Ethernet Overview

Ethernet networks are based on a development made by Xerox, Digital Equipment Corporation, and Intel Corporation. The two versions of Ethernet are commonly referred to as Ethernet I and Ethernet II (or version 2).

Ethernet uses carrier sense multiple access collision detection (CSMA/CD) to transmit frames on the wire. In an Ethernet environment, all hosts can transmit as long as no other devices are transmitting. CSMA/CD is used to detect and warn other devices of any collisions, and colliding stations use a backoff algorithm and wait a random amount of time before trying again. Colliding devices send a jam signal to advise all stations that a collision has occurred. When a jam signal is sent (a jam signal is detected by all devices, because the voltage is that of the combined colliding devices), all stations also stop transmitting. A device attempts to transmit up to 16 times before a user is notified of the collisions; typically, an application error informs the user that data could not be delivered. Microsoft's famous words are "Network is busy."

NOTE The only situation in which CSMA/CD is not used is in a full-duplex connection, because collisions are not possible when one pair of unshielded twisted-pair cable (UTP, the physical cable connection) is used to transmit data (one pair of twisted-pair cable) and receive data (a second pair of twisted-pair cable). In other words, devices connected in full-duplex mode can send and receive data at the same time without the possibility of collision.

Table 1-2 lists some of the common Ethernet media specifications and the characteristics of each.

Table 1-2 Ethernet Media Formats

Media Type

Characteristics

10BASE5*

Maximum length: 500 m Maximum stations: 1024 Speed: 10 Mbps

Minimum distance between devices: 2.5 m

10BASE2

Maximum length: 185 m, using RG58 cable types and T connectors on all end stations

Minimum distance between devices: 0.5 m Maximum devices per 185-m segment: 30 stations Speed: 10 Mbps

Up to 100 m; better-category cables longer

One device per cable; typically, only one device per segment with hubs or switches connecting all devices together

Speed: 10 Mbps

Physical topology: star

Logical topology: bus

100BASE-T

Same characteristics as 10BASE-T but operates faster, at 100 Mbps Can be fiber, as well (100BASE-FX); defined in IEEE 802.3U Physical topology: star Logical topology: bus

1000 GE

Gigabit Ethernet operating at 1000 Mbps

Can run over fiber or UTP; frame formats and CSMA/CD identical to Ethernet standards

Physical topology: star Logical topology: bus

*The word BASE refers to baseband signaling, which uses a single channel, as opposed to broadband, which uses multiple frequency channels.

*The word BASE refers to baseband signaling, which uses a single channel, as opposed to broadband, which uses multiple frequency channels.

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