Network Working Group O. Gonzalez de Dios, Ed.
Internet-Draft Telefónica I+D
Intended status: Standards Track R. Casellas, Ed.
Expires: October 13, 2013 CTTC
F. Zhang
X. Fu
D. Ceccarelli
I. Hussain
April 11, 2013

Framework and Requirements for GMPLS based control of Flexi-grid DWDM networks


This document defines a framework and the associated control plane requirements for the GMPLS based control of flexi-grid DWDM networks. To allow efficient allocation of optical spectral bandwidth for high bit-rate systems, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has extended the recommendations [G.694.1] and [G.872] to include the concept of flexible grid: a new DWDM grid has been developed within the ITU-T Study Group 15 by defining a set of nominal central frequencies, smaller channel spacings and the concept of "frequency slot". In such environment, a data plane connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum.

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Table of Contents

1. Requirements Language

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

2. Introduction

The term "Flexible grid" (flexi-grid for short) as defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Study Group 15 in the latest version of [G.694.1], refers to the updated set of nominal central frequencies (a frequency grid), channel spacings and optical spectrum management/allocation considerations that have been defined in order to allow an efficient and flexible allocation and configuration of optical spectral bandwidth for high bit-rate systems.

A key concept of flexi-grid is the "frequency slot"; a variable-sized optical frequency range that can be allocated to a data connection. As detailed later in the document, a frequency slot is characterized by its nominal central frequency and its slot width which, as per [G.694.1], is constrained to be a multiple of a given slot width granularity.

Compared to a traditional fixed grid network, which uses fixed size optical spectrum frequency ranges or "frequency slots" with typical channel separations of 50 GHz, a flexible grid network can select its media channels with a more flexible choice of slot widths, allocating as much optical spectrum as required, allowing high bit rate signals (e.g., 400G, 1T or higher) that do not fit in the fixed grid.

From a networking perspective, a flexible grid network is assumed to be a layered network [G.872][G.800] in which the flexi-grid layer (also referred to as the media layer) is the server layer and the OCh Layer (also referred to as the signal layer) is the client layer. In the media layer, switching is based on a frequency slot, and the size of a media channel is given by the properties of the associated frequency slot. In this layered network, the media channel itself can be dimensioned to contain one or more Optical Channels.

As described in [RFC3945], GMPLS extends MPLS from supporting only Packet Switching Capable (PSC) interfaces and switching to also support four new classes of interfaces and switching that include Lambda Switch Capable (LSC).

A Wavelength Switched Optical Network (WSON), addressed in [RFC6163], is a term commonly used to refer to the application/deployment of a Generalized Multi-Protocol Label Switching (GMPLS)-based control plane for the control (provisioning/recovery, etc) of a fixed grid WDM network. [editors' note: we need to think of the relationship of WSON and OCh switching. Are they equivalent? WSON includes regeneration, OCh does not? decoupling of lambda/OCh/OCC]

This document defines the framework for a GMPLS-based control of flexi-grid enabled DWDM networks (in the scope defined by ITU-T layered Optical Transport Networks [G.872], as well as a set of associated control plane requirements. An important design consideration relates to the decoupling of the management of the optical spectrum resource and the client signals to be transported. [Editor's note: a point was raised during the meeting that WSON has not made the separation between Och and Lambda (spectrum and signal are bundled). The document will consider and evaluate the relationship later].

[Editors' note: this document will track changes and evolutions of [G.694.1] [G.872] documents until their final publication. This document is not expected to become RFC until then.]

[Editor's note: -00 as agreed during IETF83, the consideration of the concepts of Super-channel (a collection of one or more frequency slots to be treated as unified entity for management and control plane) and consequently Contiguous Spectrum Super-channel (a super-channel with a single frequency slot) and Split-Spectrum super-channel (a super-channel with multiple frequency slots) is postponed until the ITU-T data plane includes such physical layer entities, e.g., an ITU-T contribution exists. ITU-T is still discussing B100G Architecture]

[Editors' note: -01 this version reflects the agreements made during IETF84, notably concerning the focus in the media layer, terminology updates post ITU-T September meeting in Geneva and the deprecation of the ROADM term, in favor of the more concrete media layer switching element (media channel matrix).]

