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...

If you have ever heard someone say they bought a quarter or half fibre pair, it means they bought the right to use a slice of laser spectrum to create transport services. Think of spectrum as clay and optical waves as the finished product. The spectrum seller places a filter on a fibre pair that ensures the buyer only gets the purchased frequencies. A quarter fibre pair means they get 25% of the pair's total usable spectrum. Similarly, for a half fibre pair.  In general, the minimum commercial purchase is 560 Gigahertz. The amount of usable bandwidth depends on the modulation or encoding scheme. Complex encoding schemes generate more zeros and ones than simple ones. And they cost proportionally more. Using standard Ciena gear, you can squeeze about 2.5 terabits of usable capacity out of the 560 Ghz allotment. Below is the basic topolgy. Richard Norris of Ciena (2025 Suboptic Spectrum Working Group) created the slides.  Spectrum is like any other service. You can lease or buy i...

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...

Coherent optics is converging to the Shannon limit. The Shannon limit represents the upper bound on information transfer for an optical channel. The gap between Shannon and various modulation schemes is shrinking. Note that the higher the transfer rate, the lower the optical reach. A lower optical reach means more OEM or optical amplification and this is an outcome carriers wish to avoid. These sort of tradeoffs permeate optical transmission. No free lunch. 

Transmission Technology: Spatial Division Multiplexing.  Length: 16,026 kilometers. Almost 10,000 US miles. Consortium Members: Google and Facebook. Type of Consortium: Open cable model.  Construction Status: Behind schedule due to permitting delays for Indonesian waters. Fifty-fifty control probably also slowed decision making.  Number of Fibre Pairs: Main trunk has 12. Estimated RFS: 1st or 2nd quarter 2025. Day One Aggregate Throughput: 144 Tbps.  Salient Features: First low latency, direct cable between Singapore and USA with no intermediate breakouts. One Indonesian branching unit. No telecom carrier consortium members. Amazon and Facebook land the cable themselves in Singapore and California.  Google announced  announced the 12 fibre pair SDM Echo project in early 2021 with a planned 2023 launch. However, permitting delays have slowed construction and the project is now expected to be RFS 2025. In addition, it is highly plausible that the 50-5...