Internet Engineering Task Force (IETF)
Request for Comments: 6163
Category: Informational
ISSN: 2070-1721
Y. Lee, Ed.
G. Bernstein, Ed.
Grotto Networking
W. Imajuku
April 2011

Framework for GMPLS and Path Computation Element (PCE) Control

of Wavelength Switched Optical Networks (WSONs)


This document provides a framework for applying Generalized Multi- Protocol Label Switching (GMPLS) and the Path Computation Element (PCE) architecture to the control of Wavelength Switched Optical Networks (WSONs). In particular, it examines Routing and Wavelength Assignment (RWA) of optical paths.

This document focuses on topological elements and path selection constraints that are common across different WSON environments; as such, it does not address optical impairments in any depth.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

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This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
   3. Wavelength Switched Optical Networks ............................6
      3.1. WDM and CWDM Links .........................................6
      3.2. Optical Transmitters and Receivers .........................8
      3.3. Optical Signals in WSONs ...................................9
           3.3.1. Optical Tributary Signals ..........................10
           3.3.2. WSON Signal Characteristics ........................10
      3.4. ROADMs, OXCs, Splitters, Combiners, and FOADMs ............11
           3.4.1. Reconfigurable Optical Add/Drop
                  Multiplexers and OXCs ..............................11
           3.4.2. Splitters ..........................................14
           3.4.3. Combiners ..........................................15
           3.4.4. Fixed Optical Add/Drop Multiplexers ................15
      3.5. Electro-Optical Systems ...................................16
           3.5.1. Regenerators .......................................16
           3.5.2. OEO Switches .......................................19
      3.6. Wavelength Converters .....................................19
           3.6.1. Wavelength Converter Pool Modeling .................21
      3.7. Characterizing Electro-Optical Network Elements ...........24
           3.7.1. Input Constraints ..................................25
           3.7.2. Output Constraints .................................25
           3.7.3. Processing Capabilities ............................26
   4. Routing and Wavelength Assignment and the Control Plane ........26
      4.1. Architectural Approaches to RWA ...........................27
           4.1.1. Combined RWA (R&WA) ................................27
           4.1.2. Separated R and WA (R+WA) ..........................28
           4.1.3. Routing and Distributed WA (R+DWA) .................28
      4.2. Conveying Information Needed by RWA .......................29
   5. Modeling Examples and Control Plane Use Cases ..................30
      5.1. Network Modeling for GMPLS/PCE Control ....................30
           5.1.1. Describing the WSON Nodes ..........................31
           5.1.2. Describing the Links ...............................34
      5.2. RWA Path Computation and Establishment ....................34
      5.3. Resource Optimization .....................................36
      5.4. Support for Rerouting .....................................36
      5.5. Electro-Optical Networking Scenarios ......................36
           5.5.1. Fixed Regeneration Points ..........................37
           5.5.2. Shared Regeneration Pools ..........................37
           5.5.3. Reconfigurable Regenerators ........................37
           5.5.4. Relation to Translucent Networks ...................38
   6. GMPLS and PCE Implications .....................................38
      6.1. Implications for GMPLS Signaling ..........................39
           6.1.1. Identifying Wavelengths and Signals ................39
           6.1.2. WSON Signals and Network Element Processing ........39
           6.1.3. Combined RWA/Separate Routing WA support ...........40
           6.1.4. Distributed Wavelength Assignment:
                  Unidirectional, No Converters ......................40
           6.1.5. Distributed Wavelength Assignment:
                  Unidirectional, Limited Converters .................40
           6.1.6. Distributed Wavelength Assignment:
                  Bidirectional, No Converters .......................40
      6.2. Implications for GMPLS Routing ............................41
           6.2.1. Electro-Optical Element Signal Compatibility .......41
           6.2.2. Wavelength-Specific Availability Information .......42
           6.2.3. WSON Routing Information Summary ...................43
      6.3. Optical Path Computation and Implications for PCE .........44
           6.3.1. Optical Path Constraints and Characteristics .......44
           6.3.2. Electro-Optical Element Signal Compatibility .......45
           6.3.3. Discovery of RWA-Capable PCEs ......................45
   7. Security Considerations ........................................46
   8. Acknowledgments ................................................46
   9. References .....................................................46
      9.1. Normative References ......................................46
      9.2. Informative References ....................................47

1. Introduction

Wavelength Switched Optical Networks (WSONs) are constructed from subsystems that include Wavelength Division Multiplexing (WDM) links, tunable transmitters and receivers, Reconfigurable Optical Add/Drop Multiplexers (ROADMs), wavelength converters, and electro-optical network elements. A WSON is a WDM-based optical network in which switching is performed selectively based on the center wavelength of an optical signal.

WSONs can differ from other types of GMPLS networks in that many types of WSON nodes are highly asymmetric with respect to their switching capabilities, compatibility of signal types and network elements may need to be considered, and label assignment can be non- local. In order to provision an optical connection (an optical path) through a WSON certain wavelength continuity and resource availability constraints must be met to determine viable and optimal paths through the WSON. The determination of paths is known as Routing and Wavelength Assignment (RWA).

Generalized Multi-Protocol Label Switching (GMPLS) [RFC3945] includes an architecture and a set of control plane protocols that can be used to operate data networks ranging from packet-switch-capable networks, through those networks that use Time Division Multiplexing, to WDM networks. The Path Computation Element (PCE) architecture [RFC4655] defines functional components that can be used to compute and suggest appropriate paths in connection-oriented traffic-engineered networks.

This document provides a framework for applying the GMPLS architecture and protocols [RFC3945] and the PCE architecture [RFC4655] to the control and operation of WSONs. To aid in this process, this document also provides an overview of the subsystems and processes that comprise WSONs and describes RWA so that the information requirements, both static and dynamic, can be identified to explain how the information can be modeled for use by GMPLS and PCE systems. This work will facilitate the development of protocol solution models and protocol extensions within the GMPLS and PCE protocol families.

Different WSONs such as access, metro, and long haul may apply different techniques for dealing with optical impairments; hence, this document does not address optical impairments in any depth. Note that this document focuses on the generic properties of links, switches, and path selection constraints that occur in many types of WSONs. See [WSON-Imp] for more information on optical impairments and GMPLS.

2. Terminology

Add/Drop Multiplexer (ADM): An optical device used in WDM networks and composed of one or more line side ports and typically many tributary ports.


Coarse Wavelength Division Multiplexing.


Dense Wavelength Division Multiplexing.

Degree: The degree of an optical device (e.g., ROADM) is given by a count of its line side ports.

Drop and continue: A simple multicast feature of some ADMs where a selected wavelength can be switched out of both a tributary (drop) port and a line side port.


Fixed Optical Add/Drop Multiplexer.


Generalized Multi-Protocol Label Switching.

Line side: In a WDM system, line side ports and links can typically carry the full multiplex of wavelength signals, as compared to tributary (add or drop) ports that typically carry a few (usually one) wavelength signals.

OXC: Optical Cross-Connect. An optical switching element in which a signal on any input port can reach any output port.

PCC: Path Computation Client. Any client application requesting a path computation to be performed by the Path Computation Element.

PCE: Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and application of computational constraints.

PCEP: PCE Communication Protocol. The communication protocol between a Path Computation Client and Path Computation Element.

ROADM: Reconfigurable Optical Add/Drop Multiplexer. A wavelength- selective switching element featuring input and output line side ports as well as add/drop tributary ports.


Routing and Wavelength Assignment.

Transparent Network: A Wavelength Switched Optical Network that does not contain regenerators or wavelength converters.

   Translucent Network:  A Wavelength Switched Optical Network that is
   predominantly transparent but may also contain limited numbers of
   regenerators and/or wavelength converters.

Tributary: A link or port on a WDM system that can carry significantly less than the full multiplex of wavelength signals found on the line side links/ports. Typical tributary ports are the add and drop ports on an ADM, and these support only a single wavelength channel.

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 Division Multiplexing.

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

3. Wavelength Switched Optical Networks

WSONs range in size from continent-spanning long-haul networks, to metropolitan networks, to residential access networks. In all these cases, the main concern is those properties that constrain the choice of wavelengths that can be used, i.e., restrict the wavelength Label Set, impact the path selection process, and limit the topological connectivity. In addition, if electro-optical network elements are used in the WSON, additional compatibility constraints may be imposed by the network elements on various optical signal parameters. The subsequent sections review and model some of the major subsystems of a WSON with an emphasis on those aspects that are of relevance to the control plane. In particular, WDM links, optical transmitters, ROADMs, and wavelength converters are examined.

3.1. WDM and CWDM Links

WDM and CWDM links run over optical fibers, and optical fibers come in a wide range of types that tend to be optimized for various applications. Examples include access networks, metro, long haul, and submarine links. International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) standards exist for various types of fibers. Although fiber can be categorized into Single-Mode Fibers (SMFs) and Multi-Mode Fibers (MMFs), the latter are typically used for short-reach campus and premise applications. SMFs are used for longer-reach applications and are therefore the primary concern of this document. The following SMF types are typically encountered in optical networks:

      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  |

Typically, WDM links operate in one or more of the approximately defined optical bands [G.Sup39]:

      Band     Range (nm)     Common Name    Raw Bandwidth (THz)
      O-band   1260-1360      Original       17.5
      E-band   1360-1460      Extended       15.1
      S-band   1460-1530      Short          9.4
      C-band   1530-1565      Conventional   4.4
      L-band   1565-1625      Long           7.1
      U-band   1625-1675      Ultra-long     5.5

Not all of a band may be usable; for example, in many fibers that support E-band, there is significant attenuation due to a water absorption peak at 1383 nm. Hence, a discontinuous acceptable wavelength range for a particular link may be needed and is modeled. Also, some systems will utilize more than one band. This is particularly true for CWDM systems.

