Spectrum Assignment Using MC Capabilities

This document explains how TransportPCE uses OpenROADM media channel (MC) capabilities to select a service frequency slot when computing an optical service path. The central method is getSpectrumAssignment in PostAlgoPathValidator.

Background

OpenROADM 2.2.1 introduced four MC capability attributes that describe what a port can support in terms of optical spectrum. These are listed directly on each ROADM degree and SRG node.

OpenROADM 7.1 introduced a named mc-capability-profile list at the device root. Nodes reference one or more profiles by name via a leaf-list mc-capability-profile-name, making profiles reusable across ports with identical characteristics. XPDR network ports gained MC capability support in this version. 7.1 also introduced min-edge-freq and max-edge-freq to describe a port’s physical frequency range.

MC capability handling is implemented for OpenROADM version 2.2.1 and 7.1 only. Capabilities are read during port mapping and stored in the tpce portmapping data store.

Attribute	Unit / default	Versions
`center-freq-granularity`	GHz, default 50	2.2.1, 7.1
`slot-width-granularity`	GHz, default 50	2.2.1, 7.1
`min-slots`	default 1	2.2.1, 7.1
`max-slots`	default 1	2.2.1, 7.1
`min-edge-freq`	THz, optional	7.1
`max-edge-freq`	THz, optional	7.1

During path computation getSpectrumAssignment reads the stored capabilities from each node on the candidate path and uses them to find a valid frequency assignment.

Summary

OpenROADM uses a 768 × 6.25 GHz grid; however, a 40-bit grid is used throughout this document for simplicity. The same principles and implementation apply to all grid sizes, regardless of the number of nodes in a service path.

Reference grid:

  ▼   : the 193.1 THz reference frequency
  |.| : a 6.25 GHz spectrum slot

192.975                              193.1                                193.225
|                                       ▼                                       |
|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|

During the frequency selection process, all the available frequencies from all nodes in the service path are merged into one coherent grid. In practice this means merging the cband frequencies-bit-set from each node into one.

See Phase 2: Build the available frequency bitmap for details.
min-edge-freq and max-edge-freq are applied. The remaining available frequencies are illustrated below with the caption “available”.

See Phase 3: Apply frequency range constraints for details.
All the nodes in the service path are being evaluated to determine whether they support the requested service slot width of 37.5 GHz by using each node’s mc-capability. In short slot-width-granularity, min-slots and max-slots are used during this process.

See Phase 4: Validate service slot width for details.
All the center-frequency-granularities from the nodes are being collected into a unique list. One center frequency granularity supported by all nodes is extracted from this list.

See Phase 5: Select center frequency for details.

The final step selects a service frequency slot; the result depends on the outcome of the steps above. The graph below illustrates the key concepts.

Example showcasing the available spectrum grid being processed in search of a suitable frequency range for a service slot width of 37.5 GHz:

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  |1| : an available 6.25 GHz spectrum slot
  | | : an occupied 6.25 GHz spectrum slot
  `-´ : possible 37.5 GHz wide service slot
  S   : iteration start point
  ↓   : 50 GHz center frequency granularity

           L                                                                       S       U
           |       ↓               ↓               ▼               ↓               ↓       |
available  |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|
             `-----------´   `-----------´   `-----------´   `-----------´   `-----------´
                   5               4               3               2               1

Finding an available service frequency slot involves scanning the grid from the highest valid center frequency downward in center-frequency-granularity steps, checking at each position whether the full service slot window is available.

This requires:

▼ - The slot index of the 193.1 THz reference frequency, measured from the grid edge
↓ - The effective center frequency granularity (in this case 50 GHz) and the corresponding nr of slots (8)
S - The slot index of the highest valid center frequency in relation to the reference frequency at ▼ — this is the
iteration start point denoted with an S (slot 37 in this example).

In the example above, service slot 1 is evaluated first going from right to left. In the end, service slot nr 3 will be selected. That is the first location where a continuous range of 6 slots (37.5 GHz) centered around a valid center frequency supported by all nodes in the path, is available.

Known limitations

The slot-width check requires the service width to be an exact multiple of each node’s slot-width-granularity. A service request is therefore rejected if any node on the path advertises a granularity that does not divide the requested width evenly — for example, a 75 GHz wide service slot on a path containing a node with slot-width-granularity=100 GHz. This check is performed by McCapabilityCollection.isCompatibleService.

The same frequencies are set up across the entire path. Even though there might be equipment out there capable of having different frequency ranges at opposite ends, TransportPCE only supports setting up the same service slot width on both ends.

