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Network Working Group                                      G. Bernstein
Internet Draft                                        Grotto Networking
Intended status: Informational                                   Y. Lee
Expires: December 2007                                           Huawei

                                                          June 25, 2007

       Applicability of GMPLS and PCE to Wavelength Switched Optical

Status of this Memo

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   Copyright (C) The IETF Trust (2007).


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   This memo examines the applicability of Generalized Multi-Protocol
   Label Switching (GMPLS) and the Path Computation Element (PCE)
   architecture to the control of wavelength switched optical networks.
   In particular we investigate how WDM based systems consisting of
   tunable laser transmitters and reconfigurable optical add/drop
   multiplexers (ROADM) or Wavelength Selective Switches (WSS) can be
   controlled with the current GMPLS/PCE protocols. Minor protocol
   extension requirements are identified where necessary.

   The three cases of full wavelength conversion, no wavelength
   conversion, and limited wavelength conversion and their impacts on
   GMPLS signaling, GMPLS routing, and PCE communications protocol are

Conventions used in this document

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

Table of Contents

   1. Introduction...................................................3
   2. Terminology....................................................4
   3. Routing and Wavelength Assignment..............................5
      3.1. Implications for GMPLS signaling..........................6
         3.1.1. No Wavelength Conversion.............................6
         3.1.2. Limited Wavelength Conversion........................7
         3.1.3. Full Wavelength Conversion...........................7
         3.1.4. Future Issues for GMPLS Signaling....................7
      3.2. Implications for GMPLS Routing............................8
         3.2.1. Need for Wavelength-Specific Maximum Bandwidth
         3.2.2. Need for Wavelength-Specific Availability Information8
         3.2.3. Describing Wavelength Conversion Capabilities........9
         3.2.4. Relationship to Link Bundling and Layering..........10
      3.3. Optical Path Computation and Implications for PCE........10
         3.3.1. No or Limited Wavelength Conversion.................10
         3.3.2. Full Wavelength Conversion..........................11
         3.3.3. PCE Discovery.......................................11
   4. Security Considerations.......................................11
   5. IANA Considerations...........................................12
   6. Conclusions...................................................12
   7. Acknowledgments...............................................12
   8. References....................................................13
      8.1. Normative References.....................................13

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      8.2. Informative References...................................13
   Author's Addresses...............................................15
   Intellectual Property Statement..................................15
   Disclaimer of Validity...........................................16

1. Introduction

   Two key optical components have emerged that are making wavelength
   switched optical networks more cost effective and dynamic. First
   introduced to reduce inventory costs, tunable optical laser
   transmitters are becoming widely deployed [Coldren04], [Buus06]. This
   allows flexibility in the wavelength used for optical transmission.
   Reconfigurable add/drop optical multiplexers (ROADM) have matured and
   are available in different forms and technologies [Basch06]. This
   allows wavelength based optical switching.

   However, another optical component, the wavelength converter, has not
   advanced as uniformly and different system designs may choose to
   utilize this component to varying degrees or not at all. Wavelength
   converters take an ingress optical signal at one wavelength and emit
   an equivalent content optical signal at another wavelength on egress.
   There are currently two approaches to building wavelength converters.
   One approach is based on optical to electrical to optical (OEO)
   conversion with tunable lasers on egress. This approach can be
   dependent upon the signal rate and format, i.e., this is basically an
   electrical regenerator combined with a tunable laser. The other
   approach performs the wavelength conversion, optically via non-linear
   optical effects, similar in spirit to the familiar frequency mixing
   used in radio frequency systems, but significantly harder to
   implement.  Such processes/effects may place limits on the range of
   achievable conversion. These may depend on the wavelength of the
   input signal and the properties of the converter as opposed to the
   only the properties of the converter in the OEO case.

