EON Network Architecture

From Fixed-Grid WDM to Elastic Optical Networks

A network architecture defines its components, functions and interfaces. Traditional fixed-grid WDM networks used fixed-width frequency channels. Elastic Optical Networks (EON) add a flexible grid in which the frequency slots assigned to a connection can have different widths. The transponder, optical switching nodes and control plane must coordinate that allocation.

EON separates client adaptation, optical switching and control functions. Its three functional groups are adaptive edge transponders, flexible core switching nodes and a control plane that computes or distributes connection parameters. In a flexi-grid, a frequency slot is identified by a nominal center frequency and a width. ITU-T G.694.1 anchors the nominal center frequencies at 193.1 THz, uses 6.25 GHz granularity for them and 12.5 GHz granularity for slot widths, so the allocated spectrum can be sized to the signal profile.

The Pillars of EON Architecture

The EON architecture is typically represented as a layered system in which client data is adapted to a flexible optical transport layer.

1. Edge Elements: Bandwidth-Variable Transponders (BVTs):A BVT interfaces with client equipment such as a high-capacity IP router and maps the client signal to an optical carrier. Within its supported modes, it can adjust modulation, symbol rate, subcarrier configuration and occupied spectrum to the connection requirements.
2. Core Elements: Bandwidth-Variable Optical Cross-Connects (BV-OXCs):A BV-OXC is a switching node in the EON core. It can route an optical channel or a contiguous block of flexible frequency slots between available input and output ports. Unlike a fixed-grid node, its switching bandwidth can follow the allocated spectrum.
3. The Control Plane: SDN, GMPLS and PCE Functions:The control plane receives connection requests and computes or distributes routing, modulation and spectrum parameters. An SDN controller or a GMPLS/PCE implementation may maintain a topology and resource view, then configure BVTs and BV-OXCs. The control architecture can be centralized, distributed or hybrid.

Architecture Components in More Detail

The Bandwidth-Variable Transponder (BVT)

The BVT is the hardware that generates a signal for an allocated flexi-grid channel. Its programmable modes can adjust:

• Modulation Format: It selects a supported format, for example from BPSK to 64-QAM, using the path conditions and target signal quality. A longer path generally requires a more impairment-tolerant, less spectrally efficient format.
• Symbol (Baud) Rate: It can tune the symbol rate, which affects the occupied bandwidth and the achievable data rate.
• Number of Optical Subcarriers: With technologies such as Optical OFDM, it can aggregate a variable number of subcarriers into a super-channel with a selected spectral width.

This allows the BVT to create a signal occupying, for example, 3 or 7 frequency slots, or another number permitted by the transponder and the grid. ITU-T G.694.1 defines a 12.5 GHz granularity for the slot width. The selected width must include the signal spectrum, filtering margin and any required guard band.

The Bandwidth-Variable Optical Cross-Connect (BV-OXC)

The BV-OXC performs flexible switching in the EON core. Its key component is a wavelength-selective switch (WSS), which may use liquid crystal on silicon (LCoS), micro-electromechanical systems (MEMS) or another implementation.

A WSS can take an incoming multi-channel WDM signal and independently route a channel or, in an EON, a contiguous block of frequency slots to an available output port. For example, the BV-OXC can:

• Route a 4-slot channel from input 1 to output 3.
• Simultaneously route a 7-slot channel from input 1 to output 5.
• Allow other channels to pass through unaffected.

This switching of variable-sized spectral blocks lets the EON core follow the spectrum allocated to each connection. If a node cannot convert the spectrum, the assigned slot must satisfy the spectrum-continuity constraint across every link.

Connection Establishment in an EON

Establishing an end-to-end EON connection coordinates the client request, physical-layer profile and spectrum state. The following example uses a 350 Gbps request between a data center in London and a corporate office in Frankfurt. The numerical profile is illustrative, because usable capacity depends on baud rate, FEC, polarization and implementation margins.

1. Step 1: The Request Arrives.The connection request specifies the source (London), destination (Frankfurt) and required bit rate (350 Gbps). It is delivered to the selected control-plane function.
2. Step 2: Path Calculation (Routing).The controller consults its topology and resource state and calculates candidate paths. One possible primary path is London → Amsterdam → Frankfurt with a total length of 950 km, together with a longer backup path.
3. Step 3: Modulation Format Selection.The controller evaluates the path model, including loss, noise and the target BER. The format decision uses the full set of transmission parameters, with distance as only one of them. In this example, its performance data indicates that 8-QAM can meet the required signal quality over the selected path.
4. Step 4: Spectrum Width Calculation.For this calculation, assume the selected transponder profile carries 37.5 Gbps in each 12.5 GHz slot with 8-QAM. This profile is used only for the example calculation. Actual capacity depends on transponder and path parameters. The controller calculates the slots needed for 350 Gbps.

   $N_{slots} = \lceil \frac{350 \text{ Gbps}}{37.5 \text{ Gbps/slot}} \rceil = \lceil 9.33 \rceil = 10 \text{ slots}$

   It then reserves guard spectrum, requiring 11 contiguous frequency slots, or 137.5 GHz of spectrum in this example.
5. Step 5: Spectrum Assignment.The controller searches for 11 contiguous slots that are free on both the London-Amsterdam and Amsterdam-Frankfurt links. Without spectrum conversion, the same slot block must remain continuous along the path. A First-Fit heuristic could select a block centered at 193.5 THz, provided the block also satisfies filtering and guard-band constraints.
6. Step 6: Hardware Configuration.The controller sends the selected parameters to the physical devices:
   • To the London BVT: configure an 8-QAM super-channel centered at 193.5 THz and occupying 11 slots.
   • To the Amsterdam BV-OXC: configure the WSS to switch the 11-slot block from the London port to the Frankfurt port.
   • To the Frankfurt BVT: tune the receiver to 193.5 THz and prepare to demodulate the 11-slot 8-QAM signal.
7. Step 7: Connection Activated.After the devices confirm their settings, the London BVT transmits and the 350 Gbps optical path becomes active.