The Spectrum Grid

TransportPCE models the optical spectrum as a BitSet of 768 slots. Each slot is 6.25 GHz wide and the grid starts at 191.325 THz:

slot 0   →  191.325 000 THz
slot 1   →  191.331 250 THz
slot 2   →  191.337 500 THz
...
slot 767 →  196.118 750 THz
(upper edge: 196.125 THz)

A bit set to 1 means the slot is available; 0 means occupied or excluded. The full grid therefore starts as a 768-bit set with all bits set.

Visualizing MC Capability Profiles

The diagrams below illustrate how mc-capability-profile attributes map onto the optical spectrum. These correspond to the constraints applied in Phases 3–5 of getSpectrumAssignment.

ROADM nodes (InterfaceMcCapability)

A ROADM node uses these mc capability properties:

center-freq-granularity
slot-width-granularity
min-slots
max-slots
min-edge-freq
max-edge-freq

   ←────────────── Full C-band (191.325–196.125 THz) ──────────────→

        min-edge-freq                             max-edge-freq
              ↓                                         ↓
──────────────┬─────────────────────────────────────────┬─────────────
              │                Usable Band              │
              ├──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┤
              │  │  │  │  │  │  │  │  │  │  │  │  │  │  │
              └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘

              │←→│  Slot-width granularity
              │←────────────→│  Min slots × granularity
              │←──────────────────────────────→│  Max slots × granularity

                   ↑      ↑      ↑      ↑      ↑
                   └──────┴──────┴──────┴──────┘
                    Center-frequency granularity

min-edge-freq / max-edge-freq: Defines the physical passband of the port. Slots outside this band are excluded from spectrum assignment (Phase 3).
slot-width-granularity: The unit step for service width. A service width must be an exact multiple of this value (Phase 4).
min-slots / max-slots: The allowed range of slot-width-granularity steps a service may occupy. A width outside [min × swg, max × swg] is rejected (Phase 4).
center-freq-granularity: The allowed spacing between center frequencies. The path-level alignment constraint is the least common multiple (LCM) of all nodes’ center-frequency granularities, ensuring that valid center frequencies lie on every node’s frequency grid (Phase 5).

XPDR nodes (XpdrMcCapability)

For 7.1 XPDRs only three attributes apply:

center-freq-granularity
min-edge-freq
max-edge-freq

   ←────────────── Full C-band (191.325–196.125 THz) ──────────────→

        min-edge-freq                               max-edge-freq
              ↓                                           ↓
──────────────┬───────────────────────────────────────────┬─────────────
              │                                           │
              │            Tunable Laser Range            │
              │                                           │
              └───────────────────────────────────────────┘
                    ↑      ↑      ↑      ↑      ↑
                    └──────┴──────┴──────┴──────┘
                       Center-Frequency Granularity
                     (minimum center-frequency spacing)

min-edge-freq / max-edge-freq: The physical tuning range of the laser. Applied in Phase 3 identically to ROADM nodes.
center-freq-granularity: The minimum spacing between selectable center frequencies. Contributes to the path-level LCM in Phase 5.

How getSpectrumAssignment Works

The method walks through five phases for each candidate path.

Phase 1: Collect path nodes

The method iterates the path’s edge list and collects the unique set of PCE nodes (by source and destination node ID of each edge). This deduplication means a node that appears in multiple edges is only processed once.

Phase 2: Build the available frequency bitmap

Starting from a full 768-bit set (all slots available), the method ANDs the frequency bitmap of each non-contentionless node into the running result:

result = all-ones (768 bits)
for each node:
    result = result AND node.frequencyBitmap

The result is the intersection of available slots across all nodes on the path. If the result is empty at this point, no frequencies are available and the method returns an empty assignment immediately.

A node marked as a contentionless SRG is skipped in this phase because contention-less switching fabrics do not share spectrum resources between add/drop ports — occupancy on one port does not exclude a slot on another.

Example

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  |1| - one available 6.25 GHz spectrum slot
  | | - one occupied 6.25 GHz spectrum slot

           L                                       ▼                                       U
Node1      |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|
Node2      |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|
Node3      |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|1|1|1|1|1|1|1|
           =================================================================================
available  |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1| | | | | | |1|1|1|1|1|

Phase 3: Apply frequency range constraints

Each node carries a FrequencyRange that describes the portion of the spectrum grid it can physically support. This is derived from min-edge-freq and max-edge-freq in the mc-capability-profile:

If both are present, McCapabilityRange maps the interval [min-edge-freq, max-edge-freq] to a BitSet of the slots that fall within that range (0 = unsupported, 1 = supported).
If either is absent, McCapabilityRange.from() falls back to EntireGridRange, which sets all 768 bits and imposes no restriction.

