Network Working Group O. Gonzalez de Dios, Ed.
Internet-Draft Telefonica I+D
Intended status: Standards Track R. Casellas, Ed.
Expires: August 18, 2014 CTTC
F. Zhang
X. Fu
D. Ceccarelli
I. Hussain
February 14, 2014

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, 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 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, 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 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, the media channel transports an Optical Tributary Signal.

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 in which media (spectrum) and signal are jointly considered

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.

3. Acronyms

EFS: Effective Frequency Slot

FS: Frequency Slot

NCF: Nominal Central Frequency

OCh: Optical Channel

OCh-P: Optical Channel Payload

OTS: Optical Tributary Signal

OCC: Optical Channel Carrier

SWG: Slot Width Granularity

4. Flexi-grid Networks

4.1. Flexi-grid in the context of OTN

[G.872] describes from a network level the functional architecture of Optical Transport Networks (OTN). The OTN 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, and the size of a media channel is given by the properties of the associated 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.

4.2. Terminology

This section presents the definition of the terms used in flexi-grid networks. These terms are included 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].

4.2.1. Frequency Slots

This subsection is focused on the frequency slot related terms.

   -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

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.

Nominal Central Frequency Granularity: It is the spacing between allowed nominal central frequencies and it is set to 6.25 GHz (note: sometimes referred to as 0.00625 THz).

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 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 3: Example Frequency slots

  • 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.

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, according to the already described rules.

                     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

4.2.2. Media Channels

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: It is a media channel that transports an Optical Tributary Signal [Editor's note: this definition goes beyond current G.870 definition, which is still tightened to a particular case of OTS, the OCh-P]

4.2.3. Media Layer Elements

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 [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.

4.2.4. Optical Tributary Signals

Optical Tributary Signal [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. One particular example of Optical Tributary Signal is an Optical Channel Payload (OCh-P) [G.872].

4.3. Flexi-grid layered network model

In the OTN layered network, the network media channel transports a single Optical Tributary Signal (see Figure 5)

  |                     Optical Tributary Signal                    |
  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

(1) - Matrix Channel

Figure 5: Simplified Layered Network Model

A particular example of Optical Tributary Signal is the OCh-P. Figure Figure 6 shows the example of the layered network model particularized for the OCH-P case, as defined in 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

(1) - Matrix Channel

Figure 6: Layered Network Model according to G.805

By definition a network media channel only supports a single Optical Tributary signal. How several Optical Tributary signals are bound together is out of the scope of the present document and is a matter of the signal layer.

4.3.1. 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 splited in other media channels.

In Figure 7 , a Media Channel has been configured and dimensioned to support two network media channels, each of them carrying one optical tributary signal.


                         Media Channel Frequency Slot 
 |                                                              |
 |       Frequency Slot                  Frequency Slot         |
 |   +------------X-----------+      +----------X-----------+   |
 |   | Opt Tributary Signal  |       | Opt Tributary Signal |   |
 |   |           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 7: Example of Media Channel / Network Media Channels and associated frequency slots

4.3.2. 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.

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. GMPLS applicability

The goal of this section is to provide an insight of the application of GMPLS to control flexi-grid networks, while specific requirements are covered in the next section. The present framework is aimed at controlling the media layer within the Optical Transport Network (OTN) hierarchy and the required adaptations of the signal layer. This document also defines the term SSON (Spectrum-Switched Optical Network) 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).

5.1. General considerations

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. Network media channels are considered a particular case of media channels when the end points are transceivers (that is, source and destination of an Optical Tributary Signal)

5.2. 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 spectrum.