Current technology subdivides the bandwidth capacity of fibers into distinct channels based on either wavelength or frequency. There are two standards covering wavelengths and channel spacing. ITU-T Recommendation G.694.1, "Spectral grids for WDM applications: DWDM frequency grid" [G.694.1], describes a DWDM grid defined in terms of frequency grids of 12.5 GHz, 25 GHz, 50 GHz, 100 GHz, and other multiples of 100 GHz around a 193.1 THz center frequency. At the narrowest channel spacing, this provides less than 4800 channels across the O through U bands. ITU-T Recommendation G.694.2, "Spectral grids for WDM applications: CWDM wavelength grid" [G.694.2], describes a CWDM grid defined in terms of wavelength increments of 20 nm running from 1271 nm to 1611 nm for 18 or so channels. The number of channels is significantly smaller than the 32-bit GMPLS Label space defined for GMPLS (see [RFC3471]). A label representation for these ITU-T grids is given in [RFC6205] and provides a common label format to be used in signaling optical paths.

Further, these ITU-T grid-based labels can also be used to describe WDM links, ROADM ports, and wavelength converters for the purposes of path selection.

Many WDM links are designed to take advantage of particular fiber characteristics or to try to avoid undesirable properties. For example, dispersion-shifted SMF [G.653] was originally designed for good long-distance performance in single-channel systems; however, putting WDM over this type of fiber requires significant system engineering and a fairly limited range of wavelengths. Hence, the following information is needed as parameters to perform basic, impairment-unaware modeling of a WDM link:

  • Wavelength range(s): Given a mapping between labels and the ITU-T grids, each range could be expressed in terms of a tuple, (lambda1, lambda2) or (freq1, freq2), where the lambdas or frequencies can be represented by 32-bit integers.
  • Channel spacing: Currently, there are five channel spacings used in DWDM systems and a single channel spacing defined for CWDM systems.

For a particular link, this information is relatively static, as changes to these properties generally require hardware upgrades. Such information may be used locally during wavelength assignment via signaling, similar to label restrictions in MPLS, or used by a PCE in providing combined RWA.

3.2. Optical Transmitters and Receivers

WDM optical systems make use of optical transmitters and receivers utilizing different wavelengths (frequencies). Some transmitters are manufactured for a specific wavelength of operation; that is, the manufactured frequency cannot be changed. First introduced to reduce inventory costs, tunable optical transmitters and receivers are deployed in some systems and allow flexibility in the wavelength used for optical transmission/reception. Such tunable optics aid in path selection.

Fundamental modeling parameters for optical transmitters and receivers from the control plane perspective are:

  • Tunable: Do the transmitters and receivers operate at variable or fixed wavelength?
  • Tuning range: This is the frequency or wavelength range over which the optics can be tuned. With the fixed mapping of labels to lambdas as proposed in [RFC6205], this can be expressed as a tuple, (lambda1, lambda2) or (freq1, freq2), where lambda1 and lambda2 or freq1 and freq2 are the labels representing the lower and upper bounds in wavelength.
  • Tuning time: Tuning times highly depend on the technology used. Thermal-drift-based tuning may take seconds to stabilize, whilst electronic tuning might provide sub-ms tuning times. Depending on the application, this might be critical. For example, thermal drift might not be usable for fast protection applications.
  • Spectral characteristics and stability: The spectral shape of a laser's emissions and its frequency stability put limits on various properties of the overall WDM system. One constraint that is relatively easy to characterize is the closest channel spacing with which the transmitter can be used.

Note that ITU-T recommendations specify many aspects of an optical transmitter. Many of these parameters, such as spectral characteristics and stability, are used in the design of WDM subsystems consisting of transmitters, WDM links, and receivers. However, they do not furnish additional information that will influence the Label Switched Path (LSP) provisioning in a properly designed system.

Also, note that optical components can degrade and fail over time. This presents the possibility of the failure of an LSP (optical path) without either a node or link failure. Hence, additional mechanisms may be necessary to detect and differentiate this failure from the others; for example, one does not want to initiate mesh restoration if the source transmitter has failed since the optical transmitter will still be failed on the alternate optical path.

3.3. Optical Signals in WSONs

The fundamental unit of switching in WSONs is intuitively that of a "wavelength". The transmitters and receivers in these networks will deal with one wavelength at a time, while the switching systems themselves can deal with multiple wavelengths at a time. Hence, multi-channel DWDM networks with single-channel interfaces are the prime focus of this document as opposed to multi-channel interfaces. Interfaces of this type are defined in ITU-T Recommendations [G.698.1] and [G.698.2]. Key non-impairment-related parameters defined in [G.698.1] and [G.698.2] are:

   (a)  Minimum channel spacing (GHz)

(b) Minimum and maximum central frequency

(c) Bitrate/Line coding (modulation) of optical tributary signals

For the purposes of modeling the WSON in the control plane, (a) and (b) are considered properties of the link and restrictions on the GMPLS Labels while (c) is a property of the "signal".

3.3.1. Optical Tributary Signals

The optical interface specifications [G.698.1], [G.698.2], and [G.959.1] all use the concept of an optical tributary signal, which is defined as "a single channel signal that is placed within an optical channel for transport across the optical network". Note the use of the qualifier "tributary" to indicate that this is a single- channel entity and not a multi-channel optical signal.

There are currently a number of different types of optical tributary signals, which are known as "optical tributary signal classes". These are currently characterized by a modulation format and bitrate range [G.959.1]:

   (a)  Optical tributary signal class Non-Return-to-Zero (NRZ) 1.25G

(b) Optical tributary signal class NRZ 2.5G

(c) Optical tributary signal class NRZ 10G

(d) Optical tributary signal class NRZ 40G

   (e)  Optical tributary signal class Return-to-Zero (RZ) 40G

Note that, with advances in technology, more optical tributary signal classes may be added and that this is currently an active area for development and standardization. In particular, at the 40G rate, there are a number of non-standardized advanced modulation formats that have seen significant deployment, including Differential Phase Shift Keying (DPSK) and Phase Shaped Binary Transmission (PSBT).

According to [G.698.2], it is important to fully specify the bitrate of the optical tributary signal. Hence, modulation format (optical tributary signal class) and bitrate are key parameters in characterizing the optical tributary signal.

3.3.2. WSON Signal Characteristics

The optical tributary signal referenced in ITU-T Recommendations [G.698.1] and [G.698.2] is referred to as the "signal" in this document. This corresponds to the "lambda" LSP in GMPLS. For signal compatibility purposes with electro-optical network elements, the following signal characteristics are considered:

  1. Optical tributary signal class (modulation format)
  1. Forward Error Correction (FEC): whether forward error correction is used in the digital stream and what type of error correcting code is used
  1. Center frequency (wavelength)
  1. Bitrate
  1. General Protocol Identifier (G-PID) for the information format

The first three items on this list can change as a WSON signal traverses the optical network with elements that include regenerators, OEO switches, or wavelength converters.

Bitrate and G-PID would not change since they describe the encoded bitstream. A set of G-PID values is already defined for lambda switching in [RFC3471] and [RFC4328].

Note that a number of non-standard or proprietary modulation formats and FEC codes are commonly used in WSONs. For some digital bitstreams, the presence of FEC can be detected; for example, in [G.707], this is indicated in the signal itself via the FEC Status Indication (FSI) byte while in [G.709], this can be inferred from whether or not the FEC field of the Optical Channel Transport Unit-k (OTUk) is all zeros.

3.4. ROADMs, OXCs, Splitters, Combiners, and FOADMs

Definitions of various optical devices such as ROADMs, Optical Cross- Connects (OXCs), splitters, combiners, and Fixed Optical Add/Drop Multiplexers (FOADMs) and their parameters can be found in [G.671]. Only a subset of these relevant to the control plane and their non- impairment-related properties are considered in the following sections.

3.4.1. Reconfigurable Optical Add/Drop Multiplexers and OXCs

ROADMs are available in different forms and technologies. This is a key technology that allows wavelength-based optical switching. A classic degree-2 ROADM is shown in Figure 1.

       Line side input    +---------------------+  Line side output
                      --->|                     |--->
                          |                     |
                          |        ROADM        |
                          |                     |
                          |                     |
                              | | | |  o o o o
                              | | | |  | | | |
                              O O O O  | | | |
      Tributary Side:   Drop (output)  Add (input)

Figure 1. Degree-2 Unidirectional ROADM

The key feature across all ROADM types is their highly asymmetric switching capability. In the ROADM of Figure 1, signals introduced via the add ports can only be sent on the line side output port and not on any of the drop ports. The term "degree" is used to refer to the number of line side ports (input and output) of a ROADM and does not include the number of "add" or "drop" ports. The add and drop ports are sometimes also called tributary ports. As the degree of the ROADM increases beyond two, it can have properties of both a switch (OXC) and a multiplexer; hence, it is necessary to know the switched connectivity offered by such a network element to effectively utilize it. A straightforward way to represent this is via a "switched connectivity" matrix A where Amn = 0 or 1, depending upon whether a wavelength on input port m can be connected to output port n [Imajuku]. For the ROADM shown in Figure 1, the switched connectivity matrix can be expressed as:

             Input    Output Port
             Port     #1 #2 #3 #4 #5
             #1:      1  1  1  1  1
             #2       1  0  0  0  0
       A =   #3       1  0  0  0  0
             #4       1  0  0  0  0
             #5       1  0  0  0  0

where input ports 2-5 are add ports, output ports 2-5 are drop ports, and input port #1 and output port #1 are the line side (WDM) ports.

For ROADMs, this matrix will be very sparse, and for OXCs, the matrix will be very dense. Compact encodings and examples, including high- degree ROADMs/OXCs, are given in [Gen-Encode]. A degree-4 ROADM is shown in Figure 2.

