Maximising submarine cable performance

SLTE or submarine line terminating equipment is the most important technology in an international cable network like the CS². This is the transmission hardware that plugs into each end of the fibre. It determines the cable’s data throughput or performance.

Terminating equipment is also the part of cable technology that sees the most change over time. Manufacturers make occasional tweaks to the physical cable, repeaters and power supply hardware and they work to improve manufacturing processes, but the real innovative development is in the SLTE.

The evolution of SLTE technology, and more importantly a significant step change in technology, delivers more capacity and that’s ultimately what the business is all about.

In recent times the biggest change in the industry has been a move towards open access cables. Cable manufacturers traditionally supplied an entire end-to-end submarine cable solution, this included using their own SLTE. Today they are often forced to supply just the wet segment. That’s the cable itself along with the repeaters and amplifiers in the water as well as the power feed equipment; everything except the SLTE. These days the SLTE part of the business is often contested.

With an open access cable, the network operator allows equipment from other vendors to install and co-exist on the system. They do this for a number of reasons. One of the biggest drivers is pressure from over-the-top customers or OTTs. These are companies like Google and Amazon who now invest in submarine cable projects. In some cases they buy and operate their own spectrum or complete fibre pairs on the cables.

In the past a network operator would aggregate all the available capacity on a cable and allocate this proportionately to members of the consortium using the cable, who would also agree to and share the cost of, future upgrades as they became available. If the operator owns a private system, like CS² or Southern Cross Cables, the operator invests in upgrading the terminating equipment to boost performance, this would give the operator more capacity to sell. However, if an OTT customer like Google has its own terminating equipment on its own spectrum or fibre pair, it gets to decide when to upgrade capacity and doesn’t need to share the cost or the capacity dividend with other consortium members.

Another force behind the move to open access cables is technical convergence. In the past the submarine cable companies would use proprietary adaptations to traditional transmission hardware to work with and manage their submarine networks. Today specialist transmission hardware companies like Ciena and Nokia offer versions of their everyday line transmission equipment along with minor tweaks making the hardware suitable for SLTE use.

At the moment most submarine cables operate with 100 wavelengths per fibre pair. The key to capacity is how much bandwidth you can squeeze out of each wavelength; in general, the more repeaters the greater the throughput possible. Typically you can push data through cable at 100 Gbps per wavelength, although the CS² will work with 180 to 200 Gbps wavelength technology as this becomes available. It may be possible to get this up to 400 Gbps in the future. Beyond that further improvement is limited by Shannon’s Law.

Shannon’s Law, sometimes called Shannon’s Limit, is a law of physics that says there is a limit to how much data can be transmitted through a channel. Sparing you from the maths, the maximum rate you can push error-free data through a channel is down to bandwidth and the signal to noise ratio of the medium. Equipment companies are coming up against the limit.

Operators get around existing limits, for example, by increasing the number of repeaters, but that increases cost so there’s a need to balance the two.