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Cabling LANs / LAN physical layer / Ethernet in the campus

LAN physical layer
5.1.1 This page describes the LAN physical layer.


Various symbols are used to represent media types. Token Ring is represented by a circle. FDDI is represented by two concentric circles and the Ethernet symbol is represented by a straight line. Serial connections are represented by a lightning bolt.

Each computer network can be built with many different media types. The function of media is to carry a flow of information through a LAN. Wireless LANs use the atmosphere, or space, as the medium. Other networking media confine network signals to a wire, cable, or fiber. Networking media are considered Layer 1, or physical layer, components of LANs.

Each type of media has advantages and disadvantages. These are based on the following factors:

• Cable length
• Cost
• Ease of installation
• Susceptibility to interference

Coaxial cable, optical fiber, and space can carry network signals. This module will focus on Category 5 UTP, which includes the Category 5e family of cables.

Many topologies support LANs, as well as many different physical media. Figure shows a subset of physical layer implementations that can be deployed to support Ethernet.

The next page explains how Ethernet is implemented in a campus environment.

Ethernet in the campus
5.1.2 This page will discuss Ethernet.


Ethernet is the most widely used LAN technology. Ethernet was first implemented by the Digital, Intel, and Xerox group (DIX). DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification, released in 1980. IEEE extended 802.3 to three new committees known as 802.3u for Fast Ethernet, 802.3z for Gigabit Ethernet over fiber, and 802.3ab for Gigabit Ethernet over UTP.

A network may require an upgrade to one of the faster Ethernet topologies. Most Ethernet networks support speeds of 10 Mbps and 100 Mbps.

The new generation of multimedia, imaging, and database products can easily overwhelm a network that operates at traditional Ethernet speeds of 10 and 100 Mbps. Network administrators may choose to provide Gigabit Ethernet from the backbone to the end user. Installation costs for new cables and adapters can make this prohibitive.

There are several ways that Ethernet technologies can be used in a campus network:

• An Ethernet speed of 10 Mbps can be used at the user level to provide good performance. Clients or servers that require more bandwidth can use 100-Mbps Ethernet.

• Fast Ethernet is used as the link between user and network devices. It can support the combination of all traffic from each Ethernet segment.

• Fast Ethernet can be used to connect enterprise servers. This will enhance client-server performance across the campus network and help prevent bottlenecks.

• Fast Ethernet or Gigabit Ethernet should be implemented between backbone devices, based on affordability.

The media and connector requirements for an Ethernet implementation are discussed on the next page.

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