[Editors' note: -01 in partial answer to Gert question on the layered model, [G.872] footnote explains that this separation is necessary to allow the description of media elements that may act on more than a single OCh-P signal. See appendix IV within.]

3. Acronyms

FS: Frequency Slot

NCF: Nominal Central Frequency

OCh: Optical Channel

OCh-P: Optical Channel Payload

OCh-O: Optical Channel Overhead

OCC: Optical Channel Carrier

SWG: Slot Width Granularity

4. Terminology

The following is a list of terms (see [G.694.1] and [G.872]) reproduced here for completeness. [Editors' note: regarding wavebands, we agreed NOT to use the term in flexi-grid. The term has been used inconsistently in fixed-grid networks and overlaps with the definition of frequency slot. If need be, a question will be sent to ITU-T asking for clarification regarding wavebands.]

Where appropriate, this documents also uses terminology and lexicography from [RFC4397].

[Editors' note: *important* these terms are not yet final and they may change / be replaced or obsoleted at any time.]

4.1. Frequency Slots

   -5 -4 -3 -2 -1  0  1  2  3  4  5     <- values of n
                   193.1 THz <- anchor frequency

Figure 1: Anchor frequency and set of nominal central frequencies

      Frequency Slot 1     Frequency Slot 2 
       -------------     -------------------  
       |           |     |                 |  
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     
       -------------     ------------------- 
             ^                    ^ 
     Central F = 193.1THz    Central F = 193.14375 THz 
     Slot width = 25 GHz     Slot width = 37.5 GHz 

Figure 2: Example Frequency slots

                     Frequency Slot 1   
             |           |    
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

          Frequency Slot 2 
          |                 |
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

        Effective Frequency Slot 
             |           |
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

Figure 3: Effective Frequency Slot

      Frequency Slot 1   
       |           |    
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

          Frequency Slot 2 
          |                 |
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

        Invalid Effective Frequency Slot - (n, m?)
          |        |
   -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11     

Figure 4: Invalid Effective Frequency Slot

  • Nominal Central Frequency Granularity: 6.25 GHz (note: sometimes referred to as 0.00625 THz).
  • Nominal Central Frequency: each of the allowed frequencies as per the definition of flexible DWDM grid in [G.694.1]. The set of nominal central frequencies can be built using the following expression f = 193.1 THz + n x 0.00625 THz, where 193.1 THz is ITU-T ''anchor frequency'' for transmission over the C band, n is a positive or negative integer including 0.
  • Slot Width Granularity: 12.5 GHz, as defined in [G.694.1].
  • Slot Width: The slot width determines the "amount" of optical spectrum regardless of its actual "position" in the frequency axis. A slot width is constrained to be m x SWG (that is, m x 12.5 GHz), where m is an integer greater than or equal to 1.
  • Frequency Slot: The frequency range allocated to a slot within the flexible grid and unavailable to other slots. A frequency slot is defined by its nominal central frequency and its slot width. Assuming a fixed and known central nominal frequency granularity, and assuming a fixed and known slot width granularity, a frequency slot is fully characterized by the values of 'n' and 'm'. Note that an equivalent characterization of a frequency slot is given by the start and end frequencies (i.e., a frequency range) which can, in turn, be defined by their respective values of 'n'.
    • The symbol '+' represents the allowed nominal central frequencies, the '--' represents the nominal central frequency granularity, and the '^' represents the slot nominal central frequency. The number on the top of the '+' symbol represents the 'n' in the frequency calculation formula. The nominal central frequency is 193.1 THz when n equals zero. Note that over a single frequency slot, one or multiple Optical Channels may be transported.
    • Note that when there are multiple optical signals within frequency slot, then each signal still has its own central frequency. That is, the term "central frequency" applies to an Optical signal and the term "nominal central frequency" applies to a frequency slot. In other words, the Frequency Slot central frequency is independent of the signals central frequencies.

