SubOptic 2025 Presentation: Subsea Cable Transmission - Part 2

Part 1 finished by noting that that the CLS in most cable systems has been downgraded to a building housing just the power feed equipment. This reflects the open cable model where each owner has fibre pairs or spectrum as opposed to a cut of the cable't lit capacity. A good example is 2Africa in Senegal. Most of the SLTEs (subsea line termination equipment) are collocated in the carrier neutral ONIX facility. But Facebook decided to put its SLTE in one of three N Plus data centers. 

Another important element is the branching unit, which originally just took the main trunk's fibre pair and allocated them into two small trunks. TAT8, the first fibre optic cable, used a branching unit to land one fibre pair in the UK and the other in France. Branching units have evolved over time and most today have the ability to switch wavelengths and more recently fibre pairs. The advantages include being able to reroute traffic in case one branch fails. The branches are typically buried on the continental shelf where most outages originate. Another advantage is that it enables the cable operator to allocate bandwidth where it is most needed. Stranded capacity is avoided as a result. In many cases the branches may have more fibre pairs than the trunk. Firmina's main trunk extends from South Caroline to Argentina. It has 16 pairs. But the Brazil branch has 24 pairs. So the branching unit incorporate a reconfigurable add-drop optical multiplexer. To be honest, it is clear me what motivates this configuration. Perhaps more fibre pairs for the landing segment allows the use of less expensive DWDM. I don't know. 

The bottom of the diagram mentions 'equalizers'. These are elements of the optical amplifiers. A problem with erbium doped optical amplifiers is that the power gain is not equal across wavelengths or spectrum frequencies. The tilt equalizers ensure that all amplified wavelengths have the power or amplitude. The shape equalizer corrects for subtle distortions that accumulate as the signal is repeatedly amplified. Together the tilt and shape equalizer are called the gain equalizer.

Diagram of the key elements of a subsea network


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