Good discussion point for optical networking interviews:
- Modern DWDM systems can achieve carrier capacities exceeding 1 Tbps per wavelength.
- This is made possible by using higher-order modulation along with advanced coherent optics.
To fully utilize the Super Channel (SCH) capacity, the optical link must be designed based on specific transmission parameters:
Key Parameters:
- Modulation order
- Channel width
- Baud rate (symbol rate)
- FEC (Forward Error Correction) mode
As explained in the diagram, the theory of operation in DWDM is as follows:
The transponder receives client traffic from devices such as switches or routers. It then encapsulates the traffic into ODU (Optical Data Unit) containers, maps them into OTU (Optical Transport Unit) frames, and finally maps the data onto an OCh (Optical Channel) for transmission over the optical network.
The modulation process converts digital data into optical symbols. By using QAM (Quadrature Amplitude Modulation), information is encoded using both the amplitude and phase of the optical signal, allowing more bits to be transmitted with each symbol.
For example, 64-QAM carries 6 bits per symbol (since (2^6 = 64)). When dual polarization (DP) is used, the transmission effectively doubles the symbol-carrying capacity by sending independent data streams on two orthogonal polarizations.
Keep in mind: Increasing the modulation order improves spectral efficiency and increases capacity, but it also reduces transmission reach because higher-order modulation formats require a higher Optical Signal-to-Noise Ratio (OSNR) to maintain the desired performance.
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