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.

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)

   ←────────────── 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.

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.

• 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)”.

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.

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)

  ▼   denotes the 193.1 THz reference frequency
  |.| denotes one 6.25 GHz spectrum slot

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

Examples

The examples below correspond directly to the test scenarios in PostAlgoPathValidatorTest. 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
ROADM-A-SRG4	6.25 GHz	12.5 GHz	3	16
ROADM-A-DEG1	6.25 GHz	12.5 GHz	3	16
ROADM-B-DEG1	100.0 GHz	12.5 GHz	3	16
ROADM-B-SRG3	6.25 GHz	12.5 GHz	3	16

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.

Example 4: min-edge-freq and max-edge-freq restricting usable band

Path: XPDR-A2 → ROADM-A-DEG1 → ROADM-B-DEG1 → ROADM-B-SRG3

Node	min-edge-freq	max-edge-freq	center-freq-gran.
XPDR-A2	191.325 THz	194.45 THz	6.25 GHz
ROADM-A-DEG1	—	—	6.25 GHz
ROADM-B-DEG1	—	—	100.0 GHz
ROADM-B-SRG3	—	—	6.25 GHz

XPDR-A2 advertises min-edge-freq=191.325 THz and max-edge-freq=194.45 THz, which maps to slots 0–499 (the lower 65 % of the grid). All ROADM nodes have full spectrum available.

Service: 100 GHz → 16 slots

Phase 3 (frequency range): XPDR-A2’s range covers slots 0–499. After ANDing all nodes’ ranges, slots 500–767 are excluded.

Phase 4 (slot-width check): XPDR uses XpdrMcCapability which always passes → compatible.

Phase 5 (center frequency): With ROADM-B-DEG1 imposing a 100 GHz center-frequency step (16 slots), the highest valid 16-slot window within slots 0–499 has its center at slot 492 (194.4 THz = 193.1 + 13 × 0.1 THz).

Result: beginIndex=484, stopIndex=499

Without the XPDR frequency restriction the same path would yield beginIndex=740, stopIndex=755 (see Example 1). The restriction shifts the assignment to the highest valid position within the supported band.

MC Capability Implementations

TransportPCE uses three McCapability implementations:

Class	Used for
InterfaceMcCapability	ROADM degrees and SRGs. Enforces slot-width-granularity, min-slots, and max-slots in the slot-width check, and applies min/max-edge-freq as a FrequencyRange.
XpdrMcCapability	XPDR network ports. Always passes the slot-width check. Applies min/max-edge-freq as a FrequencyRange.
UnconstrainedMcCapability	OTN nodes, which impose no optical spectrum constraints. Always passes all checks and returns EntireGridRange.

The FrequencyRange abstraction has two implementations:

Class	Behaviour
McCapabilityRange	Maps a [min-edge-freq, max-edge-freq] interval to a BitSet of the grid slots that fall within the interval. Slots outside the interval are cleared.
EntireGridRange	Sets all 768 bits. Used when no frequency range is advertised.

Related Classes

• PostAlgoPathValidator.getSpectrumAssignment — main entry point

• McCapabilityCollection.usableFrequencyRange — Phase 3 implementation

• McCapabilityCollection.isCompatibleService — Phase 4 implementation

• McCapabilityRange / EntireGridRange — frequency range models

• CenterFrequencyGranularityCollection — computes the effective LCM center frequency granularity across all path nodes

• PortMappingVersion710 — reads mc-capability-profile from the device and stores min/max-edge-freq in the portmapping data store (7.1 only)

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 service on a path containing a node with slot-width-granularity=100 GHz. This check is performed by McCapabilityCollection.isCompatibleService.