802.3

Protocols

802.3

This IEEE standard defines a carrier-sense, multiple access with collision detection (CSMA/CD) protocol for a bus topology. This is the technology used by Ethernet. A bus topology is depicted in Figure A.2.

Figure A.2: LAN Bus Topology

When a station on a CSMA/CD LAN wants to transmit a data packet onto the LAN, it first listens to see if there is any traffic on the LAN, that is, if anybody else is currently transmitting. If the LAN is available, the station proceeds to broadcast its data packets on to the LAN. All stations attached to the LAN read every packet transmitted on it but they process only the packets addressed to them. If the station detects traffic when it wants to transmit, it waits until the LAN is not busy.

It is possible for two (or more) stations to think the LAN is not busy and both begin transmitting data onto the LAN. This results in a packet collision. The stations running this protocol detect this collision and then reattempt to transmit their data after waiting for a (random) period of time.

There are several variants of the 802.3 standard, each designed for a different transmission media. These include

10Base2 - 10 Mbps over thin coaxial cable
10Base5 - 10 Mbps over thick coaxial cable
10Base-T - 10 Mbps over twisted pair cable
10Base-F - 10 Mbps over fiber optic cable
100Base-T - 100 Mbps over twisted pair cable

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