Subsea Cable Capacity: Where Do We Go Next?
By Geoff Bennett
Putting aside the health impact of COVID-19, the global lockdown has had a dramatic effect on our working lives. Those of us fortunate enough to still be working have had to adapt to working from home, and we are now hearing that this is resulting in increased productivity and lowering costs for businesses at a time when they need all the help they can get.
The lockdown has also had a major impact on internet traffic patterns. According to Telegeography, global internet traffic volumes began to surge in March 2020. In the past 12 months, global bandwidth rose by 35%, compared to 26% growth the previous year. Submarine network traffic has been experiencing a boom for several years, and it begs the questions, how much capacity is left in existing cables, and what technology changes are needed to cope with continuing bandwidth growth?
A Roadmap for Cable Capacity
While there are over 400 submarine cables in operation around the world, they are not all the same type of cable technology. Submarine cables have an operational design life of 25 years, but the transponder technology used on those cables typically has about a three- to four-year technology innovation cycle.
Referring to Figure 1, on the left side we see a dispersion-managed fiber system. This would have been designed in the pre-coherent age with alternative positive- and negative-dispersion fiber lengths to limit the accumulated chromatic dispersion (CD) in the cable. This was a smart decision at a time when CD was considered an undesirable fiber impairment. But modern coherent technology can fully compensate for CD, and the dispersion map for these cables often has low- or even zero-dispersion regions where nonlinear penalties are higher. This limits the ability to achieve higher spectral efficiency using high-order modulation – although it can be mitigated using techniques such as soft-decision forward error correction (SD-FEC) gain sharing.