+=======================================+
|     WSON         |          SSON      |
+=======================================+
|      OCh         |         OCh        | Signal Layer
+------------------+--------------------+
|                  |  Frequency Slot    |
| Optical Channel  |                    | Media Layer
| Carrier          |                    |
+------------------+--------------------+
|     1:1          |       N:1          | Relationship
| single layer     |    MRN/MLN         |  SL : ML
|   network        |     (* see note)   |
+------------------+--------------------+
      

Figure 8: Table Comparison WSON/SSON

A SSON (network) refers to the GMPLS control of flexi-grid enabled DWDM optical networks and it encompasses both the signal and media layers. The WSON also encompasses the signal and media layers but, since there is no formal separation between OCh and OCC (1:1) this layer separation is often not considered. A WSON is a particular case of SSON in the which all slot widths are equal and depend on the channel spacing. In other words, since there is only a 1:1 relationship between OCh : OCC there is no need to have separate controlled layers, as if both layers are collapsed into one.

Note that there is only one media layer switch matrix (one implementation is FlexGrid ROADM) in SSON, while "signal layer LSP is mainly for the purpose of management and control of individual optical signal". Signal layer LSPs (OChs) with the same attributions (such as source and destination) could be grouped into one media-layer LSP (media channel), which has advantages in spectral efficiency (reduce guard band between adjacent OChs in one FSC) and LSP management. However, assuming some network elements indeed perform signal layer switch in SSON, there must be enough guard band between adjacent OChs in one media channel, in order to compensate filter concatenation effect and other effects caused by signal layer switching elements. In such condition, the separation of signal layer from media layer cannot bring any benefit in spectral efficiency and in other aspects, but make the network switch and control more complex. If two OChs must switch to different ports, it is better to carry them by diferent FSCs and the media layer switch is enough in this scenario.

9. Control Plane Requirements

[Editor's note: The considered topology view is a layered network, in which the media layer corresponds to the server layer (flexigrid) and the signal layer corresponds to the client layer (Och). This data plane modeling considers the flexigrid and the OCh as separate layers, However, this has implications on the interop/interworking with WSON and OCh switching. We need to manage a MRN for OCh and stitching for WSON? In other words, a key part of the fwk is to define how can we have MRN/MLN hierarchical relationship with Och/FS and yet stitching 1:1 between WSON and SSON? In this line: how does OCh switching and WSON relate, actually?]

[Editor's note: formal requirements such as noted in the comments will be added in a later version of the document].

Hierarchy spectrum management decouples media and signal, but from the point of view of the control plane, such separation of concerns implies the management of a MRN/MLN network. So Control Plane needs to differentiate signal LSP and media LSP. It should also need to support Hierarchy-LSP [RFC4206] The central frequency of each hop should be same along end-to-end media or signal LSP because of Spectrum Continuity Constraint. Otherwise some nodes need to convert the central frequency along media or signal LSP.

9.1. Neighbor Discovery and Link Property Correlation

[Editors' note: text from draft-li-ccamp-grid-property-lmp-01]

Potential interworking problems between fixed-grid DWDM and flexible-grid DWDM nodes, may appear. Additionally, even two flexible-grid optical nodes may have different grid properties, leading to link property conflict.

Devices or applications that make use of the flexible-grid may not be able to support every possible slot width. In other words, applications may be defined where different grid granularity can be supported. Taking node F as an example, an application could be defined where the nominal central frequency granularity is 12.5 GHz requiring slot widths being multiple of 25 GHz. Therefore the link between two optical nodes with different grid granularity must be configured to align with the larger of both granularities. Besides, different nodes may have different slot width tuning ranges.

In summary, in a DWDM Link between two nodes, at least the following properties should be negotiated:

• Grid capability (channel spacing) - Between fixed-grid and flexible-grid nodes.
• Grid granularity - Between two flexible-grid nodes.
• Slot width tuning range - Between two flexible-grid nodes.

9.2. Path Computation / Routing and Spectrum Assignment (RSA)

Much like in WSON, in which if there is no (available) wavelength converters in an optical network, an LSP is subject to the ''wavelength continuity constraint'' (see section 4 of [RFC6163]), if the capability of shifting or converting an allocated frequency slot, the LSP is subject to the Optical ''Spectrum Continuity Constraint''.

Because of the limited availability of wavelength/spectrum converters (sparse translucent optical network) the wavelength/spectrum continuity constraint should always be considered. When available, information regarding spectrum conversion capabilities at the optical nodes may be used by RSA mechanisms.

The RSA process determines a route and frequency slot for a LSP. Hence, when a route is computed the spectrum assignment process (SA) should determine the central frequency and slot width based on the slot width and available central frequencies information of the transmitter and receiver, and the available frequency ranges information and available slot width ranges of the links that the route traverses.

9.2.1. Architectural Approaches to RSA

Similar to RWA for fixed grids, different ways of performing RSA in conjunction with the control plane can be considered. The approaches included in this document are provided for reference purposes only; other possible options could also be deployed.

