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7.2 | |
Gigabit and 10-Gigabit Ethernet | |
| 7.2.6 | |
10-Gigabit Ethernet architectures |
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As with the development of Gigabit
Ethernet, the increase in speed comes with extra requirements. The
shorter bit time duration because of increased speed requires special
considerations. For 10 GbE transmissions, each data bit duration is
0.1 nanosecond. This means there would be 1,000 GbE data bits in the
same bit time as one data bit in a 10-Mbps Ethernet data stream.
Because of the short duration of the 10 GbE data bit, it is often
difficult to separate a data bit from noise. 10 GbE data transmissions
rely on exact bit timing to separate the data from the effects of
noise on the physical layer. This is the purpose of synchronization. In response to these issues of synchronization, bandwidth, and Signal-to-Noise Ratio, 10-Gigabit Ethernet uses two separate encoding steps. By using codes to represent the user data, transmission is made more efficient. The encoded data provides synchronization, efficient usage of bandwidth, and improved Signal-to-Noise Ratio characteristics. Complex serial bit streams are used for all versions of 10GbE except for 10GBASE-LX4, which uses Wide Wavelength Division Multiplex (WWDM) to multiplex four bit simultaneous bit streams as four wavelengths of light launched into the fiber at one time. Figure
Currently, most 10GbE products are in the form of modules, or line cards, for addition to high-end switches and routers. As the 10GbE technologies evolve, an increasing diversity of signaling components can be expected. As optical technologies evolve, improved transmitters and receivers will be incorporated into these products, taking further advantage of modularity. All 10GbE varieties use optical fiber media. Fiber types include 10µ single-mode Fiber, and 50µ and 62.5µ multimode fibers. A range of fiber attenuation and dispersion characteristics is supported, but they limit operating distances. Even though
support is limited to fiber optic media, some of the maximum cable
lengths are surprisingly short.
As with 10 Mbps, 100 Mbps and 1000 Mbps versions, it is possible to modify some of the architecture rules slightly. Possible architecture adjustments are related to signal loss and distortion along the medium. Due to dispersion of the signal and other issues the light pulse becomes undecipherable beyond certain distances.
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represents
the particular case of using four slightly different wavelength, laser
sources. Upon receipt from the medium, the optical signal stream is demultiplexed into four separate optical signal streams. The four
optical signal streams are then converted back into four electronic
bit streams as they travel in approximately the reverse process back
up through the sublayers to the MAC layer. 
No
repeater is defined for 10-Gigabit Ethernet since half duplex is
explicitly not supported. 
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