Terrestrial Extension for Un-Repeatered Submarine Systems

The Issue: High Cost of Submarine Backhaul

Submarine systems terminate in landing stations which are usually as close as possible to the beach where the submarine cable comes ashore. For un-repeatered systems, the driver for this has been to minimise the distance travelled on the cable to maximise the number of wavelengths it can transmit. However, as with repeatered submarine cables, once the wavelengths are terminated at the landing station they still have to be transported to the network operators Point of Presence (POP) which is normally in a city quite far from the landing station.

Once the submarine system is terminated in the landing station, each 10Gb/s wave on the cable must be fed into a terrestrial DWDM system. This requires a terrestrial terminal at each end and, more importantly, a pair of terrestrial transponders for every wave transported (figure 1). Therefore 4 transponders are required per wavelength transmitted. This can add significantly to the cost of the system.

Figure 1: Terrestrial extension at the end of a long unrepeatered segment

The Xtera Solution: Terrestrial extension of unrepeatered submarine with XLA

Xtera’s Nu-Wave XLS submarine transmission system uses advanced Raman technology to enable an operator to transmit up to 500km (depending on the cable loss and number of wavelengths transmitted) without any repeaters. Any time the total distance between the operators POPs (including submarine and terrestrial links) is with-in the extended reach made possible with Nu-Wave XLS, the landing station can be replaced with a simple access point near the beach (Beach Man Hole) using no active equipment (figure 2). This not only saves the cost of a building at the landing station but each wavelength can be transmitted between POPs using only two transponders vs the four required in a current (or typical) design.

However, in some cases the distance between POPs is too long or the submarine cable may be of older vintage with a cable whose loss that is too high or not optimized for DWDM transmission. In other instances, the operator might like to trade-off total distance achieved for higher capacity on each fiber. For these applications, Xtera has designed a specific configuration of the Nu-Wave XLS, the XLA. The XLA takes advantage of Raman amplification's improved noise performance and ability to provide distributed gain along a fiber. In its simplest form, the XLA fits in half a rack placed near where the cable lands on the beach. This equipment amplifies all of the wavelengths on the fiber further extending the range of the system. This often eliminates the need for a terrestrial DWDM system, making it possible to transmit from POP to POP using only 2 (vs 4) transponders (figure 3).

Figure 2: Back-to-back transponders eliminated

Figure 3: XLA

Product application

In a typical application, the XLA would be deployed in a small site (or repeater cabinet) near the beach where the submarine cable reaches the shore. A full landing station is not required because of the small size and power requirements of the XLA and because unrepeatered cables do not require complex and cumbersome power feed equipment.

Once installed, the XLA will use Raman pumping in both directions (towards the POP and towards the water) to simultaneously amplify all of the wavelengths on the cable. This makes it possible to transport large numbers of wavelengths over much higher losses than previously possible.

There are many scenarios where XLA can provide a cost effective solution for challenging applications requiring extended capacity or reach across long unrepeated sections. These include:

• New installations or upgrades.
  Many new (or upgradable) un-repeatered submarine links have long terrestrial backhauls. Where the reach or capacity of the Nu-Wave XLS is exceeded, the introduction of an XLA can provide the lowest cost solution to provide high capacity between POPs. In addition to submarine applications, it is also possible to use Nu-Wave XLS with XLA on terrestrial routes to bridge very long distances between POPs while limiting the number of in-line amplification sites, for example across deserts.
• Linking un-repeatered submarine segments.
  Many submarine systems go past islands or run parallel to coasts. If no traffic is terminating, there is no need to stop at an island or terminate on the coast, and normal practice would be to install repeatered submarine systems between the two end points. However, the long reach of the Nu-Wave XLS system combined with the range and capacity extension provided by the XLA make it possible to place the XLA on land between two unrepeatered submarine segments (figure 4). This often provides a much lower cost solution than a classic repeatered submarine system.

Figure 4: XLA to link un-repeatered submarine systems