The
Ethernet System
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This chapter provides
a brief tutorial on the Ethernet system. We'll begin with the origins
of Ethernet and the Ethernet standards, and then describe the
essential features of Ethernet operation.
Ethernet is a local
area network (LAN) technology that transmits
information between computers at speeds of 10 and 100 million bits per
second (Mbps). Currently the most widely used version of Ethernet
technology is the 10-Mbps twisted-pair variety.
The 10-Mbps Ethernet
media varieties include the original thick coaxial system, as well as
thin coaxial, twisted-pair, and fiber optic systems. The most recent
Ethernet standard defines the new 100-Mbps Fast Ethernet system which
operates over twisted-pair and fiber optic media.
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Extending Ethernets with Hubs
Ethernet was designed to be easily
expandable to meet the networking needs of a given site. To help
extend Ethernet systems, networking vendors sell devices that provide
multiple Ethernet ports. These devices are known as hubs since they
provide the central portion, or hub, of a media system.
There are two major kinds of hub:
repeater hubs and switching hubs. As we've
seen, each port of a repeater hub links individual Ethernet media
segments together to create a larger network that operates as a single
Ethernet LAN. The total set of segments and repeaters in the Ethernet
LAN must meet the round trip timing specifications. The second kind of
hub provides packet switching, typically based on bridging ports as
described in Chapter 15.
The important thing to know at this
point is that each port of a packet switching hub provides a
connection to an Ethernet media system that operates as a separate
Ethernet LAN. Unlike a repeater hub whose individual ports combine
segments together to create a single large LAN, a switching hub makes
it possible to divide a set of Ethernet media systems into multiple
LANs that are linked together by way of the packet switching
electronics in the hub. The round trip timing rules for each LAN stop
at the switching hub port. This allows you to link a large number of
individual Ethernet LANs together.
A given Ethernet LAN can consist of merely a single cable segment
linking some number of computers, or it may consist of a repeater hub
linking several such media segments together. Whole Ethernet LANs can
themselves be linked together to form extended network systems using
packet switching hubs. While an individual Ethernet LAN may typically
support anywhere from a few up to several dozen computers, the total
system of Ethernet LANs linked with packet switches at a given site
may support many hundreds or thousands of machines.
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