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
802.3have 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.
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Ethernet
Ethernet uses a scheme called |
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Carrier Sense Multiple Access |
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with Collision Detection |
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Figure 3 Ethernet Topology (Coax) | ||||||||
control access to the media. A | ||||||||
station listens to the network for a |
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carrier signal which indicates that |
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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.
CTI 2572 Technical Overview Manual | 5 |