Digital communications
Digital communications

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Digital communications

4.5.1 Gigabit Ethernet

Gigabit Ethernet specifies four categories:

  • 1000BASE-SX, for transmission over multimode fibre using an LED in the 800 nm window (the wavelength is specified to be between 770 and 860 nm). The specification is for up to 275 m on 62.5/125 mm multimode fibre, or 550 m on 50/125 mm multimode fibre;

  • 1000BASE-LX, for transmission over multimode or single-mode fibre using a laser in the 1300 nm window (specified to be between 1270 and 1355 nm). The distance on both 62.5/125 mm and 50/125 mm multimode fibre is specified as 550 m, and on single-mode fibre as 5 km;

  • 1000BASE-CX specifies transmission on a shielded twisted-pair copper cable up to 25 m;

  • 1000BASE-T specifies transmission on four pairs of unshielded twisted pairs up to 100 m.

In this context 1300 nm is ‘long’ - because it is longer than 800 nm. In other contexts ‘long’ means wavelengths in the 1550 nm window in contrast to the ‘short' 1300 nm window. And you have already met references to S-band, C-band and L-band, where ‘long’ means 1565-1625 nm and ‘short’ means 1460-1530 nm. It just goes to show that you have to be very careful with terms like long and short.

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