The method ANDs all nodes’ frequency ranges into the running result:

for each node:
    result = result AND node.supportableFrequencyRange(grid parameters)

If the result is empty after this step, the path is rejected with the message “No frequencies available (restricted by McCapabilities)”.

Example

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  |1| - one available 6.25 GHz spectrum slot
  | | - one occupied 6.25 GHz spectrum slot

           L                                       ▼                                       U
available  |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1| | | | | | |1|1|1|1|1|
range      | | | | |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | |
           =================================================================================
usable     | | | | |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1| | | | | | |1|1|1| | |

Phase 4: Validate service slot width

The requested service width (in slots) must be compatible with every node’s slot-width-granularity, min-slots, and max-slots. The check is:

serviceWidthGHz  = slotCount × 6.25 GHz
slotsPerStep     = serviceWidthGHz / node.slotWidthGranularity
compatible       = serviceWidthGHz is an exact multiple of slotWidthGranularity
                   AND minSlots ≤ slotsPerStep ≤ maxSlots

This check is applied to every node in the collection. If any node fails, the method returns an empty assignment.

XPDR nodes are represented by XpdrMcCapability, which always returns true for this check. The slot-width and slot-count limits in an XPDR mc-capability-profile reflect client-side constraints, not optical line-side constraints, so they are intentionally excluded from spectrum slot-width validation.

For details see:

McCapability — per-node capability interface (interface)
InterfaceMcCapability — ROADM nodes (implements McCapability)
XpdrMcCapability — XPDR nodes (implements McCapability)

Phase 5: Select center frequency

The effective center frequency granularity for the path is the least common multiple (LCM) of all nodes’ center-freq-granularity values, expressed in slots. The assignment algorithm (high-to-low) selects the highest-indexed contiguous block of slotCount available bits whose center frequency falls on a multiple of this combined granularity relative to the ITU-T G.694.1 reference frequency of 193.1 THz.

The result is a SpectrumAssignment with beginIndex and stopIndex (both inclusive). An empty assignment (beginIndex = stopIndex = 0) signals failure.

Center-frequency granularity (CFG) anchored at 193.1 THz (ITU-T G.694.1)

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  |.| : one 6.25 GHz spectrum slot
  ↓   : center frequency granularity

         191.325   ...   193.05          193.1           193.15    ...   196.125
CFG 50GHz   |               ↓                               ↓               |
CFG 25GHz   L               ↓       ↓       ▼       ↓       ↓               U
            |.|.|  ...  |.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|.|  ...  |.|.|
            |←------------------- 768 × 6.25 GHz slots --------------------→|

Center frequency selection examples

The following examples show how the effective center-freq-granularity step is derived and how the high-to-low scan selects a slot window.

CFG = 25 GHz — service slot 4 is chosen

The example showcases two center frequency granularities in a service path; 6.25 and 25 GHz. The service slot width is 37.5 GHz. In this case, the fourth (4) attempt at locating a service slot is successful.

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  | | : an occupied 6.25 GHz spectrum slot
  |1| : an available 6.25 GHz spectrum slot
  ↓   : center frequency granularity
  ◉   : common center frequency granularity
  `-´ : possible slot
  S   : iteration start point

                                                                                 S
CFG              ◉       ◉       ◉       ◉       ◉       ◉       ◉       ◉       ◉
1. 6.25  L ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ U
2. 25    |       ↓       ↓       ↓       ↓       ▼       ↓       ↓       ↓       ↓       |
         |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|
           `-----------´   `-----------´   `-----------´   `-----------´   `-----------´
                 9               7               5               3               1
                   `-----------´   `-----------´   `-----------´   `-----------´
                         8               6               4               2

CFG = 75 GHz — service slot 2 is chosen

Compiling the unique center-freq-granularity values across all nodes on the path gives {25 GHz, 75 GHz}. LCM(25, 75) = 75 GHz = 12 slots per step.

