ssibility to extend the distance between the repeaters. By 1988 commercial fiber systems operating up to 1.7 Gbit/s with a repeater distance of 50 km had appeared. In this phase came ITU-T recommendation G.957, which specified transmitting bitrates of up to 2.5 Gbit/s targeted for use with SDH and STM 16. As a useful note, STM-N has a bitrate of exactly N X 155.52 Mbit/s, where 155.52 Mbit/s = 2430 X 64 Kbit/s (i.e. 2430 time slots, each with bitrate of the ITU-T G.711 digital voice, the global standard in digital telephony in the 1980s and 1990s). Phase 3. In this phase the target was wavelength at 1550 nm over single mode fibers, to extend the distance between repeaters in excess of 100 km. Third-generation opti- cal systems became operational at speeds of 2.5 Gbit/s in 1992. These systems were capable of operating at bitrates up to 10 Gbit/s (i.e. to carry STM-64 in SDH). Phase 4. Next came wavelength division multiplexing (WDM) systems. With WDM, optical systems' capacity started to double every six months, which was a kind of revolution at that time. By 1996 there was demonstrated transmission over optical networks of a distance of 1600 km (with the possibility to go over 21 000 km, which is near to half of the perimeter length of the equator), which brought the possibility of using submarine cables. In this phase commercial transatlantic and transpacific optical cable systems were deployed, thus providing terrestrial communication between almost all countries on the planet (which introduced the possibility to decrease delays, compared with satellite communications which travel over longer distances from Earth to the satellites in a given orbit and then backwards). This also influenced the increase of QoS due to lower end-to-end delays on a global scale. Phase 5. This is the current phase of fiber systems' development (in the 2010s and around 2020). The main target in this generation is to further increase transmission capacity over optical cables, reaching speeds of multiple Tbits/s per optical cable (by us