Gigabit Ethernet
The IEEE 802.3z Gigabit Ethernet standard was completed in 1998 and established an Ethernet LAN that increased the transmission speed over that of Fast Ethernet by a factor of 10. The goal was to define new physical layers but to again retain the frame structure and procedures of the 10 Mbps IEEE 802.3 standard.
The increase in speed by another factor of 10 put a focus on the limitations of the CSMA-CD MAC algorithm. For example, at a 1 Gbps speed, the transmission of a minimum size frame of 64 bytes can result in the transmission being completed before the sending station senses a collision. For this reason, the slot time was extended to 512 bytes. Frames smaller than 512 bytes must be extended with an additional carrier signal, in effect resulting in the same overhead as in padding the frame. In addition, an approach called packet bursting was introduced to address this scaling problem. Stations are allowed to transmit a burst of small packets, in effect to improve the key ratio a. Nevertheless, it is clear that with Gigabit Ethernet the CSMA-CD access control reached the limits of efficient operation. In fact, the standard preserves the Ethernet frame structure but operates primarily in a switched mode.
Gigabit Ethernet physical layer standards have been defined for multimode fiber with maximum length of 550 m, single-mode fiber with maximum length of 5 km, and four-pair category 5 UTP at a maximum length of up to 100 m. Table 6.4 lists the different medium alternatives.
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