   Line side-1    --->|                       |--->    Line side-2
   Input (I1)         |                       |        Output (E2)
   Line side-1    <---|                       |<---    Line side-2
   Output  (E1)       |                       |        Input (I2)
                      |         ROADM         |
   Line side-3    --->|                       |--->    Line side-4
   Input (I3)         |                       |        Output (E4)
   Line side-3    <---|                       |<---    Line side-4
   Output (E3)        |                       |        Input (I4)
                      |                       |
                      | O    | O    | O    | O
                      | |    | |    | |    | |
                      O |    O |    O |    O |
   Tributary Side:   E5 I5  E6 I6  E7 I7  E8 I8

Figure 2. Degree-4 Bidirectional ROADM

Note that this is a 4-degree example with one (potentially multi- channel) add/drop per line side port.

Note also that the connectivity constraints for typical ROADM designs are "bidirectional"; that is, if input port X can be connected to output port Y, typically input port Y can be connected to output port X, assuming the numbering is done in such a way that input X and output X correspond to the same line side direction or the same add/drop port. This makes the connectivity matrix symmetrical as shown below.

       Input     Output Port
        Port     E1 E2 E3 E4 E5 E6 E7 E8
           I1    0  1  1  1  0  1  0  0
           I2    1  0  1  1  0  0  1  0
       A = I3    1  1  0  1  1  0  0  0
           I4    1  1  1  0  0  0  0  1
           I5    0  0  1  0  0  0  0  0
           I6    1  0  0  0  0  0  0  0
           I7    0  1  0  0  0  0  0  0
           I8    0  0  0  1  0  0  0  0

where I5/E5 are add/drop ports to/from line side-3, I6/E6 are add/drop ports to/from line side-1, I7/E7 are add/drop ports to/from line side-2, and I8/E8 are add/drop ports to/from line side-4. Note that diagonal elements are zero since loopback is not supported in the example. If ports support loopback, diagonal elements would be set to one.

Additional constraints may also apply to the various ports in a ROADM/OXC. The following restrictions and terms may be used:

  • Colored port: an input or, more typically, an output (drop) port restricted to a single channel of fixed wavelength
  • Colorless port: an input or, more typically, an output (drop) port restricted to a single channel of arbitrary wavelength

In general, a port on a ROADM could have any of the following wavelength restrictions:

  • Multiple wavelengths, full range port
  • Single wavelength, full range port
  • Single wavelength, fixed lambda port
  • Multiple wavelengths, reduced range port (for example wave band switching)

To model these restrictions, it is necessary to have two pieces of information for each port: (a) the number of wavelengths and (b) the wavelength range and spacing. Note that this information is relatively static. More complicated wavelength constraints are modeled in [WSON-Info].

3.4.2. Splitters

An optical splitter consists of a single input port and two or more output ports. The input optical signaled is essentially copied (with power loss) to all output ports.

Using the modeling notions of Section 3.4.1, the input and output ports of a splitter would have the same wavelength restrictions. In addition, a splitter is modeled by a connectivity matrix Amn as follows:

              Input    Output Port
              Port     #1 #2 #3 ...   #N
        A =   #1       1  1  1  ...   1

The difference from a simple ROADM is that this is not a switched connectivity matrix but the fixed connectivity matrix of the device.

3.4.3. Combiners

An optical combiner is a device that combines the optical wavelengths carried by multiple input ports into a single multi-wavelength output port. The various ports may have different wavelength restrictions. It is generally the responsibility of those using the combiner to ensure that wavelength collision does not occur on the output port. The fixed connectivity matrix Amn for a combiner would look like:

              Input    Output Port
              Port     #1
              #1:      1
              #2       1
        A =   #3       1
              ...      1
              #N       1

3.4.4. Fixed Optical Add/Drop Multiplexers

A Fixed Optical Add/Drop Multiplexer can alter the course of an input wavelength in a preset way. In particular, a given wavelength (or waveband) from a line side input port would be dropped to a fixed "tributary" output port. Depending on the device's construction, that same wavelength may or may not also be sent out the line side output port. This is commonly referred to as a "drop and continue" operation. Tributary input ports ("add" ports) whose signals are combined with each other and other line side signals may also exist.

In general, to represent the routing properties of an FOADM, it is necessary to have both a fixed connectivity matrix Amn, as previously discussed, and the precise wavelength restrictions for all input and output ports. From the wavelength restrictions on the tributary output ports, the wavelengths that have been selected can be derived. From the wavelength restrictions on the tributary input ports, it can be seen which wavelengths have been added to the line side output port. Finally, from the added wavelength information and the line side output wavelength restrictions, it can be inferred which wavelengths have been continued.

To summarize, the modeling methodology introduced in Section 3.4.1, which consists of a connectivity matrix and port wavelength restrictions, can be used to describe a large set of fixed optical devices such as combiners, splitters, and FOADMs. Hybrid devices consisting of both switched and fixed parts are modeled in [WSON-Info].

3.5. Electro-Optical Systems

This section describes how Electro-Optical Systems (e.g., OEO switches, wavelength converters, and regenerators) interact with the WSON signal characteristics listed in Section 3.3.2. OEO switches, wavelength converters, and regenerators all share a similar property: they can be more or less "transparent" to an "optical signal" depending on their functionality and/or implementation. Regenerators have been fairly well characterized in this regard and hence their properties can be described first.

3.5.1. Regenerators

The various approaches to regeneration are discussed in ITU-T [G.872], Annex A. They map a number of functions into the so-called 1R, 2R, and 3R categories of regenerators as summarized in Table 1 below:

Table 1. Regenerator Functionality Mapped to General Regenerator

             Classes from [G.872]
   1R | Equal amplification of all frequencies within the amplification
      | bandwidth.  There is no restriction upon information formats.
      | Amplification with different gain for frequencies within the
      | amplification bandwidth.  This could be applied to both single-
      | channel and multi-channel systems.
      | Dispersion compensation (phase distortion).  This analogue
      | process can be applied in either single-channel or multi-
      | channel systems.
   2R | Any or all 1R functions.  Noise suppression.
      | Digital reshaping (Schmitt Trigger function) with no clock
      | recovery.  This is applicable to individual channels and can be
      | used for different bitrates but is not transparent to line
      | coding (modulation).
   3R | Any or all 1R and 2R functions.  Complete regeneration of the
      | pulse shape including clock recovery and retiming within
      | required jitter limits.

This table shows that 1R regenerators are generally independent of signal modulation format (also known as line coding) but may work over a limited range of wavelengths/frequencies. 2R regenerators are generally applicable to a single digital stream and are dependent upon modulation format (line coding) and, to a lesser extent, are limited to a range of bitrates (but not a specific bitrate). Finally, 3R regenerators apply to a single channel, are dependent upon the modulation format, and are generally sensitive to the bitrate of digital signal, i.e., either are designed to only handle a specific bitrate or need to be programmed to accept and regenerate a specific bitrate. In all these types of regenerators, the digital bitstream contained within the optical or electrical signal is not modified.

It is common for regenerators to modify the digital bitstream for performance monitoring and fault management purposes. Synchronous Optical Networking (SONET), Synchronous Digital Hierarchy (SDH), and Interfaces for the Optical Transport Network [G.709] all have digital signal "envelopes" designed to be used between "regenerators" (in this case, 3R regenerators). In SONET, this is known as the "section" signal; in SDH, this is known as the "regenerator section" signal; and, in G.709, this is known as an OTUk. These signals reserve a portion of their frame structure (known as overhead) for use by regenerators. The nature of this overhead is summarized in Table 2 below.

     Table 2.  SONET, SDH, and G.709 Regenerator-Related Overhead
    |Function          |       SONET/SDH      |     G.709 OTUk        |
    |                  |       Regenerator    |                       |
    |                  |       Section        |                       |
    |Signal            |       J0 (section    |  Trail Trace          |
    |Identifier        |       trace)         |  Identifier (TTI)     |
    |Performance       |       BIP-8 (B1)     |  BIP-8 (within SM)    |
    |Monitoring        |                      |                       |
    |Management        |       D1-D3 bytes    |  GCC0 (general        |
    |Communications    |                      |  communications       |
    |                  |                      |  channel)             |
    |Fault Management  |       A1, A2 framing | FAS (frame alignment  |
    |                  |       bytes          | signal), BDI (backward|
    |                  |                      | defect indication),   |
    |                  |                      | BEI (backward error   |
    |                  |                      | indication)           |
    |Forward Error     |       P1,Q1 bytes    |  OTUk FEC             |
    |Correction (FEC)  |                      |                       |

Table 2 shows that frame alignment, signal identification, and FEC are supported. By omission, Table 2 also shows that no switching or multiplexing occurs at this layer. This is a significant simplification for the control plane since control plane standards require a multi-layer approach when there are multiple switching layers but do not require the "layering" to provide the management functions shown in Table 2. That is, many existing technologies covered by GMPLS contain extra management-related layers that are essentially ignored by the control plane (though not by the management plane). Hence, the approach here is to include regenerators and other devices at the WSON layer unless they provide higher layer switching; then, a multi-layer or multi-region approach [RFC5212] is called for. However, this can result in regenerators having a dependence on the client signal type.

Hence, depending upon the regenerator technology, the constraints listed in Table 3 may be imposed by a regenerator device:

Table 3. Regenerator Compatibility Constraints

     |      Constraints            |   1R   |   2R   |   3R   |
     | Limited Wavelength Range    |    x   |    x   |    x   |
     | Modulation Type Restriction |        |    x   |    x   |
     | Bitrate Range Restriction   |        |    x   |    x   |
     | Exact Bitrate Restriction   |        |        |    x   |
     | Client Signal Dependence    |        |        |    x   |

Note that the limited wavelength range constraint can be modeled for GMPLS signaling with the Label Set defined in [RFC3471] and that the modulation type restriction constraint includes FEC.

