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Design considerations change with the length of the system.

  • For systems in which no customer is more than 100 km from either shore Cable Station, no in-line amplification is necessary. This is the simplest design, requiring no power feeding to the underwater cable system and allowing service changes (such as adding new Branches) without system downtime.
  • For systems greater than 100 km in length, but less than 300 km in length, remotely pumped amplifiers (RPA) are used. Traditional designs permit only two RPA in a system. This limits the system length to about 300 km. RPA do not require any DC power as the pump power is supplied as light through additional fibers in the cable, one pumping fiber per transmission pair.

    It increases the cable cost due to the additional fibers required for the pump power and additional terminal equipment required. While a bit more complicated in design, they require no electrical power in the underwater cable. This allows service changes without system downtime as in the first case.
  • For systems greater than 300 km in length, underwater repeaters with locally pumped amplifiers (LPA) are necessary. LPA permit lengths of many thousands of kilometers. These are ussually located every 100 km. These systems require electrical  power to the underwater cable. This allows service changes without system downtime as in the first case.

    The power for this system is provided through the center conductor of the trunk cable. Power Feed Equipment (PFE) in the cable stations provide regulated 1.6 amperes of DC current. A repeater section consisting of 100 km of cable and one repeater will dissipate 250 W. As an example, an 110 km system would operate with about 1.7 kV end-to-end and dissipate about 2.7 kW.

    When the cable station houses both ends of a system, a single grounding field (for safety and reliability) is required. If, however, system availability concerns suggest two separated cable stations, each station will require a grounding field.

    The powering system is designed to maximize safety and reliability.