes the switch to turn path io on or off from a particular initial



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   state, measured from the time the actuation energy is applied or
   removed.

   Channel uniformity (dB, Max): [G.671] The difference (in dB) between
   the powers of the channel with the most power (in dBm) and the
   channel with the least power (in dBm). This applies to a multichannel
   signal across the operating wavelength range.

B.2. Additional parameters for NEs with optical amplifiers

   Total input power range (dBm, Max, Min, Port): [G.661] The range of
   optical power levels at the input for which the corresponding output
   signal optical power lies in the specified output power range, where
   the OA performance is ensured.

   Channel input power range (dBm, Max, Min, Port): see above.

   Channel output power range (dBm, Max, Min, Port): [G.661] The range
   of optical power levels at the output of the OA for which the
   corresponding input signal power lies in the specified input power
   range, where the OA performance is ensured.

   Channel signal-spontaneous noise figure (dB, Max, Port-Port) [G.661]
   The signal-spontaneous beat noise contribution to the noise figure,
   expressed in dB.

   Input reflectance (dB, Max, Port): [G.661] The maximum fraction of
   incident optical power, at the operating wavelength and over all
   states of input light polarization, reflected by the OA from the
   input port, under nominal specified operating conditions, expressed
   in dB.

   Output reflectance (dB, Max, Port): [G.661] The fraction of incident
   optical power at the operating wavelength reflected by the OA from
   the output port, under nominal operating conditions, expressed in dB.

   Maximum reflectance tolerable at input (dB, Min, Port): [G.661] The
   maximum fraction of power, expressed in dB, exiting the optical input
   port of the OA which, when reflected back into the OA, allows the
   device to still meet its specifications.

   Maximum reflectance tolerable at output (dB, Min, Port): [G.661] The
   maximum fraction of power, expressed in dB, exiting the optical
   output port of the OA which, when reflected back into the OA, allows
   the device to still meet its specifications.




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   Maximum total output power (dBm, Max, Port): [G.661] The highest
   signal optical power at the output that can be obtained from the OA
   under nominal operating conditions.

   Channel addition/removal (steady-state) gain response (dB, Max, Min):
   [G.661] For a specified multichannel configuration, the steady-state
   change in channel gain of any one of the channels due to the
   addition/removal of one or more other channels, expressed in dB.

   Transient duration (ms, Max): [G.661] The time period from the
   addition/removal of a channel to the time when the output power level
   of that or another channel reaches and remains within +- N dB from
   its steady-state value.

   Transient gain increase (dB, Max): [G.661] For a specified
   multichannel configuration, the maximum change in channel gain of any
   one of the channels due to the addition/removal of one or more other
   channels during the transient period after channel addition/removal,
   expressed in dB.

   Transient gain reduction (dB, Max): see above.

   Channel gain (dB, Max, Min, Port-Port): [G.661] Gain for each channel
   (at wavelength wj) in a specified multichannel configuration,
   expressed in dB.

   Multichannel gain-change difference (inter-channel gain-change
   difference) (dB, Max): [G.661] For a specified channel allocation,
   the difference of change in gain in one channel with respect to the
   change in gain of another channel for two specified sets of channel
   input powers, expressed in dB.

   Multichannel gain tilt (inter-channel gain-change ratio)(dB, Max):
   [G.661] The ratio of the changes in gain in each channel to the
   change in gain at a reference channel as the input conditions are
   varied from one set of input channel powers to a second set of input
   channel powers, expressed in dB per dB.












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References

8.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [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.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.

   [Imp-Frame] G. Bernstein, Y. Lee, D. Li, A Framework for the Control
             and Measurement of Wavelength Switched Optical Networks
             (WSON) with Impairments, Work in Progress, October 2008.

   [RWA-Info]  G. Bernstein, Y. Lee, D. Li, W. Imajuku, Routing and
             Wavelength Assignment Information Model for Wavelength
             Switched Optical Networks, Work in Progress: draft-ietf-
             ccamp-rwa-info-01.txt, November 2008.





8.2. Informative References

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

   [Encode]  G. Bernstein, Y. Lee, D. Li, W. Imajuku, "Routing and
             Wavelength Assignment Information Encoding for Wavelength
             Switched Optical Networks" Work in progress: draft-
             bernstein-ccamp-wson-encode-01.txt, November 2008.






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Author's Addresses

   Greg Bernstein
   Grotto Networking
   Fremont CA, 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


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