"For example, we have deployed long-haul systems using DPSK technology and specific fiber - dispersion managed fiber - which provides capacity of 1.6 Tbps on one fiber for 9,000 km, but this is not upgradeable to 40G because of physical limitations," Gatutheron told Telecom Asia. "This is because the repeaters are spaced far apart in order to make the wet plant cost-effective."
Gatutheron says that shorter links could be upgraded to 40G with SLTE cards, but even then, it still depends on the link characteristics.
"There are many different types of cables in the water and many different generations," he says. "If you look at old systems with 2.5G WDM, you could upgrade those with SLTE that's not very state-of-the-art, because it's an old system so you can use low-cost simple terrestrial technology on it. What's more difficult is very long systems and newer links. Again, repeater spacing is quite large and variable, it's not like terrestrial where the position of the optical amplifier is fixed."
Another issue - assuming that operators upgrade some but not all of their existing waves to 40G - is the technical challenge in running waves running at different speeds on the same fiber, Gatutheron adds. "When you mix bit rates, you have interaction between wavelengths that causes degradation of the 40G channel, so you have to be careful with what modulation format you use on 40G waves."
Gatutheron also notes that SLTE-only solutions have to do more than just send a 40G wave far enough to work on long subsea links. "It also has to provide a sufficient margin for fiber aging, cable repair and performance situations."
Case by case
Assuming the technological issues can be mitigated, there is still the question of just how much demand there is for subsea 40G, regardless of the internet traffic growth hockey stick.
Some players, like Alcatel-Lucent, say subsea 40G demand will only grow once 40G takes off in the terrestrial networks.
"Terrestrial backhaul is a key driver for 40G in the submarine network," Gatutheron insists. "At the moment there's not very much deployment of 40G in the terrestrial side, so there is not strong demand today. The need for 40G in submarine will come when there's more 40G in terrestrial."
Another factor is that subsea systems in Asia are sitting tons of unlit capacity, according to TeleGeography (see chart page 23), with around 85% of total capacity unused. Pacnet - whose EAC-C2C system accounts for around half of the region's subsea total potential capacity - is planning to light up almost another terabyte later this year, says Barney.
"That's three times the size of any upgrade we've done before, and about 25% of it already sold," he said. But while Pacnet is trialing 40G, it has no concrete plans to adopt it any time soon.
Meanwhile, neither the amount of unlit capacity nor the promise of 40G upgrades at the subsea terminals is stopping carriers from investing in new systems, or even adding links to existing systems - even the ones actively trialing 40G.