CCAMP Working Group O. Gonzalez de Dios, Ed.
Internet-Draft Telefonica I+D
Intended status: Informational R. Casellas, Ed.
Expires: February 26, 2016 CTTC
August 25, 2015

Framework and Requirements for 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 its Recommendations G.694.1 and G.872 to include a new dense wavelength division multiplexing (DWDM) grid by defining a set of nominal central frequencies, channel spacings and the concept of "frequency slot". In such an environment, a data plane connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum creating what is known as a flexible grid (flexi-grid).

Given the specific characteristics of flexi-grid optical networks and their associated technology, this document defines a framework and the associated control plane requirements for the application of the existing GMPLS architecture and control plane protocols to the control of flexi-grid DWDM networks. The actual extensions to the GMPLS protocols will be defined in companion documents.

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Table of Contents

1. 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 spacing 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.

From a networking perspective, a flexible grid network is assumed to be a layered network [G.872][G.800] in which the media layer is the server layer and the optical 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, a media channel can transport more than one Optical Tributary Signals (OTSi), as defined later in this document.

A Wavelength Switched Optical Network (WSON), addressed in [RFC6163], is a term commonly used to refer to the application/deployment of a GMPLS-based control plane for the control (provisioning/recovery, etc.) of a fixed grid wavelength division multiplexing (WDM) network in which media (spectrum) and signal are jointly considered.

This document defines the framework for a GMPLS-based control of flexi-grid enabled dense wavelength division multiplexing (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.

2. Terminology

Further terminology specific to flexi-grid networks can be found in Section 3.2.

2.1. 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].

While [RFC2119] describes interpretations of these key words in terms of protocol specifications and implementations, they are used in this document to describe design requirements for protocol extensions.

2.2. Abbreviations

FS: Frequency Slot

FSC: Fiber-Switch Capable

LSR: Label Switching Router

NCF: Nominal Central Frequency

OCh: Optical Channel

OCh-P: Optical Channel Payload

OTN: Optical Transport Network

OTSi: Optical Tributary Signal

OTSiG: OTSi Group is a set of OTSi

OCC: Optical Channel Carrier

PCE: Path Computation Element

ROADM: Reconfigurable Optical Add-Drop Multiplexer

SSON: Spectrum-Switched Optical Network

SWG: Slot Width Granularity

3. Overview of Flexi-grid Networks

3.1. Flexi-grid in the Context of OTN

[G.872] describes, from a network level, the functional architecture of an OTN. It is decomposed into independent layer networks with client/layer relationships among them. A simplified view of the OTN layers is shown in Figure 1.

| Digital Layer  |
| Signal Layer   |
|  Media Layer   |

Figure 1: Generic OTN Overview

In the OTN layering context, the media layer is the server layer of the optical signal layer. The optical signal is guided to its destination by the media layer by means of a network media channel. In the media layer, switching is based on a frequency slot.

In this scope, this document uses the term flexi-grid enabled DWDM network to refer to a network in which switching is based on frequency slots defined using the flexible grid, and covers mainly the Media Layer as well as the required adaptations from the Signal layer. The present document is thus focused on the control and management of the media layer.

3.2. Flexi-grid Terminology

This section presents the definition of the terms used in flexi-grid networks. More detail about these terms can be found in the ITU-T Recommendations [G.694.1], [G.872]), [G.870], [G.8080], and [G.959.1-2013].

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

3.2.1. Frequency Slots

f = 193.1 THz + n x 0.00625 THz

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

Figure 2: 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
        -------------     -------------------
              ^                    ^
    Slot NCF = 193.1THz     Slot NCF = 193.14375 THz
    Slot width = 25 GHz     Slot width = 37.5 GHz
      n=0, m=2                n=7, m=3

Figure 3: 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 4: Effective Frequency Slot

This subsection is focused on the frequency slot and related terms.

  • Frequency Slot [G.694.1]: 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.
  • 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
  • Nominal Central Frequency Granularity: This is the spacing between allowed nominal central frequencies and it is set to 6.25 GHz [G.694.1].
  • Slot Width Granularity (SWG): 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.
    • The symbol '+' represents the allowed nominal central frequencies
    • The '--' represents the nominal central frequency granularity in units of 6.25 GHz
    • 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 to zero.

  • Effective Frequency Slot [G.870]: The effective frequency slot of a media channel is that part of the frequency slots of the filters along the media channel that is common to all of the filters' frequency slots. Note that both the Frequency Slot and Effective Frequency Slot are local terms.
  • Figure 4 shows the effect of combining two filters along a channel. The combination of frequency slot 1 and frequency slot 2 applied to the media channel is effective frequency slot shown.

3.2.2. Media Layer Elements

  • Media Element: A media element directs an 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 [G.870]. Examples of media elements include fibers, amplifiers, filters, and switching matrices.
  • 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.

3.2.3. Media Channels

This section defines concepts such as (Network) Media Channel; the mapping to GMPLS constructs (i.e., LSP) is detailed in Section 4.

