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7.2 | |
Gigabit and 10-Gigabit Ethernet | |
| 7.2.3 | |
1000BASE-SX and LX |
The IEEE 802.3 standard recommends that
Gigabit Ethernet over fiber be the preferred backbone technology.
The timing, frame format, and
transmission are common to all versions of 1000 Mbps. Two
signal-encoding schemes are defined at the physical layer.
1000BASE-X uses 8B/10B encoding converted to non-return to zero (NRZ) line encoding. NRZ encoding relies on the signal level found in the timing window to determine the binary value for that bit period. Unlike most of the other encoding schemes described, this encoding system is level driven instead of edge driven. That is the determination of whether a bit is a zero or a one is made by the level of the signal rather than when the signal changes levels. The NRZ signals are then pulsed into the fiber using either short-wavelength or long-wavelength light sources. The short-wavelength uses an 850 nm laser or LED source in multimode optical fiber (1000BASE-SX). It is the lower-cost of the options but has shorter distances. The long-wavelength 1310 nm laser source uses either single-mode or multimode optical fiber (1000BASE-LX). Laser sources used with single-mode fiber can achieve distances of up to 5000 meters. Because of the length of time to completely turn the LED or laser on and off each time, the light is pulsed using low and high power. A logic zero is represented by low power, and a logic one by high power. The Media Access Control method treats
the link as point-to-point. Since separate fibers are used for
transmitting (Tx) and receiving (Rx) the connection is inherently full
duplex. Gigabit Ethernet permits only a single repeater between two
stations. Figure
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The 8B/
10B scheme is used for optical fiber and shielded copper media, and
the pulse amplitude modulation 5 (PAM5) is used for UTP.
is a
1000BASE Ethernet media comparison chart.
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