25-05-2012, 02:30 PM
Gigabit Ethernet
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Introduction to Gigabit Ethernet
Since its inception at Xerox Corporation in the early 1970s, Ethernet has been the dominant networking protocol. Of all current networking
protocols, Ethernet has, by far, the highest number of installed ports and provides the greatest cost performance relative to Token Ring, Fiber
Distributed Data Interface (FDDI), and Asynchronous Transfer Mode (ATM) for desktop connectivity. Fast Ethernet, which increased
Ethernet speed from 10 to 100 megabits per second (Mbps), provided a simple, cost-effective option for backbone and server connectivity.
Gigabit Ethernet builds on top of the Ethernet protocol, but increases speed tenfold over Fast Ethernet to 1000 Mbps, or 1 gigabit per
second (Gbps). This protocol, which was standardized in June 1998, promises to be a dominant player in high-speed local area network
backbones and server connectivity. Since Gigabit Ethernet significantly leverages on Ethernet, customers will be able to leverage their existing
knowledge base to manage and maintain gigabit networks.
The purpose of this technology brief is to provide a technical overview of Gigabit Ethernet. This paper discusses:
• The architecture of the Gigabit Ethernet protocol, including physical interfaces, 802.3x flow control, and media connectivity options
• The Gigabit Ethernet standards effort and the timing for Gigabit Ethernet
• Comparison of Gigabit Ethernet and ATM technologies
• Gigabit Ethernet topologies
• Migration strategies to Gigabit Ethernet
Gigabit Ethernet Protocol Architecture
In order to accelerate speeds from 100 Mbps Fast Ethernet up to 1 Gbps, several changes need to be made to the physical interface. It has been
decided that Gigabit Ethernet will look identical to Ethernet from the data link layer upward. The challenges involved in accelerating to 1
Gbps have been resolved by merging two technologies together: IEEE 802.3 Ethernet and ANSI X3T11 Fibre Channel. Figure 1 shows how
key components from each technology have been leveraged to form Gigabit Ethernet.
Gigabit Ethernet Interface Converter
The Gigabit interface converter (GBIC) allows network managers to configure each gigabit port on a port-by-port basis for short wavelength
(SX), long wavelength (LX), and long-haul (LH) interfaces. LH GBICs extended the single-mode fiber distance from the standard 5 km to 10
km. Cisco views LH as a value add, although it’s not part of the 802.3z standard, allowing switch vendors to build a single physical switch or
switch module that the customer can configure for the required laser/fiber topology. As stated earlier, Gigabit Ethernet initially supports three
key media: short-wave laser, long-wave laser, and short copper. In addition, fiber-optic cable comes in three types: multimode (62.5 um),
multimode (50 um), and single mode. A diagram for the GBIC
The Fibre Channel physical medium dependent (PMD) specification currently allows for 1.062-gigabaud signaling in full duplex. Gigabit
Ethernet will increase this signaling rate to 1.25 Gbps. The 8B/10B encoding (to be discussed later) allows a data transmission rate of 1000
Mbps. The current connector type for Fibre Channel, and therefore for Gigabit Ethernet, is the SC connector for both single-mode and
multimode fiber. The Gigabit Ethernet specification calls for media support for multimode fiber-optic cable, single-mode fiber-optic cable,
and a special balanced shielded 150-ohm copper cable.