  • Effective Frequency Slot: the effective frequency slot of a media channel is the common part of the frequency slots along the media channel through a particular path through the optical network. It is a logical construct derived from the (intersection of) frequency slots allocated to each device in the path. The effective frequency slot is an attribute of a media channel and, being a frequency slot, it is described by its nominal central frequency and slot width.
  • As an example, if there are two filters having slots with the same n but different m, then the common frequency slot has the smaller of the two m values. [Editor's note: within the GMPLS label swapping paradigm, the switched resource corresponds to the local frequency slot defined by the observable filters of the media layer switching element. The GMPLS label MUST identify the switched resource locally, and (as agreed during IETF84) is locally scoped to a link, even if the same frequency slot is allocated at all the hops of the path. Note that the requested slot width and the finally allocated slot width by a given node may be different, e.g., due to restrictions in the slot width granularity of the nodes. Due to the symmetric definition of frequency slot, allocations seem to be constrained to have the same nominal central frequency. It is important to note that if n changes along the path, it cannot be guaranteed that there is a valid common frequency slot. We must determine if different n's are allowed. We need to explain this rationale. e.g. what happens when the resulting slot cannot be characterized with n and m, see Figure 3 and Figure 4.].

4.2. Media Layer, Elements and Channels

  • Media Element: a media element only directs the optical signal or affects the properties of an optical signal, it does not modify the properties of the information that has been modulated to produce the optical signal. Examples of media elements include fibers, amplifiers, filters, switching matrices[Note: the data plane component of a LSR in the media layer is a media element, but not all media elements correspond to data plane nodes in the GMPLS network model.
  • Media Channel: a media association that represents both the topology (i.e., path through the media) and the resource (frequency slot) that it occupies. As a topological construct, it represents a (effective) frequency slot supported by a concatenation of media elements (fibers, amplifiers, filters, switching matrices...). This term is used to identify the end-to-end physical layer entity with its corresponding (one or more) frequency slots local at each link filters.
  • Network Media Channel: a media channel (media association) that supports a single OCh-P network connection. It represents the concatenation of all media elements between an OCh-P source and an OCh-P sink. [TODO: |Malcolm| explain the use case rationale to support a hierarchy of media channels, where a media channel acts as "pipe" for one or more network media channels and they are both separate entities (IETF84). This may be tied to the concept of a "waveband" or express channel, as stated in [G.872] footnote 4.]
  • OCh-P Frequency Slot: The spectrum allocated to a single OCh signal supported on a Network Media Channel.

Note that by definition a network media channel only supports a single OCh-P network connection, but the architecture is flexible enough to support the case where a single Optical Channel Payload (OCh-P) is transported over multiple (N) network media channels (see Figure 5) which are non-necessarily adjacent (e.g., there may not exist an equivalent network media channel that can be represented as the union of media channels with a single, valid equivalent frequency slot). Whether a single OCh-P is transported over just one or more network media channels is an aspect that corresponds to the mapping of the signal layer to the media layer.

                                   OCh-P                Signal Layer
                                 /           \          Media Layer
                                /             \
          Network Media Channel #1     ...   Network Media Channel #N
          +------------o-----------+         +----------X-----------+   
          |                        |         |                      | 
...  +---+---+---+---+---+---+---+---+---+---+---+--+---+---+---+---+---... 

Figure 5: A single OCh-P transported over multiple network media channels

4.3. Media Layer Switching

[Editors' note: we are not discarding O/E/O. If defined in a ITU-T network reference model with trail/terminations, considering optical channels i.e. with well-defined interfaces, reference points, and architectures. The implications of O/E/O will be also addressed once we have another context that includes them. In OTN from an OCh point of view end to end means from transponder to transponder, so if there is a 3R from ingress to egress there are 2 OCh which can have different 'n' and 'm'].

