istance of up to 550 m can be obtained. Operation over single-mode fiber increases the maximum transmission distance to 3000 m. COPPER CABLE STANDARDS In addition to supporting the trans- mission of data over optical fiber, Gigabit Ethernet also supports two specifications for transmission over copper media. Those standards are 1000BASE-CX and 1000BASE-T. 1000BASE-CX The 1000BASE-CX standard is one of two Gigabit Ethernet specifications for transmission over copper cable. The 1000BASE-CX speci- fication covers the use of low-cost copper jumpers operating on high- quality shielded twisted-pair (STP) or coaxial cable. Thus, in all probability CX is a mnemonic for coaxial cable. A transmission distance of up to 25 m is supported by 100BASE-CX. Fiber in the LAN 1000BASE-T The 1000BASE-T specification defines the transmission of Gigabit Ethernet over copper cabling. The actual standard for Gigabit Ethernet is IEEE 802.3ab, which permits a transmission distance of up to 100 m. Table 5.4 summarizes the IEEE 802.3z standard for Gigabit Ether- net over optical media. ADVANTAGE OF 50-um FIBER In examining the entries in Table 5.4, we can note the primary advantage associated with the use of 50-um optical fiber, namely, its higher bandwidth in the shortwave 850-nm operating window. If you look at the modal bandwidth column in Table 5.4, you will note that 50-um fiber has a modal bandwidth approximate- ly 3 times that of 62.5-um fiber. Thus, 50-um fiber permits longer trans- mission distances at higher operating rates than does 62.5-um fiber when an 850-nm laser is used. Actually, 50-um optical fiber represents a relatively old technology, as it was developed during 1976. While the use of 50-um optical fiber was common in Germany and Japan, it was relatively slow to materialize in North America. This occurred partly because IBM adopted the newly developed 62.5-um optical fiber in its cable system in 1986. IBM selected 62.5-um fiber over 50-um fiber as it was considered at that time to be better suited for LED light sources. 