9.2.1.1. Combined RSA (R&SA)

In this case, a computation entity performs both routing and frequency slot assignment. The computation entity should have the detailed network information, e.g. connectivity topology constructed by nodes/links information, available frequency ranges on each link, node capabilities, etc.

The computation entity could reside either on a PCE or the ingress node.

9.2.1.2. Separated RSA (R+SA)

In this case, routing computation and frequency slot assignment are performed by different entities. The first entity computes the routes and provides them to the second entity; the second entity assigns the frequency slot.

The first entity should get the connectivity topology to compute the proper routes; the second entity should get the available frequency ranges of the links and nodes' capabilities information to assign the spectrum.

9.2.1.3. Routing and Distributed SA (R+DSA)

In this case, one entity computes the route but the frequency slot assignment is performed hop-by-hop in a distributed way along the route. The available central frequencies which meet the spectrum continuity constraint should be collected hop by hop along the route. This procedure can be implemented by the GMPLS signaling protocol.

9.3. Routing / Topology dissemination

In the case of combined RSA architecture, the computation entity needs to get the detailed network information, i.e. connectivity topology, node capabilities and available frequency ranges of the links. Route computation is performed based on the connectivity topology and node capabilities; spectrum assignment is performed based on the available frequency ranges of the links. The computation entity may get the detailed network information by the GMPLS routing protocol. Compared with [RFC6163], except wavelength-specific availability information, the connectivity topology and node capabilities are the same as WSON, which can be advertised by GMPLS routing protocol (refer to section 6.2 of [RFC6163]. This section analyses the necessary changes on link information brought by flexible grids.

9.3.1. Available Frequency Ranges/slots of DWDM Links

In the case of flexible grids, channel central frequencies span from 193.1 THz towards both ends of the C band spectrum with 6.25 GHz granularity. Different LSPs could make use of different slot widths on the same link. Hence, the available frequency ranges should be advertised.

9.3.2. Available Slot Width Ranges of DWDM Links

The available slot width ranges needs to be advertised, in combination with the Available frequency ranges, in order to verify whether a LSP with a given slot width can be set up or not; this is is constrained by the available slot width ranges of the media matrix Depending on the availability of the slot width ranges, it is possible to allocate more spectrum than strictly needed by the LSP.

9.3.3. Tunable Optical Transmitters and Receivers

The slot width of a LSP is determined by the transmitter and receiver that could be mapped to ADD/DROP interfaces in WSON. Moreover their central frequency could be fixed or tunable, hence, both the slot width of an ADD/DROP interface and the available central frequencies should be advertised.

9.3.4. Hierarchical Spectrum Management

[Editors' note: the part on the hierarchy of the optical spectrum could be confusing, we can discuss it]. The total available spectrum on a fiber could be described as a resource that can be divided by a media device into a set of Frequency Slots. In terms of managing spectrum, it is necessary to be able to speak about different granularities of managed spectrum. For example, a part of the spectrum could be assigned to a third party to manage. This need to partition creates the impression that spectrum is a hierarchy in view of Management and Control Plane. The hierarchy is created within a management system, and it is an access right hierarchy only. It is a management hierarchy without any actual resource hierarchy within fiber. The end of fiber is a link end and presents a fiber port which represents all of spectrum available on the fiber. Each spectrum allocation appears as Link Channel Port (i.e., frequency slot port) within fiber.

9.3.5. Information Model

Fixed DM grids can also be described via suitable choices of slots in a flexible DWDM grid. However, devices or applications that make use of the flexible grid may not be capable of supporting every possible slot width or central frequency position. Following is the definition of information model, not intended to limit any IGP encoding implementation. For example, information required for routing/path selection may be the set of available nominal central frequencies from which a frequency slot of the required width can be allocated. A convenient encoding for this information (may be as a frequency slot or sets of contiguous slices) is further study in IGP encoding document.

<Available Spectrum in Fiber for frequency slot> ::= 
    <Available Frequency Range-List>
    <Available Central Frequency Granularity >
    <Available Slot Width Granularity>
    <Minimal Slot Width>
    <Maximal Slot Width>

<Available Frequency Range-List> ::= 
    <Available Frequency Range >[< Available Frequency Range-List>]

<Available Frequency Range >::= 
  <Start Spectrum Position><End Spectrum Position> | 
  <Sets of contiguous slices>

<Available Central Frequency Granularity> ::= n × 6.25GHz, 
  where n is positive integer, such as 6.25GHz, 12.5GHz, 25GHz, 50GHz
  or 100GHz

<Available Slot Width Granularity> ::= m × 12.5GHz, 
  where m is positive integer

<Minimal Slot Width> ::= j x 12.5GHz, 
  j is a positive integer

<Maximal Slot Width> ::= k x 12.5GHz, 
    k is a positive integer (k >= j)
              
            

        e2e LSP
        +++++++++++++++++++++++++++++++++++> (LSP1-2)

                  LSP segment (flexi-LSP)
                ====================> (LSP-AB)
                    C --- E --- G
                   /|\    |   / |\
                  / | \   |  /  | \
        R1 ---- A \ |  \  | /   | / B --- R2
                   \|   \ |/    |/
                    D --- F --- H

   fixed grid --A-- flexi-grid    --B-- fixed grid