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 retransmission of the payload. 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 polarization (there are two modes)...

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. 

 The Coherent Optics Revolution: Transcending 10G Wavelengths - Part 1 The first stage of optical communication was dominated by what Mark Tinka called the simple optical detection scheme. A pulse of laser light represented a '1' and no light meant a '0' or vice versa. Cisco white papers call this 'on-off signalling'. So this approach is based on the optical power or intensity of light. The stronger a light pulse, the higher its amplitude. See the top diagram.  The Achilles of this approach is chromatic dispersion, namely that fact different frequencies of light traverse a solid medium such as fibre glass at different speeds. Now any laser pulse is a band of frequencies. It may be narrow, but it always has non-zero width. So chromatic dispersion is inevitable (like Donald Trump continuously changing tariff rates). As the fibre path distance grows, the probable outcome is that a laser might transmit a '1 0' but the light will spread over time and the opt...