1.5 Gigabit Ethernet

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Ethernet is a highly scalable LAN technology. Long available in two versions—10-Mbps Ethernet and 100-Mbps Fast Ethernet—the next version standardized by the IEEE offer another order of magnitude increase in bandwidth. Offering a raw data rate of 1,000 Mbps or 1 Gbps, the so-called Gigabit Ethernet uses the same frame format and size as previous Ethernet technologies. It also maintains full compatibility with the huge installed base of Ethernet nodes through the use of LAN hubs, switches, and routers.

Gigabit Ethernet supports full-duplex operating modes for switch-to-switch and switch-to-end-station connections and half-duplex operating modes for shared connections using repeaters and the CSMA/CD access method. Figure 1.2 illustrates the functional elements of Gigabit Ethernet.

Figure 1.2: Functional elements of Gigabit Ethernet.

The initial efforts in the standards process drew heavily on the use of Fibre Channel and other high-speed networking components. Fibre Channel encoding/decoding integrated circuits and optical components were readily available and are specified and optimized for high performance at relatively low costs. The first implementations of Gigabit Ethernet employed Fibre Channel’s high-speed, 780-nm (short wavelength) optical components for signaling over optical fiber and 8B/10B encoding/decoding schemes for serialization and deserialization. Fibre Channel technology operating at 1.063 Gbps was enhanced to run at 1.250 Gbps, thus providing the full 1,000-Mbps data rate for Gigabit Ethernet. Link distances—up to 2 km over single-mode fiber and up to 550 meters over 62.5-micron multimode fiber—were specified as well.

In mid-1999, the IEEE and the Gigabit Ethernet Alliance formally ratified the standard for Gigabit Ethernet over copper. The IEEE 802.3ab standard defines Gigabit Ethernet operation over distances of up to 100 meters (330 feet) using four pairs of Category 5 balanced copper cabling. The standard adds a Gigabit Ethernet physical layer to the original 802.3 standard, allowing for the higher speed over the existing base of Category 5 UTP wiring. It also allows for auto-negotiation between 100-Mbps and 1,000-Mbps equipment. Table 1.1 summarizes Gigabit Ethernet standards for various media.

Table 1.1: A Summary of Gigabit Ethernet Standards for Various Media Specification Transmission Facility Purpose Source: IEEE 802.3z Gigabit Task Force.

Specification	Transmission Facility	Purpose
1000BaseLX	Long-wavelength laser transceivers	Support links of up to 550m of multimode fiber or 3,000m of single-mode fiber
1000BaseSX	Short-wavelength laser transceivers operating on multimode fiber	Support links of up to 300m using 62.5-micron multimode fiber or links of up to 550m using 50-micron multimode fiber
1000BaseCX	STP cable spanning no more than 25m	Support links among devices located within a single room or equipment rack
1000BaseT	UTP cable	Support links of up to 100m using four-pair Category 5 UTP

The initial applications for Gigabit Ethernet will be for campuses or buildings requiring greater bandwidth between routers, switches, hubs and repeaters, and servers. Examples include switch-to-router, switch-to-switch, switch-to-server, and repeater-to-switch links.

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