  • Media Channel Matrixes: the media channel matrix provides flexible connectivity for the media channels. That is, it represents a point of flexibility where relationships between the media ports at the edge of a media channel matrix may be created and broken. The relationship between these ports is called a matrix channel. (Network) Media Channels are switched in a Media Channel Matrix.

                         Media Channel Frequency Slot 
     |                                                              |
     |      OCh-P Frequency Slot             OCh-P Frequency Slot   |
     |  +------------X-----------+       +----------X-----------+   |
     |   |       OCh-P           |       |      OCh-P           |   |
     |   |           o           |       |          o           |   |
     |   |           |           |       |          |           |   |
    -4  -3  -2  -1   0   1   2   3   4   5   6   7  8   9  10  11  12  
...  +---+---+---+---+---+---+---+---+---+---+---+--+---+---+---+---+---... 

...      <- Network Media Channel->     <- Network Media Channel->

...  <------------------------ Media Channel ----------------------->

     X - Frequency Slot Central Frequency

     o - signal central frequency

Figure 6: Example of Media Channel / Network Media Channels and associated frequency slots

In summary, the concept of frequency slot is a logical abstraction that represents a frequency range while the media layer represents the underlying media support. Media Channels are media associations, characterized by their (effective) frequency slot, respectively; and media channels are switched in media channel matrixes. In Figure 6 , a Media Channel has been configured and dimensioned to support two OCh-P, each transported in its own OCh-P frequency slot.

4.4. Control Plane Terms

The following terms are defined in the scope of a GMPLS control plane.

  • SSON: Spectrum-Switched Optical Network. Refers to an optical network in which a LSP is switched based on an frequency slot of a variable slot width of a media channel, rather than based on a fixed grid and fixed slot width. Please note that a Wavelength Switched Optical Network (WSON) can be seen as a particular case of SSON in which all slot widths are equal and depend on the used channel spacing.
  • RSA: Routing and Spectrum Assignment. As opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, the flexibility in SSON leads to spectral contiguous constraint, which means that when assigning the spectral resources to single connections, the resources assigned to them must be contiguous over the entire connections in the spectrum domain.

5. DWDM flexi-grid enabled network element models

Similar to fixed grid networks, a flexible grid network is also constructed from subsystems that include Wavelength Division Multiplexing (WDM) links, tunable transmitters and receivers, i.e, media elements including media layer switching elements (media matrices), as well as electro-optical network elements, all of them with flexible grid characteristics.

[Editors' Note: In the scope of this document, and despite is informal use, the term Reconfigurable Optical Add / Drop Multiplexer, (ROADM) is avoided, in favor on media matrix. This avoid ambiguity. A ROADM can be implemented in terms on media matrices. Informationally, this document may provide an appendix on possible implementations of flexi-ROADMs in terms of media layer switching elements or matrices. XF: Whether ROADM is used or not doesn't matter with GMPLS Control Plane. I suggest to delete this statement. We may check G.798. Likewise, modeling of filters is out of scope of the current document IETF84, and is also considered implementation specific.]

As stated in [G.694.1] the flexible DWDM grid defined in Clause 7 has a nominal central frequency granularity of 6.25 GHz and a slot width granularity of 12.5 GHz. However, devices or applications that make use of the flexible grid may not be capable of supporting every possible slot width or position. In other words, applications may be defined where only a subset of the possible slot widths and positions are required to be supported. For example, an application could be defined where the nominal central frequency granularity is 12.5 GHz (by only requiring values of n that are even) and that only requires slot widths as a multiple of 25 GHz (by only requiring values of m that are even).

5.1. Network element constraints

[TODO: section needs to be rewritten, remove redundancy].

Optical transmitters/receivers may have different tunability constraints, and media channel matrixes may have switching restrictions. Additionally, a key feature of their implementation is their highly asymmetric switching capability which is described in [RFC6163] in detail. Media matrices include line side ports which are connected to DWDM links and tributary side input/output ports which can be connected to transmitters/receivers.