3.5.2. OEO Switches

A common place where OEO processing may take place is within WSON switches that utilize (or contain) regenerators. This may be to convert the signal to an electronic form for switching then reconvert to an optical signal prior to output from the switch. Another common technique is to add regenerators to restore signal quality either before or after optical processing (switching). In the former case, the regeneration is applied to adapt the signal to the switch fabric regardless of whether or not it is needed from a signal-quality perspective.

In either case, these optical switches have essentially the same compatibility constraints as those described for regenerators in Table 3.

3.6. Wavelength Converters

Wavelength converters take an input optical signal at one wavelength and emit an equivalent content optical signal at another wavelength on output. There are multiple approaches to building wavelength converters. One approach is based on OEO conversion with fixed or tunable optics on output. This approach can be dependent upon the signal rate and format; that is, this is basically an electrical regenerator combined with a laser/receiver. Hence, this type of wavelength converter has signal-processing restrictions that are essentially the same as those described for regenerators in Table 3 of Section 3.5.1.

Another 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 only the properties of the converter in the OEO case. Different WSON system designs may choose to utilize this component to varying degrees or not at all.

Current or envisioned contexts for wavelength converters are:

  1. Wavelength conversion associated with OEO switches and fixed or tunable optics. In this case, there are typically multiple converters available since each use of an OEO switch can be thought of as a potential wavelength converter.
  1. Wavelength conversion associated with ROADMs/OXCs. In this case, there may be a limited pool of wavelength converters available. Conversion could be either all optical or via an OEO method.
  1. Wavelength conversion associated with fixed devices such as FOADMs. In this case, there may be a limited amount of conversion. Also, the conversion may be used as part of optical path routing.

Based on the above considerations, wavelength converters are modeled as follows:

  1. Wavelength converters can always be modeled as associated with network elements. This includes fixed wavelength routing elements.
  1. A network element may have full wavelength conversion capability (i.e., any input port and wavelength) or a limited number of wavelengths and ports. On a box with a limited number of converters, there also may exist restrictions on which ports can reach the converters. Hence, regardless of where the converters actually are, they can be associated with input ports.
  1. Wavelength converters have range restrictions that are either independent or dependent upon the input wavelength.

In WSONs where wavelength converters are sparse, an optical path may appear to loop or "backtrack" upon itself in order to reach a wavelength converter prior to continuing on to its destination. The lambda used on input to the wavelength converter would be different from the lambda coming back from the wavelength converter.

A model for an individual OEO wavelength converter would consist of:

  • Input lambda or frequency range
  • Output lambda or frequency range

3.6.1. Wavelength Converter Pool Modeling

A WSON node may include multiple wavelength converters. These are usually arranged into some type of pool to promote resource sharing. There are a number of different approaches used in the design of switches with converter pools. However, from the point of view of path computation, it is necessary to know the following:

  1. The nodes that support wavelength conversion
  1. The accessibility and availability of a wavelength converter to convert from a given input wavelength on a particular input port to a desired output wavelength on a particular output port
  1. Limitations on the types of signals that can be converted and the conversions that can be performed

To model point 2 above, a technique similar to that used to model ROADMs and optical switches can be used, i.e., matrices to indicate possible connectivity along with wavelength constraints for links/ports. Since wavelength converters are considered a scarce resource, it is desirable to include, at a minimum, the usage state of individual wavelength converters in the pool.

A three stage model is used as shown schematically in Figure 3. This model represents N input ports (fibers), P wavelength converters, and M output ports (fibers). Since not all input ports can necessarily reach the converter pool, the model starts with a wavelength pool input matrix WI(i,p) = {0,1}, where input port i can potentially reach wavelength converter p.

Since not all wavelengths can necessarily reach all the converters or the converters may have a limited input wavelength range, there is a set of input port constraints for each wavelength converter. Currently, it is assumed that a wavelength converter can only take a single wavelength on input. Each wavelength converter input port constraint can be modeled via a wavelength set mechanism.

Next, there is a state vector WC(j) = {0,1} dependent upon whether wavelength converter j in the pool is in use. This is the only state kept in the converter pool model. This state is not necessary for modeling "fixed" transponder system, i.e., systems where there is no sharing. In addition, this state information may be encoded in a much more compact form depending on the overall connectivity structure [Gen-Encode].

After that, a set of wavelength converter output wavelength constraints is used. These constraints indicate what wavelengths a particular wavelength converter can generate or are restricted to generating due to internal switch structure.

Finally, a wavelength pool output matrix WE(p,k) = {0,1} indicates whether the output from wavelength converter p can reach output port k. Examples of this method being used to model wavelength converter pools for several switch architectures are given in [Gen-Encode].

      I1   +-------------+                       +-------------+ E1
     ----->|             |      +--------+       |             |----->
      I2   |             +------+ WC #1  +-------+             | E2
     ----->|             |      +--------+       |             |----->
           | Wavelength  |                       |  Wavelength |
           | Converter   |      +--------+       |  Converter  |
           | Pool        +------+ WC #2  +-------+  Pool       |
           |             |      +--------+       |             |
           | Input       |                       |  Output     |
           | Connection  |           .           |  Connection |
           | Matrix      |           .           |  Matrix     |
           |             |           .           |             |
           |             |                       |             |
      IN   |             |      +--------+       |             | EM
     ----->|             +------+ WC #P  +-------+             |----->
           |             |      +--------+       |             |
           +-------------+   ^               ^   +-------------+
                             |               |
                             |               |
                             |               |
                             |               |

Input wavelength Output wavelength

constraints for constraints for

each converter each converter

Figure 3. Schematic Diagram of Wavelength Converter Pool Model

Figure 4 shows a simple optical switch in a four-wavelength DWDM system sharing wavelength converters in a general shared "per-node" fashion.

                 +-----------+ ___________                +------+
                 |           |--------------------------->|      |
                 |           |--------------------------->|  C   |
           /|    |           |--------------------------->|  o   | E1
     I1   /D+--->|           |--------------------------->|  m   |
         + e+--->|           |                            |  b   |====>
    ====>| M|    |  Optical  |    +-----------+  +----+   |  i   |
         + u+--->|   Switch  |    |  WC Pool  |  |O  S|-->|  n   |
          \x+--->|           |    |  +-----+  |  |p  w|-->|  e   |
           \|    |           +----+->|WC #1|--+->|t  i|   |  r   |
                 |           |    |  +-----+  |  |i  t|   +------+
                 |           |    |           |  |c  c|   +------+
           /|    |           |    |  +-----+  |  |a  h|-->|      |
     I2   /D+--->|           +----+->|WC #2|--+->|l   |-->|  C   | E2
         + e+--->|           |    |  +-----+  |  |    |   |  o   |
    ====>| M|    |           |    +-----------+  +----+   |  m   |====>
         + u+--->|           |                            |  b   |
          \x+--->|           |--------------------------->|  i   |
           \|    |           |--------------------------->|  n   |
                 |           |--------------------------->|  e   |
                 |___________|--------------------------->|  r   |
                 +-----------+                            +------+

Figure 4. An Optical Switch Featuring a Shared Per-Node Wavelength

                Converter Pool Architecture

In this case, the input and output pool matrices are simply:

              +-----+       +-----+
              | 1 1 |       | 1 1 |
          WI =|     |,  WE =|     |
              | 1 1 |       | 1 1 |
              +-----+       +-----+

Figure 5 shows a different wavelength pool architecture known as "shared per fiber". In this case, the input and output pool matrices are simply:

               +-----+       +-----+
               | 1 1 |       | 1 0 |
           WI =|     |,  WE =|     |
               | 1 1 |       | 0 1 |
               +-----+       +-----+
                 +-----------+                            +------+
                 |           |--------------------------->|      |
                 |           |--------------------------->|  C   |
           /|    |           |--------------------------->|  o   | E1
     I1   /D+--->|           |--------------------------->|  m   |
         + e+--->|           |                            |  b   |====>
    ====>| M|    |  Optical  |    +-----------+           |  i   |
         + u+--->|   Switch  |    |  WC Pool  |           |  n   |
          \x+--->|           |    |  +-----+  |           |  e   |
           \|    |           +----+->|WC #1|--+---------->|  r   |
                 |           |    |  +-----+  |           +------+
                 |           |    |           |           +------+
           /|    |           |    |  +-----+  |           |      |
     I2   /D+--->|           +----+->|WC #2|--+---------->|  C   | E2
         + e+--->|           |    |  +-----+  |           |  o   |
    ====>| M|    |           |    +-----------+           |  m   |====>
         + u+--->|           |                            |  b   |
          \x+--->|           |--------------------------->|  i   |
           \|    |           |--------------------------->|  n   |
                 |           |--------------------------->|  e   |
                 |___________|--------------------------->|  r   |
                 +-----------+                            +------+

Figure 5. An Optical Switch Featuring a Shared Per-Fiber Wavelength

               Converter Pool Architecture

3.7. Characterizing Electro-Optical Network Elements

In this section, electro-optical WSON network elements are characterized by the three key functional components: input constraints, output constraints, and processing capabilities.

                             WSON Network Element
          WSON Signal     |      |         |      |    WSON Signal
                          |      |         |      |
        --------------->  |      |         |      | ----------------->
                          |      |         |      |
                          <-----> <-------> <----->
                          Input   Processing Output

Figure 6. WSON Network Element

3.7.1. Input Constraints

Sections 3.5 and 3.6 discuss the basic properties of regenerators, OEO switches, and wavelength converters. From these, the following possible types of input constraints and properties are derived:

  1. Acceptable modulation formats
   2.  Client signal (G-PID) restrictions
  1. Bitrate restrictions
  1. FEC coding restrictions
   5.  Configurability: (a) none, (b) self-configuring, (c) required

These constraints are represented via simple lists. Note that the device may need to be "provisioned" via signaling or some other means to accept signals with some attributes versus others. In other cases, the devices may be relatively transparent to some attributes, e.g., a 2R regenerator to bitrate. Finally, some devices may be able to auto-detect some attributes and configure themselves, e.g., a 3R regenerator with bitrate detection mechanisms and flexible phase locking circuitry. To account for these different cases, item 5 has been added, which describes the device's configurability.

