EON vs Fixed Grid Comparison

Rigid channels: The spectrum follows a selected fixed-grid plan. ITU-T G.694.1 specifies reference spacings including 12.5, 25, 50, and 100 GHz, plus wider options, while a deployed system supports a particular subset. The spacing and channel profile do not by themselves define the bit rate.

Unused capacity: A low-rate request can occupy a channel profile whose supported line rate is higher. For example, a 20 Gbit/s service may use a profile built for a 100 Gbit/s channel, leaving part of that profile unused. A 110 Gbit/s request may need two 100 Gbit/s profiles. The exact result depends on modulation, coding, symbol rate, and reach, not on spacing alone.

Separation between channels: Channel spacing and filtering require a design margin that limits interference. The margin is equipment and system dependent. It should not be treated as one universal fixed guard band sized identically in every network.

Granular slots: Flex-grid uses frequency slots identified by a nominal central frequency and a slot width. The central-frequency grid has 6.25 GHz granularity, while slot widths are multiples of 12.5 GHz. Implementations support selected ranges rather than every possible value.

Channels assembled from slots: An optical connection is assigned a contiguous block whose width matches a supported signal profile. The number of slots depends on modulation, symbol rate, coding, transponder capability, and path constraints. A lower-rate service can require fewer slots, but no rate-to-slot mapping is universal.

Configured separation: Flex-grid does not remove the need for filtering and system margins. The required separation depends on the transponder, switching node, neighbouring channels, and operating conditions. A single empty slot is not a general rule for every deployment.

Coarse granularity: Service offerings are bounded by the available fixed-rate transponders and channels. Common profiles include 10 Gbit/s, 40 Gbit/s, and 100 Gbit/s, but the exact catalogue depends on the equipment and operator.

Provisioning effort: If the required rate or optical interface is not installed, a new transponder or other field work may be needed at the endpoints. The resulting provisioning time depends on the inventory, management system, and operational process. It is not inherently a fixed number of weeks or months.

Electronic aggregation: When client traffic does not match the optical profile, IP/MPLS equipment can aggregate smaller streams before transmission. This adds electronic ports, processing, power, and operational complexity, although the need depends on the service mix.

Fine granularity: EON can offer a range of service rates when the BVT, frequency slots, and switching nodes support them. Allocating more or fewer slots does not create an arbitrary rate with unchanged hardware. Symbol rate, modulation, coding, reach, and transponder tuning range remain limiting factors.

Software-assisted provisioning: A compatible connection can be established, released, or resized through a central SDN Controller. The control plane still has to perform path computation, signaling, configuration, and verification, so the time is equipment and process dependent.

Optical bypass: When the transponders and optical switching nodes support the requested signal, a service can avoid electronic regeneration or aggregation at an intermediate node. This may reduce ports, power, and latency, but bypass is not guaranteed for every service or route.

Configured modulation: A classic fixed-grid transponder may be designed for one modulation profile, such as DP-QPSK on a 100 Gbit/s product. Other fixed-grid products can be tunable, so the actual limitation is the configured equipment rather than the grid label alone. Planning must respect the reach of that profile.

Unused margin: On a shorter link, a long-haul profile may have more signal margin than required. A fixed configuration may not be able to use that margin to raise the rate or change the spectrum width. This depends on whether the installed transponder supports another mode.

Planning changes: The transponder profile and channel plan are normally selected during engineering and installation. Changing them later can require a new configuration, compatible cards, or field work, depending on the platform.

Adaptive modulation: A Bandwidth-Variable Transponder (BVT) can select among its supported modulation formats and symbol rates. A lower-order format such as BPSK can provide more tolerance to noise on a long path, while a higher-order format such as 64-QAM can use spectrum more efficiently on a short path when the link budget permits.

Rate and reach selection: EON can choose a rate and slot width per path from the modes supported by the BVT and line system. A short link may support a higher rate than a long link, but the rates, slot counts, and reuse of hardware are product-specific. A flex-grid label alone does not guarantee a particular capacity.

Conditional adaptation: If a fiber link changes condition and the platform supports in-service reconfiguration, a controller can request a lower-order modulation or another supported profile. This may preserve the connection at a lower rate. The action requires monitoring, compatible nodes, and an operator policy.