   The presence and amount of wavelength conversion available at a
   wavelength switching interface has an impact on the information that
   needs to be transferred by the control plane (Generalized
   Multiprotocol Label Switching - GMPLS) and the Path Computation
   Element (PCE) architecture. Figure 1, below, summarizes the current
   capabilities of GMPLS signaling, GMPLS routing and the PCE
   architecture to support the control of switched optical networks
   consisting of (a) full wavelength conversion capabilities, (b) no
   wavelength conversion capabilities, and (c) limited wavelength
   conversion capabilities.

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                        Signaling   Routing     PCE
         Full        |  Yes      |  Yes      |  Yes     |
         Conversion  |           |           |          |
         No          |  Yes      |  No       |  Partial |
         Conversion  |           |           |          |
         Limited     |  Yes      |  No       |  Partial |
         Conversion  |           |           |          |

     Figure 1 Current support for wavelength switching in GMPLS & PCE.

   The full wavelength conversion case occurs when all ROADMs or WSSs
   have wavelength converters available on every interface. This, for
   example, would occur in the case of OEO switches with WDM interfaces
   featuring tunable lasers. Limited wavelength conversion capabilities
   exist in a network when either wavelength conversion is either not
   present on every port or not present at every switching node.
   Finally, in the case of no conversion, none of the wavelength
   switching nodes has wavelength conversion capability.

2. Terminology

   ROADM: Reconfigurable optical add/drop multiplexer. A reduced port
   count wavelength selective switching element featuring ingress and
   egress line side ports as well as add/drop side ports.

   Wavelength Conversion/Converters: The process of converting an
   information bearing optical signal centered at a given wavelength to
   one with "equivalent" content centered at a different wavelength.
   Wavelength conversion can be implemented via an optical-electronic-
   optical (OEO) process or via a strictly optical process.

   Wavelength Switched Optical Networks: Wavelength Division Multiplex
   (WDM) based optical networks in which switching is performed
   selectively based on the center wavelength of an optical signal.

   Wavelength Selective Switch (WSS): A general, multi-port, switch used
   in wavelength switched optical networks. Switches data based on
   ingress port and ingress lambda. May or may not have wavelength
   conversion capabilities.

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3. Routing and Wavelength Assignment

   In wavelength switched optical networks consisting of tunable lasers
   and wavelength selective switches with wavelength converters on every
   interface, path selection is similar to the MPLS and TDM circuit
   switched cases in that the labels, in this case wavelengths
   (lambdas), have only local significance. That is, a wavelength-
   convertible network with full wavelength-conversion capability at
   each node is equivalent to a circuit-switched TDM network with full
   time slot interchange capability; thus, the routing problem needs to
   be addressed only at the level of the TE link choice, and wavelength
   assignment can be resolved by the switches on a hop-by-hop basis.

   However, in the limiting case of an optical network with no
   wavelength converters, a light path (optical channel - OCh -) needs a
   route from source to destination and must pick a single wavelength
   that can be used along that path without "colliding" with the
   wavelength used by any other light path that may share an optical
   span. This is sometimes referred to as a "wavelength continuity
   constraint". To ease up on this constraint while keeping network
   costs in check a limited number of wavelength converters maybe
   introduce at key points in the network [Chu03].

   In the general case of limited or no wavelength converters this
   computation is known as the Routing and Wavelength Assignment (RWA)
   problem [HZang00]. The "hardness" of this problem is well documented,
   however, there exists a number of reasonable approximate methods for
   its solution [HZang00].

   The inputs to the basic RWA problem are the requested light paths
   source and destination, the networks topology, the locations and
   capabilities of any wavelength converters, and the wavelengths
   available on each optical link. The output from an algorithm solving
   the RWA problem is an explicit route through ROADMs or WSSs, a
   wavelength for the optical transmitter, and a set of locations
   (generally associated with switches) where wavelength conversion is
   to occur and the new wavelength to be used on each component link
   after that point in the route.

   It is to be noted that the RWA algorithm is out of the scope for this
   document. This document discusses GMPLS signaling and routing
   requirements and PCE requirements that enable RWA aware light path
   computation and the establishment of the LSPs in wavelength switched
   optical networks.

