Ethernet Overview

Ethernet networks are based on a development made by Xerox, Digital, and Intel. 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 will use a back off 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 will attempt to transmit up to 16 times before a user is notified of the collisions; typically, an application error will inform the user that data could not be delivered. Microsoft's famous words are "Network is busy."

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

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

Table 2-2 Ethernet Media Formats

Media Type

Characteristics

10Base5

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

Minimum distance between devices is 2.5 m

10Base2

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

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

Physical topology star, logical topology bus

100BaseT

Same characteristics as 10BaseT but operates faster, at 100 Mbps Can be fibre, as well (100BaseFx); defined in IEEE 802.3U Physical topology star, logical topology bus

1000 GE

Gigabit Ethernet operating at 1000 Mbps

Can run over fibre 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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