A set of common constraints can be defined:

  • The minimum and maximum slot width.
  • Granularity: the optical hardware may not be able to select parameters with the lowest granularityy (e.g. 6.25 GHz for nominal central frequencies or 12.5 GHz for slot width granularity).
  • Available frequency ranges: the set or union of frequency ranges that are not allocated (i.e. available). The relative grouping and distribution of available frequency ranges in a fiber is usually referred to as ''fragmentation''.
  • Available slot width ranges: the set or union of slot width ranges supported by media matrices. It includes the following information.
    • Slot width threshold: the minimum and maximum Slot Width supported by the media matrix. For example, the slot width can be from 50GHz to 200GHz.
    • Step granularity: the minimum step by which the optical filter bandwidth of the media matrix can be increased or decreased. This parameter is typically equal to slot width granularity (i.e. 12.5GHz) or integer multiples of 12.5GHz.

[Editor's note: different configurations such as C/CD/CDC will be added later. This section should state specifics to media channel matrices, ROADM models need to be moved to an appendix].

6. Layered Network Model

 OCh AP                     Trail (OCh)                        OCh AP
  O- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
  |                                                                  |
 --- OCh-P                                                    OCh-P ---
 \ / source                                                   sink  \ /
  +                                                                  +
  | OCh-P                 OCh-P Network Connection             OCh-P |
  O TCP - - - - - - - - - - - - - - - - - - - - - - - - - - - - -TCP O
  |                                                                  |
  |Channel Port            Network Media Channel        Channel Port |
  O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  O
  |                                                                  |
+--------+                 +-----------+                   +----------+
|  \ (1) |  OCh-P LC       |    (1)    |  OCh-P LC         |   (1)  / |
|   \----|-----------------|-----------|-------------------|-------/  |
+--------+ Link Channel    +-----------+  Link Channel     +----------+
Media Channel              Media Channel                   Media Channel
  Matrix                     Matrix                          Matrix

(1) - Matrix Channel

Figure 7: Layered Network Model according to G.805

This section presents an overview of the OCh / flexi-grid layered network model defined by ITU-T. [Editors' note: OTN hierarchy is not fully covered. It is important to understand, where the FSC sits in the OTN hierarchy. This is also important from control plane perspective as this layer becomes the connection end points of optical layer service].

[Editors' note: we are replicating the figure here for reference, until the ITU-T document is official.

7. GMPLS applicability

The goal of this section is to provide an insight of the application of GMPLS to flexi-grid networks, while specific requirements are covered in the next section. The present framework is aimed at controlling the so called media layer within the OTN hierarchy. Specifically, the GMPLS control of the media layer deals with the establishment of media channels, which are switched in media channel matrixes. GMPLS labels locally represent the media channel and its associated frequency slot. [Editors'note: As agreed during IETF84, current focus is on the media layer.]

Also, this sections provides a mapping of the mapping ITU-T G.872 architectural aspects to GMPLS/Control plane terms, and see the relationship between the architectural concept/construct of media channel and its control plane representations (e.g. as a TE link).

7.1. Considerations on TE Links

From a theoretical / abstract point of view, a fiber can be modeled has having a frequency slot that ranges from (-inf, +inf). This representation helps understand the relationship between frequency slots / ranges.

The frequency slot is a local concept that applies locally to a component / element. When applied to a media channel, we are referring to its effective frequency slot as defined in [G.872].

The association of a filter, a fiber and a filter is a media channel in its most basic form, which from the control plane perspective may modeled as a (physical) TE-link with a contiguous optical spectrum at start of day. A means to represent this is that the portion of spectrum available at time t0 depends on which filters are placed at the ends of the fiber and how they have been configured. Once filters are placed we have the one hop media channel. In practical terms, associating a fiber with the terminating filters determines the usable optical spetrum.