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3.1. Implications for GMPLS signaling

   In [RFC3471] a wavelength label is just a 32 bit integer that at a
   minimum must have significance to the two neighbors, i.e., maps to a
   specific wavelength or frequency. To set up a transparent network it
   makes more sense to map labels to wavelengths at the network (domain)
   level so we have an easy and consistent way to describe them in GMPLS
   signaling. ITU-T recommendation [G.694.1] describes a WDM grid
   defined in terms of frequency spacing of 12.5GHz, 25GHz, 50GHz,
   100GHz, and other multiples of 100GHz. To see that the 32 bit GMPLS
   label currently allocated is sufficient, consider a wideband fiber
   such as that specified in [G.656] which is capable of operating over
   a wavelength range of 1460-1625nm. This would correspond to a
   frequency range of approximately 53.44THz, and with the currently
   finest grid spacing of 12.5GHz would require approximately 4276 <
   2^13 labels. This is far less than the possible 2^32 lambda labels
   available via GMPLS hence a simplistic network wide map of
   wavelengths to labels is feasible.  An alternative to a global
   network map of labels to wavelengths would be to use LMP to assign
   the map for each link then convey that information to any path
   computation entities, e.g., label switch routers or stand alone PCEs.

   For use in GMPLS RSVP-TE path messages GMPLS already has the lambda
   (value 9) LSP encoding type [RFC3471], or for G.709 compatible
   optical channels, the LSP encoding type (value = 13) "G.709 Optical
   Channel" from [RFC4328].

3.1.1. No Wavelength Conversion

   Given a system-wide mapping between labels and lambdas and assuming
   that the RWA problem has been solved to yield a path as a series of
   links traversed by a single wavelength(explicit route). We can then
   use the GMPLS signaling procedures [RFC3471] to set up the light path
   with an appropriate interpretation of the parameters made at each
   ROADM. In particular, the source of the light path would originate a
   path message containing a label set consisting of a single label
   (that corresponds to the assigned lambda). Upon reception at the
   first ROADM or WSS this wavelength is confirmed to not be used on the
   selected outgoing interface (fiber). Per [RFC3471] procedures for the
   non-wavelength converter case the incoming label set (consisting of a
   single label) forms the basis for the out-going label set and in this
   way a path can be set up for the assigned wavelength and any
   potential lambda collisions can be caught by GMPLS signaling
   processing. Hence current GMPLS signaling can support the case with
   no wavelength conversion.

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3.1.2. Limited Wavelength Conversion

   When the optical network contains a limited number of wavelength
   converters, the solution to the RWA problem will consist of a route
   from the source to destination along with the wavelengths
   (generalized labels) to be used along portions of the path. Current
   GMPLS signaling supports an explicit route object (ERO) and within an
   ERO an ERO Label subobject can be use to indicate the wavelength to
   be used at a particular node. Hence current GMPLS signaling supports
   the case of limited wavelength conversion.

3.1.3. Full Wavelength Conversion

   When the optical network consists of full wavelength converters,
   label assignment is strictly a link local matter and wavelength
   assignment is not an issue beyond the local link, i.e., one doesn't
   have to solve the wavelength assignment portion of the RWA problem.
   Hence current GMPLS signaling (local) label assignment techniques can
   be used and the current GMPLS signaling supports the case of full
   wavelength conversion.

3.1.4. Future Issues for GMPLS Signaling

   Although Non-Return to Zero (NRZ) is currently the dominant form of
   optical modulation, new modulation formats are being researched
   [Winzer06] and deployed. With a choice in modulation formats we no
   longer have a one to one relationship between digital bandwidth in
   bytes or bits per second and the amount of optical spectrum (optical
   bandwidth) consumed. To simplify the specification of optical signals
   the ITU-T, in recommendation G.959.1, combined a rate bound and
   modulation format designator [G.959.1]. For example, two of the
   signal classes defined in [G.959.1] are:

   Optical tributary signal class NRZ 1.25G:

     "Applies to continuous digital signals with non-return to zero line
     coding, from nominally 622 Mbit/s to nominally 1.25 Gbit/s. Optical
     tributary signal class NRZ 1.25G includes a signal with STM-4 bit
     rate according to ITU-T Rec. G.707/Y.1322."