Note that such input constraints also apply to the termination of the WSON signal.

3.7.2. Output Constraints

None of the network elements considered here modifies either the bitrate or the basic type of the client signal. However, they may modify the modulation format or the FEC code. Typically, the following types of output constraints are seen:

  1. Output modulation is the same as input modulation (default)
  1. A limited set of output modulations is available
  1. Output FEC is the same as input FEC code (default)
  1. A limited set of output FEC codes is available

Note that in cases 2 and 4 above, where there is more than one choice in the output modulation or FEC code, the network element will need to be configured on a per-LSP basis as to which choice to use.

3.7.3. Processing Capabilities

A general WSON network element (NE) can perform a number of signal processing functions including:

   (A) Regeneration (possibly different types)

(B) Fault and performance monitoring

   (C) Wavelength conversion
   (D) Switching

An NE may or may not have the ability to perform regeneration (of one of the types previously discussed). In addition, some nodes may have limited regeneration capability, i.e., a shared pool, which may be applied to selected signals traversing the NE. Hence, to describe the regeneration capability of a link or node, it is necessary to have, at a minimum:

   1.  Regeneration capability: (a) fixed, (b) selective, (c) none
   2.  Regeneration type: 1R, 2R, 3R
  1. Regeneration pool properties for the case of selective regeneration (input and output restrictions, availability)

Note that the properties of shared regenerator pools would be essentially the same as that of wavelength converter pools modeled in Section 3.6.1.

Item B (fault and performance monitoring) is typically outside the scope of the control plane. However, when the operations are to be performed on an LSP basis or on part of an LSP, the control plane can be of assistance in their configuration. Per-LSP, per-node, and fault and performance monitoring examples include setting up a "section trace" (a regenerator overhead identifier) between two nodes or intermediate optical performance monitoring at selected nodes along a path.

4. Routing and Wavelength Assignment and the Control Plane

From a control plane perspective, a wavelength-convertible network with full wavelength-conversion capability at each node can be controlled much like a packet MPLS-labeled network or a circuit- switched Time Division Multiplexing (TDM) network with full-time slot interchange capability is controlled. In this case, the path selection process needs to identify the Traffic Engineered (TE) links to be used by an optical path, and wavelength assignment can be made on a hop-by-hop basis.

However, in the case of an optical network without wavelength converters, an optical path needs to be routed from source to destination and must use a single wavelength that is available along that path without "colliding" with a wavelength used by any other optical path that may share an optical fiber. This is sometimes referred to as a "wavelength continuity constraint".

In the general case of limited or no wavelength converters, the computation of both the links and wavelengths is known as RWA.

The inputs to basic RWA are the requested optical path's source and destination, the network topology, the locations and capabilities of any wavelength converters, and the wavelengths available on each optical link. The output from an algorithm providing RWA is an explicit route through ROADMs, a wavelength for optical transmitter, and a set of locations (generally associated with ROADMs or 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 choice of a specific RWA algorithm is out of the scope of this document. However, there are a number of different approaches to dealing with RWA algorithms that can affect the division of effort between path computation/routing and signaling.

4.1. Architectural Approaches to RWA

Two general computational approaches are taken to performing RWA. Some algorithms utilize a two-step procedure of path selection followed by wavelength assignment, and others perform RWA in a combined fashion.

In the following sections, three different ways of performing RWA in conjunction with the control plane are considered. The choice of one of these architectural approaches over another generally impacts the demands placed on the various control plane protocols. The approaches are provided for reference purposes only, and other approaches are possible.

4.1.1. Combined RWA (R&WA)

In this case, a unique entity is in charge of performing routing and wavelength assignment. This approach relies on a sufficient knowledge of network topology, of available network resources, and of network nodes' capabilities. This solution is compatible with most known RWA algorithms, particularly those concerned with network optimization. On the other hand, this solution requires up-to-date and detailed network information.

Such a computational entity could reside in two different places:

  • In a PCE that maintains a complete and updated view of network state and provides path computation services to nodes
  • In an ingress node, in which case all nodes have the R&WA functionality and network state is obtained by a periodic flooding of information provided by the other nodes

4.1.2. Separated R and WA (R+WA)

In this case, one entity performs routing while a second performs wavelength assignment. The first entity furnishes one or more paths to the second entity, which will perform wavelength assignment and final path selection.

The separation of the entities computing the path and the wavelength assignment constrains the class of RWA algorithms that may be implemented. Although it may seem that algorithms optimizing a joint usage of the physical and wavelength paths are excluded from this solution, many practical optimization algorithms only consider a limited set of possible paths, e.g., as computed via a k-shortest path algorithm. Hence, while there is no guarantee that the selected final route and wavelength offer the optimal solution, reasonable optimization can be performed by allowing multiple routes to pass to the wavelength selection process.

The entity performing the routing assignment needs the topology information of the network, whereas the entity performing the wavelength assignment needs information on the network's available resources and specific network node capabilities.

4.1.3. Routing and Distributed WA (R+DWA)

In this case, one entity performs routing, while wavelength assignment is performed on a hop-by-hop, distributed manner along the previously computed path. This mechanism relies on updating of a list of potential wavelengths used to ensure conformance with the wavelength continuity constraint.

As currently specified, the GMPLS protocol suite signaling protocol can accommodate such an approach. GMPLS, per [RFC3471], includes support for the communication of the set of labels (wavelengths) that may be used between nodes via a Label Set. When conversion is not performed at an intermediate node, a hop generates the Label Set it sends to the next hop based on the intersection of the Label Set received from the previous hop and the wavelengths available on the node's switch and ongoing interface. The generation of the outgoing Label Set is up to the node local policy (even if one expects a consistent policy configuration throughout a given transparency domain). When wavelength conversion is performed at an intermediate node, a new Label Set is generated. The egress node selects one label in the Label Set that it received; additionally, the node can apply local policy during label selection. GMPLS also provides support for the signaling of bidirectional optical paths.

Depending on these policies, a wavelength assignment may not be found, or one may be found that consumes too many conversion resources relative to what a dedicated wavelength assignment policy would have achieved. Hence, this approach may generate higher blocking probabilities in a heavily loaded network.

This solution may be facilitated via signaling extensions that ease its functioning and possibly enhance its performance with respect to blocking probability. Note that this approach requires less information dissemination than the other techniques described.

The first entity may be a PCE or the ingress node of the LSP.

4.2. Conveying Information Needed by RWA

The previous sections have characterized WSONs and optical path requests. In particular, high-level models of the information used by RWA process were presented. This information can be viewed as either relatively static, i.e., changing with hardware changes (including possibly failures), or relatively dynamic, i.e., those that can change with optical path provisioning. The time requirement in which an entity involved in RWA process needs to be notified of such changes is fairly situational. For example, for network restoration purposes, learning of a hardware failure or of new hardware coming online to provide restoration capability can be critical.

Currently, there are various methods for communicating RWA relevant information. These include, but are not limited to, the following:

   o  Existing control plane protocols, i.e., GMPLS routing and
      signaling.  Note that routing protocols can be used to convey both
      static and dynamic information.
  • Management protocols such as NetConf, SNMPv3, and CORBA.
  • Methods to access configuration and status information such as a command line interface (CLI).
  • Directory services and accompanying protocols. These are typically used for the dissemination of relatively static information. Directory services are not suited to manage information in dynamic and fluid environments.
   o  Other techniques for dynamic information, e.g., sending
      information directly from NEs to PCEs to avoid flooding.  This
      would be useful if the number of PCEs is significantly less than
      the number of WSON NEs.  There may be other ways to limit flooding
      to "interested" NEs.

Possible mechanisms to improve scaling of dynamic information include:

  • Tailoring message content to WSON, e.g., the use of wavelength ranges or wavelength occupation bit maps
  • Utilizing incremental updates if feasible

5. Modeling Examples and Control Plane Use Cases

This section provides examples of the fixed and switched optical node and wavelength constraint models of Section 3 and use cases for WSON control plane path computation, establishment, rerouting, and optimization.

5.1. Network Modeling for GMPLS/PCE Control

Consider a network containing three routers (R1 through R3), eight WSON nodes (N1 through N8), 18 links (L1 through L18), and one OEO converter (O1) in a topology shown in Figure 7.

                       +--+    +--+             +--+       +--------+
                  +-L3-+N2+-L5-+  +--------L12--+N6+--L15--+   N8   +
                  |    +--+    |N4+-L8---+      +--+       ++--+---++
                  |            |  +-L9--+|                  |  |   |
      +--+      +-+-+          ++-+     ||                  | L17 L18
      |  ++-L1--+   |           |      ++++      +----L16---+  |   |
      |R1|      | N1|           L7     |R2|      |             |   |
      |  ++-L2--+   |           |      ++-+      |            ++---++
      +--+      +-+-+           |       |        |            +  R3 |
                  |    +--+    ++-+     |        |            +-----+
                  +-L4-+N3+-L6-+N5+-L10-+       ++----+
                       +--+    |  +--------L11--+ N7  +
                               +--+             ++---++
                                                 |   |
                                                L13 L14
                                                 |   |
                                                ++-+ |

Figure 7. Routers and WSON Nodes in a GMPLS and PCE Environment

5.1.1. Describing the WSON Nodes

The eight WSON nodes described in Figure 7 have the following properties:

  • Nodes N1, N2, and N3 have FOADMs installed and can therefore only access a static and pre-defined set of wavelengths.
  • All other nodes contain ROADMs and can therefore access all wavelengths.
  • Nodes N4, N5, N7, and N8 are multi-degree nodes, allowing any wavelength to be optically switched between any of the links. Note, however, that this does not automatically apply to wavelengths that are being added or dropped at the particular node.
  • Node N4 is an exception to that: this node can switch any wavelength from its add/drop ports to any of its output links (L5, L7, and L12 in this case).
  • The links from the routers are only able to carry one wavelength, with the exception of links L8 and L9, which are capable to add/drop any wavelength.
  • Node N7 contains an OEO transponder (O1) connected to the node via links L13 and L14. That transponder operates in 3R mode and does not change the wavelength of the signal. Assume that it can regenerate any of the client signals but only for a specific wavelength.

