Primer on Optical Fibre & Subsea Versus Terrestrial Network Architecture

Each fibre strand consists of the core and the cladding. The core is like a one way road for the light to traveland cladding are the guardrails that prevent it from escaping. The refractive index measures how fast light traverses a given medium. It is defined as the ratio of the speed of light in a vacuum to the velocity of light via the medium. The core has a higher refractive index than the cladding. As long as the refractive index differential is large enough, photons that hit the cladding at a critical angle or less will bounce back into the core. Physicists call this bouncing of light internal reflection. So the core/cladding structure is designed to preserve the optical power or amplitude of the wavelengths. It minimizes optical attenuation, the great enemy of optical networking along with chromatic and polar dispersion. 

The bottom middle chart shows that optical attenuation, which reflects opposing forces, reaches a minimum in the C band. The C band is defined as wavelengths in the 1530 nm to 1565 range. Not only is attenuation as a function of distance minimized in this range, but it is also compatible with erbium doped optical amplifiers. This obviates the need for OEO (Optical-electrical-optical) regeneration. Both long haul terrestrial and subsea networks rely on the C band as their work horse spectrum. The key difference is that power and weight limit the number of pairs on submarine cables. It is believed 48 pairs is close to the upper limit for long distance repeated cables. In contrast, there is no upper limit to a terrestrial fibre trunk and fact that labor is 80% of the cost makes high fibre pair trunk counts (144 to thousands) the profit maximizing outcome. The result is that subsea networks are optimized for extremely high throughput per pair because fibre pairs are relatively scarce. This means a lot of expensive and high performance dense wave division multiplexing equipment. This naturally leads to a small oligopoly of equipment vendors that extract high profit margins from their service provider clients. In contrast, on land long haul fibre is so plentiful that instead of the vertical subsea networks architecture, we see flatter networks with fewer wavelengths per fibre pair. The American tech giants run largely flat networks that minimize the use of DWDM versus less costly and easier to manage CWDM.