   Optical tributary signal class RZ 40G:

     "Applies to continuous digital signals with return to zero line
     coding, from nominally 9.9 Gbit/s to nominally 43.02 Gbit/s.
     Optical tributary signal class RZ 40G includes a signal with STM-
     256 bit rate according to ITU-T Rec. G.707/Y.1322 and OTU3 bit rate
     according to ITU-T Rec. G.709/Y.1331."

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   Hence, as was done in reference [RFC4606] for SONET/SDH, in the
   future it maybe worthwhile to define traffic parameters for lambda
   LSPs that include a signal type field that includes modulation format

3.2. Implications for GMPLS Routing

   GMPLS routing [RFC4202] currently defines an interface capability
   descriptor for "lambda switch capable" which we can use to describe
   the interfaces on a ROADM or other type of wavelength selective

3.2.1. Need for Wavelength-Specific Maximum Bandwidth Information

   Difficulties are encountered when trying to use the bandwidth
   accounting methods of [RFC4202] and [RFC3630] to describe the
   availability of wavelengths on a WDM link. The current RFCs give
   three link resource measures: Maximum Bandwidth, Maximum Reservable
   Bandwidth, and Unreserved Bandwidth. Although these can be used to
   describe a WDM span they do not provide the fundamental information
   needed for RWA. We are not given the maximum bandwidth per wavelength
   for the span. If we did then we could use the aforementioned measures
   to tell us the maximum wavelength count and the number of available

   For example, suppose we have a 32 channel WDM span, and that the
   system in general supports ITU-T NRZ signals up to NRZ 10Gbps.
   Further suppose that the first 20 channels are carrying 1Gbps
   Ethernet, then the maximum bandwidth would be 320Gbps and the maximum
   reservable bandwidth would be 120Gbps (12 wavelengths).
   Alternatively, consider the case where the first 8 channels are
   carrying 2.5Gbps SDH STM-16 channels, then the maximum bandwidth
   would still be 320Gbps and the maximum reservable bandwidth would be
   240Gbps (24 wavelengths).

3.2.2. Need for Wavelength-Specific Availability Information

   Even if we know the number of available wavelengths on a link, we
   actually need to know which specific wavelengths are available and
   which are occupied so we can assign a wavelength that can be used
   across the entire path from source to destination. This is currently
   not possible with GMPLS routing extensions.

   In the routing extensions for GMPLS [RFC4202], requirements for
   layer-specific TE attributes are discussed. The RWA problem for

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   optical networks without wavelength converters imposes an additional
   requirement for the lambda (or optical channel) layer: that of
   knowing which specific wavelengths are in use. Note that current
   dense WDM (DWDM) systems range from 16 channels to 128 channels with
   advanced laboratory systems with as many as 300 channels. Given these
   channel limitations and if we take the approach of a global
   wavelength to label mapping or furnishing the local mappings to the
   PCEs then representing the use of wavelengths via a simple bit-map is

   In the GMPLS extensions for OSPF [RFC4203] the interface capability
   descriptor sub-TLV contains a subfield that contains switching
   capability specific information and is one possible place for a bit
   map of available lambdas. However, current GMPLS routing extensions
   do not provide enough information for the solution of the RWA

3.2.3. Describing Wavelength Conversion Capabilities

   Topology, switching capabilities and resource status information are
   typically disseminated via GMPLS extensions to routing. From the
   point of view of an algorithm for RWA we are interested in the
   following features associated with an interface to a wavelength

   1. The number of wavelengths that can be converted, i.e., out of the
      N channels supported by the WDM link how many can be converted to
      a new lambda.

   2. The range of conversion for a given lambda. In all optical
      wavelength conversion this is typically a function of the input
      lambda. In electro-optic wavelength conversion it is just a
      property of the egress tunable laser.

   A switching node may share a pool of wavelength converters amongst
   many ports hence it would be appropriate to feed this overall node
   constraint to a RWA algorithm particularly in the case of batch
   processing of multiple light paths. See [TE-NODE] for examples of
   currently shared TE node capabilities.