Given the above restrictions, the node information for the eight nodes can be expressed as follows (where ID = identifier, SCM = switched connectivity matrix, and FCM = fixed connectivity matrix):

      +ID+SCM                    +FCM                    +
      |  |   |L1 |L2 |L3 |L4 |   |   |L1 |L2 |L3 |L4 |   |
      |  |L1 |0  |0  |0  |0  |   |L1 |0  |0  |1  |0  |   |
      |N1|L2 |0  |0  |0  |0  |   |L2 |0  |0  |0  |1  |   |
      |  |L3 |0  |0  |0  |0  |   |L3 |1  |0  |0  |1  |   |
      |  |L4 |0  |0  |0  |0  |   |L4 |0  |1  |1  |0  |   |
      |  |   |L3 |L5 |   |   |   |   |L3 |L5 |   |   |   |
      |N2|L3 |0  |0  |   |   |   |L3 |0  |1  |   |   |   |
      |  |L5 |0  |0  |   |   |   |L5 |1  |0  |   |   |   |
      |  |   |L4 |L6 |   |   |   |   |L4 |L6 |   |   |   |
      |N3|L4 |0  |0  |   |   |   |L4 |0  |1  |   |   |   |
      |  |L6 |0  |0  |   |   |   |L6 |1  |0  |   |   |   |
      |  |   |L5 |L7 |L8 |L9 |L12|   |L5 |L7 |L8 |L9 |L12|
      |  |L5 |0  |1  |1  |1  |1  |L5 |0  |0  |0  |0  |0  |
      |N4|L7 |1  |0  |1  |1  |1  |L7 |0  |0  |0  |0  |0  |
      |  |L8 |1  |1  |0  |1  |1  |L8 |0  |0  |0  |0  |0  |
      |  |L9 |1  |1  |1  |0  |1  |L9 |0  |0  |0  |0  |0  |
      |  |L12|1  |1  |1  |1  |0  |L12|0  |0  |0  |0  |0  |
      |  |   |L6 |L7 |L10|L11|   |   |L6 |L7 |L10|L11|   |
      |  |L6 |0  |1  |0  |1  |   |L6 |0  |0  |1  |0  |   |
      |N5|L7 |1  |0  |0  |1  |   |L7 |0  |0  |0  |0  |   |
      |  |L10|0  |0  |0  |0  |   |L10|1  |0  |0  |0  |   |
      |  |L11|1  |1  |0  |0  |   |L11|0  |0  |0  |0  |   |
      |  |   |L12|L15|   |   |   |   |L12|L15|   |   |   |
      |N6|L12|0  |1  |   |   |   |L12|0  |0  |   |   |   |
      |  |L15|1  |0  |   |   |   |L15|0  |0  |   |   |   |
      |  |   |L11|L13|L14|L16|   |   |L11|L13|L14|L16|   |
      |  |L11|0  |1  |0  |1  |   |L11|0  |0  |0  |0  |   |
      |N7|L13|1  |0  |0  |0  |   |L13|0  |0  |1  |0  |   |
      |  |L14|0  |0  |0  |1  |   |L14|0  |1  |0  |0  |   |
      |  |L16|1  |0  |1  |0  |   |L16|0  |0  |1  |0  |   |
      |  |   |L15|L16|L17|L18|   |   |L15|L16|L17|L18|   |
      |  |L15|0  |1  |0  |0  |   |L15|0  |0  |0  |1  |   |
      |N8|L16|1  |0  |0  |0  |   |L16|0  |0  |1  |0  |   |
      |  |L17|0  |0  |0  |0  |   |L17|0  |1  |0  |0  |   |
      |  |L18|0  |0  |0  |0  |   |L18|1  |0  |1  |0  |   |

5.1.2. Describing the Links

For the following discussion, some simplifying assumptions are made:

  • It is assumed that the WSON node supports a total of four wavelengths, designated WL1 through WL4.
  • It is assumed that the impairment feasibility of a path or path segment is independent from the wavelength chosen.

For the discussion of RWA operation, to build LSPs between two routers, the wavelength constraints on the links between the routers and the WSON nodes as well as the connectivity matrix of these links need to be specified:

   +Link+WLs supported    +Possible output links+
   | L1 | WL1             | L3                  |
   | L2 | WL2             | L4                  |
   | L8 | WL1 WL2 WL3 WL4 | L5 L7 L12           |
   | L9 | WL1 WL2 WL3 WL4 | L5 L7 L12           |
   | L10| WL2             | L6                  |
   | L13| WL1 WL2 WL3 WL4 | L11 L14             |
   | L14| WL1 WL2 WL3 WL4 | L13 L16             |
   | L17| WL2             | L16                 |
   | L18| WL1             | L15                 |

Note that the possible output links for the links connecting to the routers is inferred from the switched connectivity matrix and the fixed connectivity matrix of the Nodes N1 through N8 and is shown here for convenience; that is, this information does not need to be repeated.

5.2. RWA Path Computation and Establishment

The calculation of optical impairment feasible routes is outside the scope of this document. In general, optical impairment feasible routes serve as an input to an RWA algorithm.

For the example use case shown here, assume the following feasible routes:

    +Endpoint 1+Endpoint 2+Feasible Route        +
    |  R1      | R2       | L1 L3 L5 L8          |
    |  R1      | R2       | L1 L3 L5 L9          |
    |  R1      | R2       | L2 L4 L6 L7 L8       |
    |  R1      | R2       | L2 L4 L6 L7 L9       |
    |  R1      | R2       | L2 L4 L6 L10         |
    |  R1      | R3       | L1 L3 L5 L12 L15 L18 |
    |  R1      | N7       | L2 L4 L6 L11         |
    |  N7      | R3       | L16 L17              |
    |  N7      | R2       | L16 L15 L12 L9       |
    |  R2      | R3       | L8 L12 L15 L18       |
    |  R2      | R3       | L8 L7 L11 L16 L17    |
    |  R2      | R3       | L9 L12 L15 L18       |
    |  R2      | R3       | L9 L7 L11 L16 L17    |

Given a request to establish an LSP between R1 and R2, an RWA algorithm finds the following possible solutions:

    +WL  + Path          +
    | WL1| L1 L3 L5 L8   |
    | WL1| L1 L3 L5 L9   |
    | WL2| L2 L4 L6 L7 L8|
    | WL2| L2 L4 L6 L7 L9|
    | WL2| L2 L4 L6 L10  |

Assume now that an RWA algorithm yields WL1 and the path L1 L3 L5 L8 for the requested LSP.

Next, another LSP is signaled from R1 to R2. Given the established LSP using WL1, the following table shows the available paths:

    +WL  + Path          +
    | WL2| L2 L4 L6 L7 L9|
    | WL2| L2 L4 L6 L10  |

Assume now that an RWA algorithm yields WL2 and the path L2 L4 L6 L7 L9 for the establishment of the new LSP.

An LSP request -- this time from R2 to R3 -- cannot be fulfilled since the four possible paths (starting at L8 and L9) are already in use.

5.3. Resource Optimization

The preceding example gives rise to another use case: the optimization of network resources. Optimization can be achieved on a number of layers (e.g., through electrical or optical multiplexing of client signals) or by re-optimizing the solutions found by an RWA algorithm.

Given the above example again, assume that an RWA algorithm should identify a path between R2 and R3. The only possible path to reach R3 from R2 needs to use L9. L9, however, is blocked by one of the LSPs from R1.

5.4. Support for Rerouting

It is also envisioned that the extensions to GMPLS and PCE support rerouting of wavelengths in case of failures.

For this discussion, assume that the only two LSPs in use in the system are:

   LSP1: WL1 L1 L3 L5 L8
   LSP2: WL2 L2 L4 L6 L7 L9

Furthermore, assume that the L5 fails. An RWA algorithm can now compute and establish the following alternate path:

   R1 -> N7 -> R2

Level 3 regeneration will take place at N7, so that the complete path looks like this:

   R1 -> L2 L4 L6 L11 L13 -> O1 -> L14 L16 L15 L12 L9 -> R2

5.5. Electro-Optical Networking Scenarios

In the following subsections, various networking scenarios are considered involving regenerators, OEO switches, and wavelength converters. These scenarios can be grouped roughly by type and number of extensions to the GMPLS control plane that would be required.

5.5.1. Fixed Regeneration Points

In the simplest networking scenario involving regenerators, regeneration is associated with a WDM link or an entire node and is not optional; that is, all signals traversing the link or node will be regenerated. This includes OEO switches since they provide regeneration on every port.

There may be input constraints and output constraints on the regenerators. Hence, the path selection process will need to know the regenerator constraints from routing or other means so that it can choose a compatible path. For impairment-aware routing and wavelength assignment (IA-RWA), the path selection process will also need to know which links/nodes provide regeneration. Even for "regular" RWA, this regeneration information is useful since wavelength converters typically perform regeneration, and the wavelength continuity constraint can be relaxed at such a point.

Signaling does not need to be enhanced to include this scenario since there are no reconfigurable regenerator options on input, output, or processing.

5.5.2. Shared Regeneration Pools

In this scenario, there are nodes with shared regenerator pools within the network in addition to the fixed regenerators of the previous scenario. These regenerators are shared within a node and their application to a signal is optional. There are no reconfigurable options on either input or output. The only processing option is to "regenerate" a particular signal or not.

In this case, regenerator information is used in path computation to select a path that ensures signal compatibility and IA-RWA criteria.