  • 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 frequency slot (an 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: [G.870] defines the Network Media Channel as a media channel that transports a single OTSi, defined next.

3.2.4. Optical Tributary Signals

  • Optical Tributary Signal (OTSi) [G.959.1-2013]: The optical signal that is placed within a network media channel for transport across the optical network. This may consist of a single modulated optical carrier or a group of modulated optical carriers or subcarriers. To provide a connection between the OTSi source and the OTSi sink the optical signal must be assigned to a network media channel.
  • OTSi Group (OTSiG): The set of OTSi that are carried by a group of network media channels. Each OTSi is carried by one network media channel. From a management perspective it SHOULD be possible to manage both the OTSiG and a group of Network Media Channels as single entities.

3.2.5. Composite Media Channels

  • It is possible to construct an end-to-end media channel as a composite of more than one network media channels. A composite media channel carries a group of OTSi (i.e., OTSiG). Each OTSi is carried by one network media channel. This group of OTSi are carried over a single fibre.
  • In this case, the effective frequency slots may be contiguous (i.e., there is no spectrum between them that can be used for other media channels) or non-contiguous.
  • It is not currently envisaged that such composite media channels may be constructed from slots carried on different fibers whether those fibers traverse the same hop-by-hop path through the network or not.
  • Furthermore, it is not considered likely that a media channel may be constructed from a different variation of slot composition on each hop. That is, the slot composition (i.e., the group of OTSi carried by the composite media channel) must be the same from one end to the other of the media channel even if the specific slot for each OTSi and the spacing among slots may vary hop by hop.
  • How the signal is carried across such groups of network media channels is out of scope for this document.

3.3. Hierarchy in the Media Layer

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. From the control and management perspective, a media channel can be logically split into network media channels.

In Figure 5, a media channel has been configured and dimensioned to support two network media channels, each of them carrying one OTSi.


                          Media Channel Frequency Slot
  |                                                              |
  |       Frequency Slot                  Frequency Slot         |
  |   +-----------X-----------+       +----------X-----------+   |
  |   |         OTSi          |       |         OTSi         |   |
  |   |           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 5: Example of Media Channel / Network Media Channels and Associated Frequency Slots

3.4. Flexi-grid Layered Network Model

In the OTN layered network, the network media channel transports a single OTSi (see Figure 6)

  |                            OTSi                                 |
  O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
  |                                                                 |
  | Channel Port         Network Media Channel         Channel Port |
  O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
  |                                                                 |
+--------+                 +-----------+                   +--------+
|  \ (1) |                 |    (1)    |                   | (1)  / |
|   \----|-----------------|-----------|-------------------|-----/  |
+--------+ Link Channel    +-----------+  Link Channel     +--------+
  Media Channel            Media Channel                Media Channel
  Matrix                   Matrix                       Matrix

The symbol (1) indicates a Matrix Channel


Figure 6: Simplified Layered Network Model

Note that a particular example of OTSi is the OCh-P. Figure 7 shows this specific example as defined in G.805 [G.805].

 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

 The symbol (1) indicates a Matrix Channel


Figure 7: Layered Network Model According to G.805

3.4.1. DWDM Flexi-grid Enabled Network Element Models

A flexible grid network is constructed from subsystems that include WDM links, tunable transmitters, and receivers, (i.e, media elements including media layer switching elements that are media matrices) as well as electro-optical network elements. This is just the same as in a fixed grid network except that each element has flexible grid characteristics.

As stated in Clause 7 of [G.694.1] the flexible DWDM grid 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 might 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).

4. GMPLS Applicability

The goal of this section is to provide an insight into the application of GMPLS as a control mechanism in flexi-grid networks. Specific control plane requirements for the support of flexi-grid networks are covered in Section 5. This framework is aimed at controlling the media layer within the OTN hierarchy, and controlling the required adaptations of the signal layer. This document also defines the term Spectrum-Switched Optical Network (SSON) to refer to a Flexi-grid enabled DWDM network that is controlled by a GMPLS/PCE control plane.

This section provides a mapping of the ITU-T G.872 architectural aspects to GMPLS/Control plane terms, and considers the relationship between the architectural concept/construct of media channel and its control plane representations (e.g., as a TE link, as defined in [RFC3945]).

4.1. General Considerations

The GMPLS control of the media layer deals with the establishment of media channels that are switched in media channel matrices. GMPLS labels are used to locally represent the media channel and its associated frequency slot. Network media channels are considered a particular case of media channels when the end points are transceivers (that is, source and destination of an OTSi).

4.2. Consideration of TE Links

From a theoretical / abstract point of view, a fiber can be modeled as having a frequency slot that ranges from minus infinity to plus infinity. This representation helps understand the relationship between frequency slots and ranges.

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

The association sequence of the three components (i.e., a filter, a fiber, and a filter), is a media channel in its most basic form. From the control plane perspective this may modeled as a (physical) TE-link with a contiguous optical spectrum. This can be represented by saying 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 a one-hop media channel. In practical terms, associating a fiber with the terminating filters determines the usable optical spectrum.