Subsea Cables & Internet Infrastructure

Picture below shows subsea amplfiers aboard a cable ship. They cost roughly $250K a piece, can support up to 40 Tbps, and are spaced every 70 kilometers or 42 US miles. These remarks apply to these specific amplifiers. As noted in Part 1 of this series, erbium doped fibre eliminated optical-electrical-optical (OEO) conversions for amplifiers. So the limiting factor of computer hardware involved in the conversions was eliminated. The result was a 'transparent' technology that placed no limits on submarine termination equipment upgrades. Subsea cables became highly scable because the amplifiers could easily handle higher throughput achieved via DWDM improvements. These amplifiers are tough buggers. They must withstand severe pressure at depths as great 8,000 meters for 25 years as well as nasty seawater corrosion. The amplifier hull is generally made of titanium or a special beryllium copper alloy, C17200, which conducts heat and electricity very well and is highly resistant to ...

TAT-8 was the first Atlantic optical cable. It was RFS in 1988 and signified a bandwidth revolution with its capacity 10x that of its coaxial predecessors. By modern day standards it was a pygmy with 280 megabits total throughput. But it heralded the beginning of a long period of rapid throughput growth. Optical repeaters were spaced every 50 kilometers or 30 US miles. Photo diodes received the weak incoming signals, converted them into a digital representation of zeros and ones, and then laser diodes generated fresh light signals. A copper conductor provided power. Lots of redundancy in terms of components were built into this amplifiers to avoid repairs. However, this approach had a severe drawback, namely the throughput could not be increased in a time where Moore's law was rapidly increasing transmission rates. The pace of the optical-electrical-optical conversion was set in stone because an upgrade would require replacing all the hardware on all the amplifiers. TAT-8's ca...

Long haul fibre optic bandwidth ranges from a few terabits per second into the low thirties with the equipment and operating expense sharply rising as transmission rates go up. Repeatered subsea cables generally lie in the 12 to 25 Tbps window with most spatial division multiplexing deployments pushing 12 to 20 Tbps whereas the traditional 6 to 8 pair coherent optics deployments transmit at least 20 Tbps or higher per strand.  The key factor determining the optical transmission rate is attenuation, which refers to the fact that a photon or wavelength's intensity or energy diminishes as it travels through fibre optic glass or any other medium. Light is scattered, reflected backwards or absorbed. Other variables that affect transmission rates include the number of distinct wavelength bands (dense wave division multiplexing) that can serve as distinct optical channels in a given spectrum range (usually the C band). The more channels, the higher the transmission rate. Chromatic dispers...