Subsea Cables & Internet Infrastructure

The FEC (forward error correction) encoder takes the bit stream and modifies it so the FEC decoder can detect and correct bit errors due to optical or electronic noise. To accomplish this, FEC adds extra redundant bits to the overhead that the decoder can use to identify and fix bit errors. The redundant bits are a mathematical function of the data payload values. This scheme enables transmission to keep bit errors very low without payload retransmission. It is similar in philosophy to the checksum field in IPV4. The difference is that the IPV4 field is strictly limited to detecting overhead errors and the routers respond to checksum failure by dropping the packet and forcing retransmission. In contrast, FEC is designed to detect and fix payload errors without retransmission.  The next interesting step is the bit to symbol conversion. Coherent optics represents bit patterns using combinations of optical properties like phase, amplitude, and the two polarization modes. So the bit pa...

Lasers produce coherent light. This means the electromagnetic waves have the same frequency, move in the same direction, and their phase repeats in a regular pattern. The practical implication is that the laser light remains in a tight, focused band as opposed to spreading over time. This prevents the signal strength or optical power from rapidly diminishing. However, in telecommunications, coherent optics is more than just laser light. It has two key ingredients, advanced modulation schemes that use combinations of amplitude and phase to create higher bandwidth. The other component are digital processing chips (DSPs). Laser light is subject to nonlinear errors due to chromatic and polar dispersion. The DSPs can detect these errors and recover the original pristine signal. Digital signal processing detects nonlinear errors using mathematical algorithms called forward error correction.