Bandwidth-Variable Transponder in an EON
In a traditional optical network, a transponder maps client data to a prescribed channel in a fixed-grid . It receives client data from an Ethernet switch or router, converts it to an optical signal and sends it on the fiber. In a fixed-grid design, the transponder and channel rate may be fixed, for example 10 or 100 Gbps, even when the requested data rate differs. The unused part of such a channel cannot automatically be reassigned to another connection, because the channel width and transponder profile are fixed by the design.
The Bandwidth-Variable Transponder (BVT) is a configurable transponder for . It can adjust modulation, symbol rate, subcarrier configuration and occupied spectrum within its supported modes. A BVT is one enabling element of a flexible-grid network, which also requires compatible optical switches and control functions. The term bandwidth-variable describes the configurable optical signal and its occupied spectrum. Each profile has limits set by hardware, signal-quality targets and available frequency slots.
Core Capabilities of a BVT
A BVT achieves elasticity through programmable control of several transmission parameters. The selected profile must satisfy the client rate, the available spectrum and the physical-layer limits of the path. The transmitter and receiver must use compatible settings, and the control plane must reserve the same spectrum along the optical path when spectrum conversion is unavailable.
- Adaptive Modulation Format: The transponder can change the of the optical signal. In a simplified comparison, BPSK carries 1 bit per symbol and 64-QAM carries 6 bits per symbol. Higher-order formats usually require higher OSNR for a target error rate, which can reduce reach or margin. The net bit rate is lower than the raw symbol calculation when FEC overhead and implementation margins are included. Polarization multiplexing can also change the capacity of a practical profile.
- Tunable Baud Rate: The Baud Rate, or , determines how many symbols are sent per second. A BVT can adjust this rate. For a fixed pulse-shaping ratio, a higher baud rate increases occupied bandwidth and can provide a higher data rate with the same modulation format.
- Flexible Carrier Count & Spacing: Modern BVTs are often based on . This allows a signal to use multiple closely spaced subcarriers instead of one carrier. The BVT can change the supported number of subcarriers and their spacing to produce a selected spectral width while preserving orthogonality.
Combining these capabilities lets a BVT generate a signal with the spectrum needed for the client data rate and a modulation profile that meets the target quality. Guard bands, FEC and the path budget are included in the configuration.
Relationship Between Data Rate and Reach
A BVT selects the modulation format, symbol rate, and occupied spectrum for the parameters of a provisioned path. Higher-order modulation carries more bits per symbol but usually requires a higher OSNR to reach a given error rate. The profile is a physical-layer trade-off that accounts for the client rate and the path conditions.
Increasing the number of bits per symbol may therefore reduce available reach, but the scale of this change is not fixed. There is no universal halving distance law.
| Modulation Format | Bits Per Symbol | Required OSNR | Typical Effect on Reach |
|---|---|---|---|
| BPSK | 1 | Lowest among the listed formats | Highest among the listed formats |
| QPSK | 2 | Moderate | Usually higher than for higher-order QAM |
| 8-QAM | 3 | Elevated | Depends on link parameters |
| 16-QAM | 4 | High | Usually lower than for QPSK |
| 32-QAM | 5 | Very high | Lower than for 16-QAM |
| 64-QAM | 6 | Very high | Lowest among the listed formats |
The reach of a particular format must be determined from the link budget. Relevant factors include symbol rate, polarization count, FEC, target error rate, slot width, optical power, fiber and amplifier parameters, and the applied DSP. The table expresses qualitative ordering. Exact OSNR thresholds and reach values belong to a defined transceiver implementation and line system.
The BVT in the EON Architecture
In the EON architecture, the BVT is the edge device that adapts client data to the network's flexible optical transport parameters. Its configuration must agree with the slot width and center frequency selected by the control plane, as well as with the receiver's supported demodulation modes.
A connection can be established through the following control sequence:
- A connection request arrives, for example to create a 175 Gbps link between two data centers.
- The control plane performs Routing and Spectrum Assignment (RSA), often together with modulation selection. It chooses a route that satisfies reach, spectrum-continuity and capacity constraints, then selects a supported modulation profile for the path. In standards terminology, the combined routing, modulation and spectrum decision is often called RMSA or RMLSA.
- The controller calculates the number of frequency slots needed for the 175 Gbps signal using the selected profile and guard spectrum.
- Finally, the controller sends configuration parameters to the relevant devices:
- It instructs the source BVT to generate an optical signal with the selected modulation, occupying the calculated slots and centered on the assigned frequency.
- It instructs all the intermediate BV-OXCs along the path to configure their switches for a continuous path through that slot block.
- It instructs the destination BVT to tune its receiver to the assigned frequency and demodulate the selected format.
Once the devices confirm their settings, the BVT begins transmitting and the elastic connection is established. The provisioning time depends on the control-plane implementation and device interfaces, while the same procedure supports dynamic, on-demand allocation of optical spectrum.