The original Ethernet was developed as an experimental
coaxial cable network in the 1970s by Xerox Corporation to operate with a data
rate of 3 Mbps using a carrier sense multiple access collision detect (CSMA/CD)
protocol for LANs with sporadic but occasionally heavy traffic requirements.
Success with that project attracted early attention and led to the 1980 joint
development of the 10-Mbps Ethernet Version 1.0 specification by the
three-company consortium: Digital Equipment Corporation, Intel Corporation, and
Xerox Corporation.
The original IEEE 802.3 standard was based on, and was very
similar to, the Ethernet Version 1.0 specification. The draft standard was
approved by the 802.3 working group in 1983 and was subsequently published as an
official standard in 1985 (ANSI/IEEE Std. 802.3-1985). Since then, a number of
supplements to the standard have been defined to take advantage of improvements
in the technologies and to support additional network media and higher data rate
capabilities, plus several new optional network access control features.
Throughout the rest of this chapter, the terms
Ethernet and
802.3 will refer
exclusively to network implementations compatible with the IEEE 802.3 standard.
Ethernet Network Elements
Ethernet LANs consist of network nodes and interconnecting
media. The network nodes fall into two major classes:
� Data
terminal equipment (DTE)�Devices that are either the source or the
destination of data frames. DTEs are typically devices such as PCs,
workstations, file servers, or print servers that, as a group, are all often
referred to as end stations.
�Data
communication equipment (DCE)�Intermediate network devices that receive
and forward frames across the network. DCEs may be either standalone devices
such as repeaters, network switches, and routers, or communications interface
units such as interface cards and modems.
Throughout this chapter, standalone intermediate network
devices will be referred to as either
intermediate nodes
or DCEs. Network
interface cards will be referred to as
NICs.
The current Ethernet media options include two general
types of copper cable: unshielded twisted-pair (UTP) and shielded twisted-pair (STP),
plus several types of optical fiber cable.
Ethernet Network Topologies and Structures
LANs take on many topological configurations, but
regardless of their size or complexity, all will be a combination of only three
basic interconnection structures or network building blocks.
The simplest structure is the point-to-point
interconnection, shown in Figure 7-1. Only two network units are involved, and
the connection may be DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in
point-to-point interconnections is known as a network link. The maximum
allowable length of the link depends on the type of cable and the transmission
method that is used.
Figure 7-1 Example Point-to-Point Interconnection
The original Ethernet networks were implemented with a
coaxial bus structure, as shown in Figure 7-2. Segment lengths were limited to
500 meters, and up to 100 stations could be connected to a single segment.
Individual segments could be interconnected with repeaters, as long as multiple
paths did not exist between any two stations on the network and the number of
DTEs did not exceed 1024. The total path distance between the most-distant pair
of stations was also not allowed to exceed a maximum prescribed value. | 