To set up an LSP that utilizes a regenerator from a node with a shared regenerator pool, it is necessary to indicate that regeneration is to take place at that particular node along the signal path. Such a capability does not currently exist in GMPLS signaling.

5.5.3. Reconfigurable Regenerators

This scenario is concerned with regenerators that require configuration prior to use on an optical signal. As discussed previously, this could be due to a regenerator that must be configured to accept signals with different characteristics, for regenerators with a selection of output attributes, or for regenerators with additional optional processing capabilities.

As in the previous scenarios, it is necessary to have information concerning regenerator properties for selection of compatible paths and for IA-RWA computations. In addition, during LSP setup, it is necessary to be able to configure regenerator options at a particular node along the path. Such a capability does not currently exist in GMPLS signaling.

5.5.4. Relation to Translucent Networks

Networks that contain both transparent network elements such as Reconfigurable Optical Add/Drop Multiplexers (ROADMs) and electro- optical network elements such as regenerators or OEO switches are frequently referred to as translucent optical networks.

Three main types of translucent optical networks have been discussed:

  1. Transparent "islands" surrounded by regenerators. This is frequently seen when transitioning from a metro optical subnetwork to a long-haul optical subnetwork.
  1. Mostly transparent networks with a limited number of OEO ("opaque") nodes strategically placed. This takes advantage of the inherent regeneration capabilities of OEO switches. In the planning of such networks, one has to determine the optimal placement of the OEO switches.
  1. Mostly transparent networks with a limited number of optical switching nodes with "shared regenerator pools" that can be optionally applied to signals passing through these switches. These switches are sometimes called translucent nodes.

All three types of translucent networks fit within the networking scenarios of Sections 5.5.1 and 5.5.2. Hence, they can be accommodated by the GMPLS extensions envisioned in this document.

6. GMPLS and PCE Implications

The presence and amount of wavelength conversion available at a wavelength switching interface have an impact on the information that needs to be transferred by the control plane (GMPLS) and the PCE architecture. Current GMPLS and PCE standards address the full wavelength conversion case, so the following subsections will only address the limited and no wavelength conversion cases.

6.1. Implications for GMPLS Signaling

Basic support for WSON signaling already exists in GMPLS with 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]. However, a number of practical issues arise in the identification of wavelengths and signals and in distributed wavelength assignment processes, which are discussed below.

6.1.1. Identifying Wavelengths and Signals

As previously stated, a global-fixed mapping between wavelengths and labels simplifies the characterization of WDM links and WSON devices. Furthermore, a mapping like the one described in [RFC6205] provides fixed mapping for communication between PCE and WSON PCCs.

6.1.2. WSON Signals and Network Element Processing

As discussed in Section 3.3.2, a WSON signal at any point along its path can be characterized by the (a) modulation format, (b) FEC, (c) wavelength, (d) bitrate, and (e) G-PID.

Currently, G-PID, wavelength (via labels), and bitrate (via bandwidth encoding) are supported in [RFC3471] and [RFC3473]. These RFCs can accommodate the wavelength changing at any node along the LSP and can thus provide explicit control of wavelength converters.

In the fixed regeneration point scenario described in Section 5.5.1, no enhancements are required to signaling since there are no additional configuration options for the LSP at a node.

In the case of shared regeneration pools described in Section 5.5.2, it is necessary to indicate to a node that it should perform regeneration on a particular signal. Viewed another way, for an LSP, it is desirable to specify that certain nodes along the path perform regeneration. Such a capability does not currently exist in GMPLS signaling.

The case of reconfigurable regenerators described in Section 5.5.3 is very similar to the previous except that now there are potentially many more items that can be configured on a per-node basis for an LSP.

Note that the techniques of [RFC5420] that allow for additional LSP attributes and their recording in a Record Route Object (RRO) could be extended to allow for additional LSP attributes in an Explicit Route Object (ERO). This could allow one to indicate where optional

3R regeneration should take place along a path, any modification of LSP attributes such as modulation format, or any enhance processing such as performance monitoring.

6.1.3. Combined RWA/Separate Routing WA support

In either the combined RWA case or the separate routing WA case, the node initiating the signaling will have 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 used to indicate the wavelength to be used at a particular node. In case the local label map approach is used, the label subobject entry in the ERO has to be interpreted appropriately.

6.1.4. Distributed Wavelength Assignment: Unidirectional, No Converters

GMPLS signaling for a unidirectional optical path LSP allows for the use of a Label Set object in the Resource Reservation Protocol - Traffic Engineering (RSVP-TE) path message. Processing of the Label Set object to take the intersection of available lambdas along a path can be performed, resulting in the set of available lambdas being known to the destination, which can then use a wavelength selection algorithm to choose a lambda.

6.1.5. Distributed Wavelength Assignment: Unidirectional, Limited


In the case of wavelength converters, nodes with wavelength converters would need to make the decision as to whether to perform conversion. One indicator for this would be that the set of available wavelengths that is obtained via the intersection of the incoming Label Set and the output links available wavelengths is either null or deemed too small to permit successful completion.

At this point, the node would need to remember that it will apply wavelength conversion and will be responsible for assigning the wavelength on the previous lambda-contiguous segment when the RSVP-TE RESV message is processed. The node will pass on an enlarged label set reflecting only the limitations of the wavelength converter and the output link. The record route option in RSVP-TE signaling can be used to show where wavelength conversion has taken place.

6.1.6. Distributed Wavelength Assignment: Bidirectional, No Converters

There are cases of a bidirectional optical path that require the use of the same lambda in both directions. The above procedure can be used to determine the available bidirectional lambda set if it is interpreted that the available Label Set is available in both directions. According to [RFC3471], Section 4.1, the setup of bidirectional LSPs is indicated by the presence of an upstream label in the path message.

However, until the intersection of the available Label Sets is determined along the path and at the destination node, the upstream label information may not be correct. This case can be supported using current GMPLS mechanisms but may not be as efficient as an optimized bidirectional single-label allocation mechanism.

6.2. Implications for GMPLS Routing

GMPLS routing [RFC4202] currently defines an interface capability descriptor for "Lambda Switch Capable" (LSC) that can be used to describe the interfaces on a ROADM or other type of wavelength selective switch. In addition to the topology information typically conveyed via an Interior Gateway Protocol (IGP), it would be necessary to convey the following subsystem properties to minimally characterize a WSON:

  1. WDM link properties (allowed wavelengths)
   2.  Optical transmitters (wavelength range)
   3.  ROADM/FOADM properties (connectivity matrix, port wavelength
  1. Wavelength converter properties (per network element, may change if a common limited shared pool is used)

This information is modeled in detail in [WSON-Info], and a compact encoding is given in [WSON-Encode].

6.2.1. Electro-Optical Element Signal Compatibility

In network scenarios where signal compatibility is a concern, it is necessary to add parameters to our existing node and link models to take into account electro-optical input constraints, output constraints, and the signal-processing capabilities of an NE in path computations.

Input constraints:

  1. Permitted optical tributary signal classes: A list of optical tributary signal classes that can be processed by this network element or carried over this link (configuration type)
  1. Acceptable FEC codes (configuration type)
  1. Acceptable bitrate set: a list of specific bitrates or bitrate ranges that the device can accommodate. Coarse bitrate info is included with the optical tributary signal-class restrictions.
  1. Acceptable G-PID list: a list of G-PIDs corresponding to the "client" digital streams that is compatible with this device

Note that the bitrate of the signal does not change over the LSP. This can be communicated as an LSP parameter; therefore, this information would be available for any NE that needs to use it for configuration. Hence, it is not necessary to have "configuration type" for the NE with respect to bitrate.

Output constraints:

  1. Output modulation: (a) same as input, (b) list of available types
  1. FEC options: (a) same as input, (b) list of available codes

Processing capabilities:

   1.  Regeneration: (a) 1R, (b) 2R, (c) 3R, (d) list of selectable
       regeneration types
   2.  Fault and performance monitoring: (a) G-PID particular
       capabilities, (b) optical performance monitoring capabilities.

Note that such parameters could be specified on (a) a network- element-wide basis, (b) a per-port basis, or (c) a per-regenerator basis. Typically, such information has been on a per-port basis; see the GMPLS interface switching capability descriptor [RFC4202].

6.2.2. Wavelength-Specific Availability Information

For wavelength assignment, it is necessary to know which specific wavelengths are available and which are occupied if a combined RWA process or separate WA process is run as discussed in Sections 4.1.1 and 4.1.2. This is currently not possible with GMPLS routing.

In the routing extensions for GMPLS [RFC4202], requirements for layer-specific TE attributes are discussed. RWA for 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 DWDM systems range from 16 channels to 128 channels, with advanced laboratory systems with as many as 300 channels. Given these channel limitations, if the approach of a global wavelength to label mapping or furnishing the local mappings to the PCEs is taken, representing the use of wavelengths via a simple bitmap is feasible [Gen-Encode].

6.2.3. WSON Routing Information Summary

The following table summarizes the WSON information that could be conveyed via GMPLS routing and attempts to classify that information according to its static or dynamic nature and its association with either a link or a node.

     Information                         Static/Dynamic       Node/Link
     Connectivity matrix                 Static               Node
     Per-port wavelength restrictions    Static               Node(1)
     WDM link (fiber) lambda ranges      Static               Link
     WDM link channel spacing            Static               Link
     Optical transmitter range           Static               Link(2)
     Wavelength conversion capabilities  Static(3)            Node
     Maximum bandwidth per wavelength    Static               Link
     Wavelength availability             Dynamic(4)           Link
     Signal compatibility and processing Static/Dynamic       Node


  1. These are the per-port wavelength restrictions of an optical device such as a ROADM and are independent of any optical constraints imposed by a fiber link.
  1. This could also be viewed as a node capability.
  1. This could be dynamic in the case of a limited pool of converters where the number available can change with connection establishment. Note that it may be desirable to include regeneration capabilities here since OEO converters are also regenerators.
  1. This is not necessarily needed in the case of distributed wavelength assignment via signaling.

