SubOptic 2025 Presentation: Subsea Cable Transmission - Part 1

Benoit Kowalski gave the presentation at the event. The diagram below shows the standard subsea network architecture. The fibre optic cable and optical amplifiers are collectively called the wet segment. The rule of thumb is to bury the cable in waters a thousand meters or less deep. Approaching shore one has a choice. One can give bring the cable ashore using small boats. The cable initially lies exposed on the beach up to the beach manhole. Then the cable is buried from some point in the water up to the manhole where it is spliced into the front haul fibre that carries the signal to the cable landing station. From the CLS back haul fibre goes to a carrier neutral data center that serves as a point of presence (POP) or interconnection point. The other platinum-plated approach uses horizontal drilling to install a bore pipe from the manhole to a point on the sea floor offshore. This is much more expensive, but better protects the cable. 

A couple of things to note. In the beginning was the PTT, which was the only telecom network providing service in a country. Since the wet segment needs high voltage power, it made sense to build a facility called a cable landing station where the power feed equipment was located. And naturally the submarine line termination equipment with the lasers and DWDM gear also would be installed there. When PTT consortiums arose, the corporate culture of centralized control led to a model where the consortium owned the wet and dry segments. Common ownership meant that each consortium member got a percentage of lit capacity based on their total economic contribution to the project, which included invested funds, landing rights, cable landing stations, etc.

Over time this monolithic architecture or vertical integration has unraveled. Some consortium members wanted their own fibre pairs so a subset of the consortium might hold shares of lit capacity and other members might own fibre pairs that they managed themselves including the SLTEs. An early example of this is the ASE cable where Telekom Malaysia took two fibre pairs whereas the rest of the consortium had shares of the lit capacity on the other fibre pairs.

The American Tech Giant completed this unbundling of network elements. They wished to minimize their cost per bit. This required big capacity subsea cables. Often in excess of their needs. So the model arose where the OTTs built very high capacity systems based on the new technology of spatial division multiplexing to exploit all economies of scale. Then they sold the excess capacity to third parties, typically competitive wholesale providers and also PTTs. Capacity sales meant IRUs on either spectrum or fibre pairs. And since the OTTs have very different network architectures from the carriers, it was best to allow everyone to select their own SLTEs or DWDM gear and where it should be installed.

Diagram of Fibre Optic Subsea Cable Network Architecture


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