   Currently the wavelength conversion capabilities/properties of a
   lambda switch capable interface are not defined in GMPLS routing
   extensions [RFC4202]. In reference [RFC4202] an interface can be
   denoted as lambda switching capable (LSC), but the default assumption
   seems to be that no constraints on wavelength conversion exist. A
   simple way to indicate that a wavelength selective switch has no
   wavelength conversion capabilities would be desirable. Note that OSPF

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   extensions for GMPLS [RFC4203] does provide a placeholder for
   "switching capability" specific information that could be used for
   this purpose.

3.2.4. Relationship to Link Bundling and Layering

   When dealing with static DWDM systems, particularly from a SONET/SDH
   or G.709 digital wrapper layer, each lambda looks like a separate
   link. Typically a bunch of unnumbered links, as supported in GMPLS
   routing extensions [RFC4202], would be used to describe a static DWDM
   system. In addition these links can be bundled into a TE link
   ([RFC4202], [RFC4201]) for more efficient dissemination of resource
   information. However, in the case discussed here we want to control a
   dynamic WDM layer and must deal with wavelengths as labels and not
   just as links or component links from the perspective of an upper
   (client) layer. In addition, a typical point to point optical cable
   contains many optical fibers and hence it may be desirable to bundle
   these separate fibers into a TE link. Note that in the no wavelength
   conversion or limited wavelength conversion situations that we will
   need information on wavelength usage on the individual component

3.3. Optical Path Computation and Implications for PCE

   As previously noted the RWA problem can be computationally intensive
   [HZang00]. Such computationally intensive path computations and
   optimizations were part of the impetus for the PCE (path computation
   element) architecture.

3.3.1. No or Limited Wavelength Conversion

   A network that consists of switches with no wavelength conversion is
   referred to as a transparent optical network. From the perspective of
   path computation, this type of network imposes an additional
   constraint; that is, a wavelength continuity constraint. It is not
   sufficient for a path that has available lambda channels on every
   link to be considered as a candidate path. At least one channel of
   the same wavelength must be available on every link of the path
   within a transparency domain.

   When the optical network contains a limited number of wavelength
   converters, the complexity of path computation increases. That is,
   the PCE needs to compute a route for a given source-destination pair
   along with the wavelengths to be used over some segments of the

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   At a minimum to solve the RWA problem one needs the following
   information: (a) tuning range of the source laser, (b) network
   topology, (c) network resource availability (wavelengths in
   use/available on particular spans), and (d) location and capabilities
   of any wavelength converters. These attributes must be made available
   to the path computation engine via configuration or advertising.

   Tuning ranges of lasers can vary with product model and more
   specifically are usually associated with specific optical bands such
   as C band (1530-1562nm) and L band (1570-1605nm). Note that if we set
   up a mapping between the system frequency grid and labels then we can
   represent the tuning range of a laser by a range of labels.

   With respect to the PCE architecture the tuning range of the source
   laser could be configured via management or as a constraint furnished
   to the PCE in a PCEP request message. After the PCE has performed the
   RWA computation and wants to return the result to the PCC, it needs
   an object/TLV in which to send back the assigned wavelength (label)in
   the case of no conversion or a set of wavelengths corresponding to
   the egress wavelengths at the wavelength converters. This can be done
   with the ERO object in conjunction with the ERO label subobject given
   that there is either a global mapping of labels to lambdas known to
   the PCE or the PCE has a collection of local label to lambda mappings
   for each interface.

3.3.2. Full Wavelength Conversion

   When the optical network consists of full wavelength converters, only
   the routing problem needs to be addressed, and wavelength assignment
   can be handled locally. In this case the PCE would not necessarily
   need to be involved with lambda/label assignments.

3.3.3. PCE Discovery

   The algorithms and network information needed for solving the RWA are
   somewhat specialized and computationally intensive hence not all PCEs
   within a domain would necessarily need or want this capability.
   Hence, it would be useful via the mechanisms being established for
   PCE discovery [DISCO] to indicate that a PCE has the ability to deal
   with the RWA problem. Reference [DISCO] indicates that a sub-TLV
   could be allocated for this purpose.