While the full complement of the information from the previous table is needed in the Combined RWA and the separate Routing and WA architectures, in the case of Routing + Distributed WA via Signaling, only the following information is needed:

     Information                         Static/Dynamic       Node/Link
     Connectivity matrix                 Static               Node
     Wavelength conversion capabilities  Static(3)            Node
   Information models and compact encodings for this information are
   provided in [WSON-Info], [Gen-Encode], and [WSON-Encode].

6.3. Optical Path Computation and Implications for PCE

As previously noted, RWA can be computationally intensive. Such computationally intensive path computations and optimizations were part of the impetus for the PCE architecture [RFC4655].

The Path Computation Element Communication Protocol (PCEP) defines the procedures necessary to support both sequential [RFC5440] and Global Concurrent Optimization (GCO) path computations [RFC5557]. With some protocol enhancement, the PCEP is well positioned to support WSON-enabled RWA computation.

Implications for PCE generally fall into two main categories: (a) optical path constraints and characteristics, (b) computation architectures.

6.3.1. Optical Path Constraints and Characteristics

For the varying degrees of optimization that may be encountered in a network, the following models of bulk and sequential optical path requests are encountered:

  • Batch optimization, multiple optical paths requested at one time (PCE-GCO)
  • Optical path(s) and backup optical path(s) requested at one time (PCEP)
  • Single optical path requested at a time (PCEP)

PCEP and PCE-GCO can be readily enhanced to support all of the potential models of RWA computation.

Optical path constraints include:

  • Bidirectional assignment of wavelengths
  • Possible simultaneous assignment of wavelength to primary and backup paths
  • Tuning range constraint on optical transmitter

6.3.2. Electro-Optical Element Signal Compatibility

When requesting a path computation to PCE, the PCC should be able to indicate the following:

   o  The G-PID type of an LSP
  • The signal attributes at the transmitter (at the source): (i) modulation type, (ii) FEC type
  • The signal attributes at the receiver (at the sink): (i) modulation type, (ii) FEC type

The PCE should be able to respond to the PCC with the following:

  • The conformity of the requested optical characteristics associated with the resulting LSP with the source, sink, and NE along the LSP
  • Additional LSP attributes modified along the path (e.g., modulation format change)

6.3.3. Discovery of RWA-Capable PCEs

The algorithms and network information needed for RWA are somewhat specialized and computationally intensive; hence, not all PCEs within a domain would necessarily need or want this capability. Therefore, it would be useful to indicate that a PCE has the ability to deal with RWA via the mechanisms being established for PCE discovery [RFC5088]. [RFC5088] indicates that a sub-TLV could be allocated for this purpose.

Recent progress on objective functions in PCE [RFC5541] would allow operators to flexibly request differing objective functions per their need and applications. For instance, this would allow the operator to choose an objective function that minimizes the total network cost associated with setting up a set of paths concurrently. This would also allow operators to choose an objective function that results in the most evenly distributed link utilization.

This implies that PCEP would easily accommodate a wavelength selection algorithm in its objective function to be able to optimize the path computation from the perspective of wavelength assignment if chosen by the operators.

7. Security Considerations

This document does not require changes to the security models within GMPLS and associated protocols. That is, the OSPF-TE, RSVP-TE, and PCEP security models could be operated unchanged.

However, satisfying the requirements for RWA using the existing protocols may significantly affect the loading of those protocols. This may make the operation of the network more vulnerable to denial- of-service attacks. Therefore, additional care maybe required to ensure that the protocols are secure in the WSON environment.

Furthermore, the additional information distributed in order to address RWA represents a disclosure of network capabilities that an operator may wish to keep private. Consideration should be given to securing this information. For a general discussion on MPLS- and GMPLS-related security issues, see the MPLS/GMPLS security framework [RFC5920].

8. Acknowledgments

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

9. References

9.1. Normative References

   [RFC3471]     Berger, L., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Signaling Functional Description",
                 RFC 3471, January 2003.
   [RFC3473]     Berger, L., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Signaling Resource ReserVation
                 Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
                 3473, January 2003.
   [RFC3945]     Mannie, E., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Architecture", RFC 3945, October
   [RFC4202]     Kompella, K., Ed., and Y. Rekhter, Ed., "Routing
                 Extensions in Support of Generalized Multi-Protocol
                 Label Switching (GMPLS)", RFC 4202, October 2005.
   [RFC4328]     Papadimitriou, D., Ed., "Generalized Multi-Protocol
                 Label Switching (GMPLS) Signaling Extensions for G.709
                 Optical Transport Networks Control", RFC 4328, January
   [RFC4655]     Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                 Computation Element (PCE)-Based Architecture", RFC
                 4655, August 2006.
   [RFC5088]     Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
                 R. Zhang, "OSPF Protocol Extensions for Path
                 Computation Element (PCE) Discovery", RFC 5088, January
   [RFC5212]     Shiomoto, K., Papadimitriou, D., Le Roux, JL.,
                 Vigoureux, M., and D. Brungard, "Requirements for
                 GMPLS-Based Multi-Region and Multi-Layer Networks
                 (MRN/MLN)", RFC 5212, July 2008.
   [RFC5557]     Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
                 Computation Element Communication Protocol (PCEP)
                 Requirements and Protocol Extensions in Support of
                 Global Concurrent Optimization", RFC 5557, July 2009.
   [RFC5420]     Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and
                 A. Ayyangarps, "Encoding of Attributes for MPLS LSP
                 Establishment Using Resource Reservation Protocol
                 Traffic Engineering (RSVP-TE)", RFC 5420, February
   [RFC5440]     Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
                 Computation Element (PCE) Communication Protocol
                 (PCEP)", RFC 5440, March 2009.
   [RFC5541]     Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
                 Objective Functions in the Path Computation Element
                 Communication Protocol (PCEP)", RFC 5541, June 2009.

9.2. Informative References

   [Gen-Encode]  Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "General Network Element Constraint Encoding for GMPLS
                 Controlled Networks", Work in Progress, December 2010.
   [G.652]       ITU-T Recommendation G.652, "Characteristics of a
                 single-mode optical fibre and cable", November 2009.
   [G.653]       ITU-T Recommendation G.653, "Characteristics of a
                 dispersion-shifted single-mode optical fibre and
                 cable", July 2010.
   [G.654]       ITU-T Recommendation G.654, "Characteristics of a cut-
                 off shifted single-mode optical fibre and cable", July
   [G.655]       ITU-T Recommendation G.655, "Characteristics of a non-
                 zero dispersion-shifted single-mode optical fibre and
                 cable", November 2009.
   [G.656]       ITU-T Recommendation G.656, "Characteristics of a fibre
                 and cable with non-zero dispersion for wideband optical
                 transport", July 2010.
   [G.671]       ITU-T Recommendation G.671, "Transmission
                 characteristics of optical components and subsystems",
                 January 2009.
   [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 interfaces",
                 November 2009.
   [G.698.2]     ITU-T Recommendation G.698.2, "Amplified multichannel
                 dense wavelength division multiplexing applications
                 with single channel optical interfaces ", November
   [G.707]       ITU-T Recommendation G.707, "Network node interface for
                 the synchronous digital hierarchy (SDH)", January 2007.
   [G.709]       ITU-T Recommendation G.709, "Interfaces for the Optical
                 Transport Network (OTN)", December 2009.
   [G.872]       ITU-T Recommendation G.872, "Architecture of optical
                 transport networks", November 2001.
   [G.959.1]     ITU-T Recommendation G.959.1, "Optical transport
                 network physical layer interfaces", November 2009.
   [G.Sup39]     ITU-T Series G Supplement 39, "Optical system design
                 and engineering considerations", December 2008.
   [Imajuku]     Imajuku, W., Sone, Y., Nishioka, I., and S. Seno,
                 "Routing Extensions to Support Network Elements with
                 Switching Constraint", Work in Progress, July 2007.
   [RFC6205]     Otani, T., Ed. and D. Li, Ed., "Generalized Labels of
                 Lambda-Switch Capable (LSC) Label Switching Routers",
                 RFC 6205, March 2011.
   [RFC5920]     Fang, L., Ed., "Security Framework for MPLS and GMPLS
                 Networks", RFC 5920, July 2010.
   [WSON-Encode] Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "Routing and Wavelength Assignment Information Encoding
                 for Wavelength Switched Optical Networks", Work in
                 Progress, March 2011.
   [WSON-Imp]    Lee, Y., Bernstein, G., Li, D., and G. Martinelli, "A
                 Framework for the Control of Wavelength Switched
                 Optical Networks (WSON) with Impairments", Work in
                 Progress, April 2011.
   [WSON-Info]   Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "Routing and Wavelength Assignment Information Model
                 for Wavelength Switched Optical Networks", Work in
                 Progress, July 2008.


   Snigdho Bardalai
   Diego Caviglia
   Via A. Negrone 1/A 16153
   Phone: +39 010 600 3736
   Daniel King
   Old Dog Consulting
   Itaru Nishioka
   NEC Corp.
   1753 Simonumabe, Nakahara-ku
   Kawasaki, Kanagawa 211-8666
   Phone: +81 44 396 3287
   Lyndon Ong
   Pierre Peloso
   Route de Villejust, 91620 Nozay
   Jonathan Sadler
   Dirk Schroetter

Jonas Martensson
Electrum 236
16440 Kista

Authors' Addresses

Young Lee (editor)
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075

   Phone: (972) 509-5599 (x2240)

Greg M. Bernstein (editor)
Grotto Networking
Fremont, CA

   Phone: (510) 573-2237
   Wataru Imajuku
   NTT Network Innovation Labs
   1-1 Hikari-no-oka, Yokosuka, Kanagawa
   Phone: +81-(46) 859-4315