*Statistics from Telegeography.
Those are serious questions, but let’s start with a transatlantic cable that most people agree represents the state of the art today – the MAREA cable, which is owned jointly by Telxius, Facebook and Microsoft.
MAREA is so highly regarded because it consists of an optical fibre highly optimised for coherent transmission at high modulation order.
16QAM (quadrature amplitude modulation) is used on at least one of its fibre pairs – a first for production grade transatlantic transmission. In addition, MAREA’s optical amplifiers operate at very high-power levels and are more closely spaced than normal at around 55km.
By using extremely high performance transponders, such as Infinera’s ICE4, the commercial capacity on a single fibre pair on MAREA is at least 24Tbps. Note that up to 26.2Tbps of capacity was achieved in trials of ICE4, but operators will decide the optical budget safety margin required.
MAREA was deployed with eight fibre pairs and it’s interesting to note that each one of these pairs delivers as much capacity as all the operational transatlantic cables in service at the time. Even so, the demand for subsea capacity is so great that we must plan now to meet the needs of the near future.
Why not simply deploy more fibre pairs in such a cable?
The answer is that the amplifier chains on submarine cables must be powered by setting up huge voltages at each end of the cable – positive at one end and negative at the other. Based on the power level used in MAREA’s amps, and the fact that the amps are so closely spaced, there simply isn’t a way to get enough power into the cable to supply more fibre pairs.
When MAREA was designed, the goal was to maximise the capacity per fibre pair – and this was a great success. But for the next generation of transoceanic cables the focus is shifting to maximise the total cable capacity, not necessarily the capacity per fibre pair. So how do we do this?
The limiting factor is total electrical power. One step would be to lower the power level of the amplifiers and increase the spacing. This would mean that conventional modulations like 16QAM would not be able to close the link.
In the past we would need to dial down the modulation to 8QAM or QPSK (quadrature phase shift keying), but modern coherent technology introduces a capability called probabilistic constellation shaping (PCS) that can smoothly optimise the modulation efficiency to the optical budget of the link.
By operating in a lower power regime, a new cable architecture emerges, known as Space Division Multiplexing (SDM). The rationale is that, while there is a small reduction in the capacity per fibre pair with SDM, it’s possible to increase the number of pairs by 50-100 percent and achieve a much higher total cable capacity in future submarine deployments.
Google’s Dunant cable
Google recently announced plans to bring the first such cable into service next year – the Dunant cable between the East coast of the USA and a landing point in Western France. The total capacity for Dunant is forecast to be 250Tbps over 12 fibre pairs, compared to a potential 192Tbps for MAREA’s eight fibre pairs.
Steve Grubb, global optical architect for Facebook, has recently described some of the advances needed to achieve a transatlantic cable capacity of 1Pbps using SDM techniques. So, it does appear that the next wave of subsea cables will operate with an SDM architecture. But what about current generations of subsea cables?
For systems like MAREA, with large area, positive dispersion fibres, there may still be some room to increase capacity – perhaps by 25% to 50% with Infinera’s next generation of optical engines.
Before cables like MAREA were deployed there were tens of “dispersion managed” submarine cables in operation – including a very early transatlantic cable that first went into service in 2003.
Recently Infinera’s ICE4 technology was used to double the existing capacity on that cable with the result that the commercial life will be extended yet again.
Similarly, there are hundreds of small cables that are laid without amplifiers to cover short underwater distances across straits, between islands, or laid in festoon routes along coastlines between cities.
The capacity of one such cable, operated by OTEGLOBE, was also doubled recently using ICE4 technology.
The demand for subsea capacity continues to soar, and we must meet that demand. In the longer term, SDM promises a great way to increase cable capacity for future long-distance installations. Meanwhile, advances in high-performance submarine transponder technology continue to boost the capacity of all types of existing cable.
