Flexible Grid Technology

The need for flexibility in WDM networks

Traditional operate on a fixed grid. Each channel has a predefined spacing, for example 50 GHz. A low-data-rate service occupies the entire channel, while a higher-data-rate service may need two channels, even though the allocated spectrum is used differently in the two cases.

Fixed channel width makes it harder to match an allocation to the signal requirement and leaves part of the spectrum unused. A flexible grid builds a channel from adjacent spectrum units with 12.5 GHz granularity. Elastic Optical Networks (EON) can then assign spectrum according to traffic demand, modulation format, and required reach.

Flexible-Grid Parameters

In a flexible grid, an optical channel is described by its nominal central frequency and frequency-slot width. The first parameter identifies the position of the allocated spectrum, and the second gives its width. The allocation can therefore match the signal requirement.

Frequency Slot

According to ITU-T G.694.1, a frequency slot is defined by two independent parameters. They are the nominal central frequency $f_c$ and the slot width $\Delta f$ .

Slot width: $\Delta f = m \times 12.5 \text{ GHz}$ , where $m$ is a positive integer. The slot-width granularity is 12.5 GHz, so allowed widths are integer multiples of 12.5 GHz.
Nominal central frequency: $f_c = 193.1 \text{ THz} + n \times 6.25 \text{ GHz}$ , where $n$ is a positive or negative integer, including zero. The 6.25 GHz spacing applies to possible central-frequency positions, not to the slot width.

Constructing an optical channel

With a flexible grid, an optical channel is established on demand by assigning a specified number of adjacent spectrum units. The allocation size follows the required bit rate, modulation format, and transmission parameters. RFC 7698 describes RSA as selecting a route and assigning a central frequency and slot width from the request and the spectrum available on the links along that route.

Contiguous allocation: The $N$ frequency slots assigned to a single connection must be , meaning they form an unbroken, continuous block of spectrum. You cannot assign slots 1, 2, and 5 to the same channel while skipping 3 and 4.
Variable width: The total width (bandwidth) of the channel is determined by the number of slots ( $N$ ) it occupies.
$\text{Channel Bandwidth} = N \times \Delta f_{slot}$
Here, $N$ counts adjacent base units, and $\Delta f_{slot} = 12.5 \text{ GHz}$ is the unit granularity.
Guard band: A gap may be left between adjacent channels to limit inter-channel interference. It may contain one or more unused spectrum units. Annex I of ITU-T G.694.1 also shows a case where assigning an additional 50 GHz slot leaves 6.25 GHz of spectrum unallocated. A is implemented by leaving unused units between the blocks assigned to two different connections.

Spectrum allocation constraints

A flexible grid has two allocation constraints. The first concerns adjacent slots on one link, and the second concerns the effective frequency range along the path.

1. Spectrum contiguity on a link

The slots assigned to one connection on any fiber link must form one contiguous, unbroken block. If a connection needs slots 1, 2, and 5, the assignment fails because slots 3 and 4 are missing. Optical switches and filters are designed to handle one continuous frequency range.

2. Spectrum continuity along a path

If the nodes cannot shift or convert the allocated spectrum, an optical connection crossing multiple links must retain the same effective frequency range along the path. RFC 7698 defines this as the spectrum continuity constraint. Local $n$ and $m$ values may differ between links when the locally assigned slots have an intersection that forms a valid effective range. Hardware constraints determine which values are available. The end-to-end effective range must remain valid for the connection request. For a connection from Node A through B and C to D, this range must be available on every link when the nodes do not convert spectrum.

In the example route from London to Rome and the M25 motorway, number 3 identifies the selected resource. In an optical network, it represents the index of the effective spectrum range, which remains the same on successive links without spectrum conversion.

Reason for the constraint: RFC 7698 points to the limited availability of spectrum converters as the reason to account for continuity. An optical switch (BV-OXC) can redirect a light block from input to output, but moving its position in the spectrum requires conversion. Finding a common, contiguous block of free slots over a long, multi-hop path remains part of the problem, and scattered free blocks can lead to spectrum fragmentation.