Spectrum assignment in elastic optical networks
When a request for a new optical connection arrives, the network operator must select a feasible path through the topology and reserve spectrum on every link of that path. Selecting the path is Routing. Selecting the frequency resource on the selected path is Spectrum Assignment (SA). Together, these operations are known as Routing and Spectrum Assignment (RSA). In a flex-grid network, the two decisions are coupled because the requested slot width must be available along the complete route.
In an , the available spectrum is represented as small frequency resources called frequency slots. A connection requests a specific number of adjacent slots, that is, a contiguous block. The Spectrum Assignment algorithm must select a block that is free on every link of the route, obeys spectrum continuity when no converter is available, and does not overlap an existing allocation. It should also leave a usable spectrum layout for later requests.
Spectrum fragmentation
A central challenge in Spectrum Assignment is spectrum fragmentation. In a dynamic network, connections are established and released at different times. Releasing a connection returns its frequency slots, but later requests may occupy only parts of the resulting gaps. Repeated allocation and release can leave many scattered free blocks that are too narrow for a new request.
Fragmentation is important because a large total amount of free spectrum does not guarantee a usable allocation. A new request may still be rejected when no single block of slots is wide enough on all links of a candidate route. The effect is evaluated together with route availability, slot width, and the continuity constraint.
Consequences of fragmentation
Poorly managed spectrum assignment can increase fragmentation and the resulting blocking probability. A connection is "blocked" when the network cannot find a suitable route and spectrum block, even though the sum of free slots may appear sufficient. Blocking probability is a network-performance measure. Its effect on revenue or customer experience depends on service contracts, traffic demand, and the operator's admission policy.
Spectrum assignment heuristics
Finding an optimal assignment for a changing set of requests is a computationally difficult () task. Network controllers therefore use heuristics, which apply simple rules and return a feasible allocation quickly. A heuristic trades a possible loss of optimality for predictable computation time.
Consider a fiber link whose slot indexes run from 0 to 319. A request needs four adjacent slots. The currently free blocks are 10-15 with six slots, 40-43 with four slots, and 90-95 with six slots. The allocation rule determines which eligible block is selected.
Common Allocation Strategies
First-Fit (FF)
The Rule: Scan the spectrum starting from the lowest frequency (slot 0) upwards and allocate the first available contiguous block of slots that is large enough to accommodate the request.
In our example: The algorithm would scan, find the free block at slots 10-15, see that it is large enough , and allocate slots 10, 11, 12, and 13 to the new connection.
Implications: FF is simple and fast to implement. It often fills lower-indexed slots first, which can leave larger blocks at higher indexes, although the result depends on traffic duration, topology, and request widths. Blocking performance must therefore be measured for the target network rather than assumed from the rule alone.
Last-Fit (LF)
The Rule: The opposite of First-Fit. Scan the spectrum starting from the highest frequency downwards and allocate the first block that is large enough.
In our example: The algorithm would scan from the top, find the free block at 90-95, and allocate slots 92, 93, 94, and 95 (or 90-93).
Implications: This strategy packs connections at the high-index end of the spectrum and leaves free space at the bottom. Its performance is the counterpart of FF under a reversed scan order, but the outcome still depends on traffic, topology, and the usable frequency range of the equipment.
Random-Fit (RF)
The Rule: Identify all available contiguous blocks that are large enough for the request, and then choose one of them at random.
In our example: The algorithm would identify blocks , , and as candidates and randomly select one for the four-slot connection.
Implications: Random placement can spread allocations across the spectrum and may create more separated gaps. Whether this improves or worsens blocking depends on the traffic model, topology, and comparison policy, so RF requires evaluation rather than a universal ranking against FF.
Exact-Fit
The Rule: Search for a free block that has exactly the same size as the request. If no exact block exists, the implementation must apply a defined fallback, such as selecting a larger block with another policy, or reject the request.
In our example: The algorithm would first search for a 4-slot block. It would find the block at 40-43 and immediately select it, leaving the larger blocks (10-15 and 90-95) untouched for potentially larger future requests.
Implications: The exact match can preserve larger blocks when the fallback is chosen carefully. It adds a search condition and does not guarantee lower blocking for every traffic pattern. Its benefit should be checked against the selected fallback and network workload.
Constraints beyond the allocation rule
Beyond a slot-selection rule, a practical Spectrum Assignment system must consider optical constraints and the capabilities signaled by the transponders and switching nodes. A control plane can carry the requested central frequency and slot width in its signaling, while the exact procedure depends on the control architecture.
Modulation and reach
The required slot width is not fixed. It depends on the symbol rate, coding, and modulation format, while the acceptable format depends on the path loss, noise, nonlinear effects, and required reach.
Consider a 100 Gbit/s request as an equipment-specific example:
- On a short path, an equipment profile might support higher-order 16-QAM and assign three slots. The exact slot width depends on the symbol rate, coding, and transponder design.
- On a longer path, the same profile might select a lower-order format such as BPSK to obtain a larger noise margin. It could then require eight slots for the same client rate. These figures are illustrative and require a link budget for validation.
An SA algorithm therefore cannot search only for "a free block." It must derive a feasible signal profile from the path and transponder constraints, then search for a contiguous block wide enough for that profile. The allocation must also remain available across the route.
Spectrum defragmentation
When fragmentation limits future allocations, one option is spectrum defragmentation. Existing connections are moved to other valid frequency ranges so that small free blocks can be consolidated into larger ones. The procedure must preserve the signal constraints of every affected path and coordinate the nodes along those paths.
Live traffic requires make-before-break or another hitless mechanism, together with control-plane coordination. If the equipment cannot shift a channel without interruption, defragmentation can require a planned service change. The choice depends on transponder tuning, switching support, and the operator's protection policy.