  L   : spectrum grid lower edge frequency
  U   : spectrum grid upper edge frequency
  ▼   : the 193.1 THz reference frequency
  | | : an occupied 6.25 GHz spectrum slot
  |1| : an available 6.25 GHz spectrum slot
  ↓   : center frequency granularity
  ◉   : common center frequency granularity
  `-´ : possible slot
  S   : iteration start point

                                                                         S
CFG                      ◉                       ◉                       ◉
1. 25    L       ↓       ↓       ↓       ↓       ▼       ↓       ↓       ↓       ↓       U
2. 75    |               ↓                       ▼                       ↓               |
         |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | | |1|1|1|1|1|
                  `-------------´         `-------------´         `-------------´
                         3                       2                       1

See CenterFrequencyGranularityCollection — Phase 5 for the corresponding API usage.

End-to-end Examples

All paths assume the full 768-slot bitmap is available on every node unless stated otherwise.

Example 1: Uniform ROADM path — 100 GHz service

Path: ROADM-A-SRG4 → ROADM-A-DEG1 → ROADM-B-DEG1 → ROADM-B-SRG3

Node	center-freq-gran.	slot-width-gran.	min-slots	max-slots	100 GHz service
ROADM-A-SRG4	6.25 GHz	12.5 GHz	3	16	Pass
ROADM-A-DEG1	6.25 GHz	12.5 GHz	3	16	Pass
ROADM-B-DEG1	100.0 GHz	12.5 GHz	3	16	Pass
ROADM-B-SRG3	6.25 GHz	12.5 GHz	3	16	Pass

Service: 100 GHz → 16 slots × 6.25 GHz

Phase 2 (bitmap): All nodes report full spectrum → result is all 768 bits.

Phase 3 (frequency range): No min/max-edge-freq on any node → no restriction.

Phase 4 (slot-width check): 100 GHz / 12.5 GHz = 8 steps per slot-width unit; 3 ≤ 8 ≤ 16 on all nodes → compatible.

Phase 5 (center frequency): LCM(6.25, 6.25, 100.0, 6.25) = 100 GHz = 16 slots per step. Starting from the top of the grid the algorithm finds center slot 748 (196.0 THz = 193.1 + 29 × 0.1 THz), giving a 16-slot window at slots 740–755.

Result: beginIndex=740, stopIndex=755

Example 2: Mixed center-freq-granularity — 62.5 GHz service

Path: ROADM-B-SRG13 → ROADM-B-DEG2 → ROADM-C-DEG2 → ROADM-C-SRG13

Node	center-freq-gran.	slot-width-gran.	min-slots	max-slots	62.5 GHz service
ROADM-B-SRG13	6.25 GHz	12.5 GHz	1	20	Pass
ROADM-B-DEG2	75.0 GHz	12.5 GHz	4	8	Pass
ROADM-C-DEG2	6.25 GHz	12.5 GHz	1	20	Pass
ROADM-C-SRG13	6.25 GHz	12.5 GHz	1	20	Pass

Service: 62.5 GHz → 10 slots × 6.25 GHz

Phase 4 (slot-width check): 62.5 GHz / 12.5 GHz = 5; 4 ≤ 5 ≤ 8 on all nodes → compatible.

Phase 5 (center frequency): LCM(6.25, 75.0, 6.25, 6.25) = 75 GHz = 12 slots per step. The algorithm selects the highest 10-slot block whose center aligns to a multiple of 75 GHz from 193.1 THz.

Result: beginIndex=747, stopIndex=756

Compare this to Example 1: even though 62.5 GHz < 100 GHz, the 75 GHz center frequency constraint from ROADM-B-DEG2 limits where the block can be placed.

Example 3: min-slots and max-slots reject a service

Path: ROADM-B-SRG13 → ROADM-B-DEG2 → ROADM-C-DEG2 → ROADM-C-SRG13 → ROADM-C-SRG12

Node	center-freq-gran.	slot-width-gran.	min-slots	max-slots	62.5 GHz service
ROADM-B-SRG13	6.25 GHz	12.5 GHz	1	4	Fail
ROADM-B-DEG2	75.0 GHz	12.5 GHz	4	8	Pass
ROADM-C-DEG2	6.25 GHz	12.5 GHz	1	20	Pass
ROADM-C-SRG12	50.0 GHz	50.0 GHz	1	1	Fail

A node that advertises slot-width-granularity=12.5 GHz, min-slots=1, max-slots=4 supports service widths from 12.5 GHz to 50 GHz in 12.5 GHz increments. ROADM-B-SRG13 therefore fails the phase 4 slot-width check.
A node that advertises slot-width-granularity=50 GHz, max-slots=1 is saying it can carry exactly one 50 GHz channel and nothing wider or narrower. ROADM-C-SRG12 therefore fails the phase 4 slot-width check.

The service request must fit within all nodes’ slot ranges simultaneously.