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IEEE 802.3z specified
several media options to support different grades of fiber-optic
cable and one version to support a new copper cable implementation.
The fiber-optic options vary according to the size of the fiber and
the modal bandwidth. The table in the graphic summarizes the options
and the distances supported by each.
The 1000BASE-SX Ethernet
format uses the short wavelength of 850 nanometers (nm). Although
this is a laser-based system, the distances supported are generally
shorter than for 1000BASE-LX. This results from the interaction of
the light with the fiber cable at this wavelength. Why use
1000BASE-SX then? Because the components are less expensive than for
1000BASE-LX. Use this least-expensive method for short distances (for
example, within an equipment rack).
In fiber-optic systems,
light sources differ in the type of device (LED or laser) generating
the optical signal and in the wavelength they generate. Wavelength
correlates to the frequency of radio frequency (RF) systems. In the
case of optics, we specify the wavelength rather than the frequency.
In practical terms, this corresponds to the color of the light.
Typical wavelengths are
850 and 1300 nm; 850-nm light is visible to the human eye as red,
whereas 1300 nm is invisible. The 1000BASE-LX Ethernet format uses
1300-nm optical sources. In fact, the L of LX stands for long
wavelength. The 1000BASE-LX Ethernet uses laser sources. Use the LX
option for longer-distance requirements. If you need to use single
mode, you must use LX. Be careful when using fiber-optic systems. Do
not look into the port or the end of a fiber! It can be hazardous to
your eye.
Not included in the
table in the figure to the left is
a copper media option. The 1000BASE-CX Ethernet format uses a 150-ohm
balanced shielded copper cable. This new cable type is not
well-known in the industry, but is necessary to support
high-bandwidth data over copper. The 1000BASE-CX Ethernet format
supports transmissions up to 25 meters. It is intended to be used to
interconnect devices collocated within an equipment rack. This is
appropriate when Catalyst switches are stacked in a rack and you
want a high-speed link between them, but you don't want to spend the
money for fiber-optic interfaces.
Another copper version
is the 1000BASE-T standard, which uses Category 5 twisted-pair cable.
It supports up to 100 meters, and uses all four pairs in the cable.
This offers another low-cost alternative to 1000BASE-SX and
1000BASE-LX and does not depend upon the special cable used with
1000BASE-CX. This standard is under the purview of the IEEE 802.3ab
committee.
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