2.3 Gigabit Ethernet
2.3.4 Gigabit media options

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.