4. Security Considerations


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5. IANA Considerations


6. Conclusions


7. Acknowledgments

   The authors would like to thank Adrian Farrel for many helpful
   comments that greatly improved the contents of this draft.

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

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8. References

8.1. Normative References

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

   [RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
             (GMPLS) Signaling Functional Description", RFC 3471,
             January 2003.

   [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
             (TE) Extensions to OSPF Version 2", RFC 3630, September

   [RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in
             MPLS Traffic Engineering (TE)", RFC 4201, October 2005.

   [RFC4202] Kompella, K. and Y. Rekhter, "Routing Extensions in Support
             of Generalized Multi-Protocol Label Switching (GMPLS)", RFC
             4202, October 2005.

   [RFC4203] Kompella, K. and Y. Rekhter, "OSPF Extensions in Support of
             Generalized Multi-Protocol Label Switching (GMPLS)", RFC
             4203, October 2005.

   [RFC4328] Papadimitriou, D., "Generalized Multi-Protocol Label
             Switching (GMPLS) Signaling Extensions for G.709 Optical
             Transport Networks Control", RFC 4328, January 2006.

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

   [DISCO]  J.L. Le Roux, J.P. Vasseur, Yuichi Ikejiri, and Raymond
             Zhang, "OSPF protocol extensions for Path Computation
             Element (PCE) Discovery", work in progress, draft-ietf-pce-
             disco-proto-ospf-05.txt, May 2007.

8.2. Informative References

   [TE-NODE] J.P. Vasseur and J.L. Le Roux (eds), "IGP Routing Protocol
             Extensions for Discovery of Traffic Engineering Node
             Capabilities", work in progress, draft-ietf-ccamp-te-node-
             cap-05.txt, April 2007.

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   [HZang00] H. Zang, J. Jue and B. Mukherjeee, "A review of routing and
             wavelength assignment approaches for wavelength-routed
             optical WDM networks", Optical Networks Magazine, January

   [Coldren04]    Larry A. Coldren, G. A. Fish, Y. Akulova, J. S.
             Barton, L. Johansson and C. W. Coldren, "Tunable
             Seiconductor Lasers: A Tutorial", Journal of Lightwave
             Technology, vol. 22, no. 1, pp. 193-202, January 2004.

   [Chu03]   Xiaowen Chu, Bo Li and Chlamtac I, "Wavelength converter
             placement under different RWA algorithms in wavelength-
             routed all-optical networks", IEEE Transactions on
             Communications, vol. 51, no. 4, pp. 607-617, April 2003.

   [Buus06]    Jens Buus EJM, "Tunable Lasers in Optical Networks",
             Journal of Lightware Technology, vol. 24, no. 1, pp. 5-11,
             January 2006.

   [Basch06] E. Bert Bash, Roman Egorov, Steven Gringeri and Stuart
             Elby, "Architectural Tradeoffs for Reconfigurable Dense
             Wavelength-Division Multiplexing Systems", IEEE Journal of
             Selected Topics in Quantum Electronics, vol. 12, no. 4, pp.
             615-626, July/August 2006.

   [Winzer06]    Peter J. Winzer and Rene-Jean Essiambre, "Advanced
             Optical Modulation Formats", Proceedings of the IEEE, vol.
             94, no. 5, pp. 952-985, May 2006.

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

   [RFC4606] Mannie, E. and D. Papadimitriou, "Generalized Multi-
             Protocol Label Switching (GMPLS) Extensions for Synchronous
             Optical Network (SONET) and Synchronous Digital Hierarchy
             (SDH) Control", RFC 4606, August 2006.

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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
   Huawei Technologies
   1700 Alma Drive, Suite 100
   Plano, TX 75075

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

Intellectual Property Statement

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Internet-Draft   Wavelength Switched Optical Networks         June 2007

Disclaimer of Validity

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