Description of FLAG
The FLAG Cable System was launched in September 1997. FLAG offers a speed of 10 Gbit/s and uses synchronous digital hierarchy technology. It carries over 120,000 voice channels via 27,000 km of mostly undersea cable. FLAG uses erbium-doped fiber amplifiers and was jointly supplied by AT&T Submarine Systems and KDD-Submarine Cable Systems. With respect to design, development, installation and service FLAG conforms to all ISO 9000 quality standards. FLAG provides a link between the European end of high-density transatlantic crossings and the Asian end of the transpacific crossings.
FLAG consists of several undersea cable segments and two terrestrial crossings. The segments can be either direct point-to-point links or multipoint links which are attained through branching units. At each landing point a FLAG cable station is located. The total route length exceeds 27,000 km and comprises 1020 km of terrestrial crossings. Approximately 6600 km of the submerged cable is buried 1 m below the sea bed. Cable burial was performed by either utilizing a submersible plough as the cable was laid or jetting the laid cable into the sea bed via remotely operated vehicles (ROVs).
Over several years the route has evolved as new branches and feeder systems were considered and realized. FLAG includes two terrestrial crossings, one in Egypt and the other in Thailand. Each of these land crossings is totally duplicated on fully different routes. As a result, any fault within one route will cause automatic protection switching to the other route within a time period of less than 50 ms.
Like other global undersea networks, FLAG uses erbium-doped fibre amplifiers. EDFAs boost the optical signals instead of the optical/electrical conversion which is generally used in regenerative technology. These amplifiers use short, gain-specific lengths of fibre which are doped with erbium ions and spliced in-line with the transmission fibre. The signal power is amplified by pumping the erbium-doped fibre (EDF) with 1480 nm laser light which is attached through an optical coupler. The majority of the repeater components are passive. These include EDF, fused-fibre optical couplers and optical isolators. Active components include laser pump assemblies and associated controls. The total number of components within the repeater is lesser than that of regenerative systems.
The FLAG terrestrial crossings do not contain repeaters for reliability reasons. The terminal stations in land crossings use optical amplifiers, high performance transmitter/receivers and forward error correction to cross the large distances without repeaters. Amplification at the terminal output provides output signal power as high as 17 dBm, and optical amplification at the receiver improves the receiver sensitivity as high as 8 dBm.
The route between Alexandria and Cairo is 223 km long and hence requires remote pumping in order to meet FLAG performance requirements. Remotely pumped amplifiers can be regarded as repeaters without active modules. This technology comprises short lengths of EDF spliced into the land cable. The erbium-doped sections are situated within the cable span and are pumped by 1480 nm pump lasers which are based at the station.
Read more about this topic: Fiber-Optic Link Around The Globe
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