SubOptic 2025 Presentation - Benoit Kowalski - Subsea Network Design Primer

Dr Kowalski is a Nokia employee who collaborates with ASN. His presentation on the first day of the conference is a good subsea cable primer. 

The transmission equipment is known as the submarine line termination equipment or SLTE. It has three key components. Encoding involves taking the digital input and converting it into a series of laser commands according to the modulation scheme. FEC or forward error correction adds redundant bits called parity bits that help the far end FEC to detect and correct payload mistakes. It is a bit like a router creating the checksum field in the IP packet header. Once quality is control is completed, the laser sends the optical signal. Its ever weakening light travels the fibre until it is passively boosted by erbium doped fibre in the amplifier that has been raised to a high energy level by pump lasers. At the far end of the linear transmission the steps are reversed. The optical receiver takes the light signal and converts it into the appropriate digital form. Then forward error correction looks for mistakes using the parity bits and replaces suspect data with its estimate of its true or original value. The standard is no more than one error bit in a billion. 

The most crude form of forward error correction would be to send the same data payload three times. If there are no discrepancies, the data is considered error free. If there are discrepancies, then various algorithms are used to extract the signal from the error noise. For example, if two copies are identical, but the third copy differs, then the two identical copies are considered to be error free.

Diagram Of Subsea Network Architecture



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