CTI 2572 Technical Overview Manual
5
2.3 Ethernet and IEEE 802.3
Officially, "Ethernet" is the name of a CSMA/CD network developed at Xerox. It was jointly
documented by Digital Equipment Corporation (DEC), Intel, and Xerox in a 1982 specification
for Ethernet Version 2. This specification is commonly called Ethernet II or "Blue Book"
Ethernet for the color of the specification cover. Later, the IEEE developed a specification for a
CSMA/CD local area network based on Ethernet II. This specification, called 802.3, is fully
compatible with Ethernet II at the physical layer (e.g. hardware, connectors, and media).
However, there are some formatting differences in the data link packet (discussed in the next
section).
Although the specifications are not totally compatible, the distinction between "Ethernet" and
802.3 have become blurred. Consequently both are commonly referred to as Ethernet. In this
document, except where specifically noted, the term Ethernet will refer to both the "Blue Book"
(Version 2) and the 802.3 specification.
Ethernet stations are connected in
a bus arrangement, all stations
share a common connection.
When a message is transmitted on
the network, all attached devices
receive it. The Ethernet standard
supports a variety of media,
including coaxial cable, twisted
pair, and fiber optics.
Ethernet uses a scheme called
Carrier Sense Multiple Access
with Collision Detection
(abbreviated CSMA/CD) to
control access to the media. A
station listens to the network for a
carrier signal which indicates that
another station is transmitting. If the signal is detected, the station waits until it detects no
carrier, then transmits. It is possible that when there is no carrier (no one is transmitting) more
than one station can attempt to transmit data at the same time. This condition is referred to as a
collision. Ethernet stations can detect this condition (Collision Detection), wait a random
amount of time, then retry.
Ethernet has a number of important benefits. Since it is widely used, Ethernet adapters are very
inexpensive when compared to other alternatives. It has a very flexible topology and media
selection (coaxial cable tap, twisted pair hub, or fiber optic hub). In addition, it provides
excellent response and throughput at typical utilization levels.
PC
PC
PC
Server
PC
PC
Ethernet
Figure 3 Ethernet Topology (Coax)
