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Ethernet

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A previous article introduced Ethernet and briefly touched on the range of data rates that have been standardized. The classical 10 Mbps is of limited use in today's high-volume environment, and market attention is focused on 100 Mbps, 1 Gbps, and 10 Gbps products. This article provides an overview of the standards in these areas.

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A previous article introduced Ethernet and briefly touched on the range of data rates that have been standardized. The classical 10 Mbps is of limited use in today's high-volume environment, and market attention is focused on 100 Mbps, 1 Gbps, and 10 Gbps products. This article provides an overview of the standards in these areas.

Fast Ethernet

Fast Ethernet refers to a set of specifications developed by the IEEE 802.3 committee to provide a low-cost, Ethernet-compatible LAN operating at 100 Mbps. The blanket designation for these standards is 100BASE-T. The committee defined a number of alternatives to be used with different transmission media, each of which has its own designation:

  • 100BASE-X refers to a set of options that use two physical links between nodes—one for transmission and one for reception.

  • 100BASE-TX makes use of shielded twisted pair (STP) or high-quality (Category 5) unshielded twisted pair (UTP).

  • 100BASE-FX uses optical fiber.

For all of these schemes, the distance involved between hubs and stations is on the order of a maximum of 100 to 200 meters (m).

In many buildings, any of the 100BASE-X options requires the installation of new cable. To minimize costs for buildings that don't have the required cable in place, 100BASE-T4 defines a lower-cost alternative that can use Category-3, voice-grade UTP in addition to the higher-quality Category 5 UTP. To achieve the 100 Mbps data rate over lower-quality cable, 100BASE-T4 dictates the use of four twisted pair lines between nodes, with the data transmission making use of three pairs in one direction at a time.

100BASE-X

For all of the transmission media specified under 100BASE-X, a unidirectional data rate of 100 Mbps is achieved when transmitting over a single link (single twisted pair, single optical fiber). For all these media, an efficient and effective signal encoding scheme is required. The one chosen is referred to as 4B/5B-NRZI. This encoding technique is more efficient than the Manchester technique used for 10 Mbps Ethernet, and is therefore desirable at the higher data rate.

The 100BASE-X designation includes two physical medium specifications: one for twisted pair, known as 100BASE-TX, and one for optical fiber, known as 100BASE-FX. 100BASE-TX makes use of two pairs of twisted-pair cable, one pair used for transmission and one for reception. Both STP and Category 5 UTP are allowed. 100BASE-FX makes use of two optical fiber cables, one for transmission and one for reception.

100BASE-T4

100BASE-T4 is designed to produce a 100 Mbps data rate over lower-quality Category 3 cable, thus taking advantage of the large installed base of Category 3 cable in office buildings. The specification also indicates that the use of Category 5 cable is optional.

For 100BASE-T4 using voice-grade Category 3 cable, it's not reasonable to expect to achieve 100 Mbps on a single twisted pair. Instead, 100BASE-T4 specifies that the data stream to be transmitted is split up into three separate data streams, each with an effective data rate of 33 1/3 Mbps. Four twisted pairs are used. Data is transmitted using three pairs and received using three pairs. Thus, two of the pairs must be configured for bi-directional transmission.

Mixed Configuration

One of the strengths of the Fast Ethernet approach is that it readily supports a mixture of existing 10 Mbps LANs and newer 100 Mbps LANs. For example, the 100 Mbps technology can used as a backbone LAN, with many of the stations attached to 10 Mbps hubs. These hubs are in turn connected to switching hubs that conform to 100BASE-T and that can support both 10 Mbps and 100 Mbps links. Additional high-capacity workstations and servers attach directly to these 10/100 switches. These mixed-capacity switches are in turn connected to 100 Mbps hubs using 100 Mbps links. The 100 Mbps hubs provide a building backbone and are also connected to a router that provides connection to an outside WAN.

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