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Local area networks (LANs) are typically shared by a number of attached systems, and only one system at a time may use the network cable to transmit data. An access method defines how a system gains access to a shared network in a cooperative way so its transmissions do not interfere with the transmissions of other systems. Simultaneous access to the cable is either prevented by using a token-passing method or controlled with a carrier sensing and collision detection method.

The primary access methods listed below. The first is used by Ethernet. The last is used by Token ring networks.

• CSMA/CD (Carrier Sense Multiple Access/Collision Detection)    Carrier sensing implies that network nodes listen for a carrier tone on the cable and send information when other devices are not transmitting. Multiple access means that many devices share the same cable. If two or more devices sense that the network is idle, they will attempt to access it simultaneously (contention), causing collisions. Each station must then back off and wait a certain amount of time before attempting to retransmit. Contention may be reduced by dividing networks with bridges or using switches. See "CSMA/CD (Carrier Sense Multiple Access/Collision Detection)" and "Ethernet" for more information.


• CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)    This access method is a variation on the CSMA/CD method. Nodes estimate when a collision might occur and avoid transmission during that period. This method is cheaper to implement, since collision detection circuitry is not required; however, it imposes more delay and can slow network throughput.


• Token Passing    ARCNET and token ring networks use the token-passing access method. A workstation must have possession of a token before it can begin transmission. The token is passed around the network. Any station that needs to transmit must grab the token. See "Token and Token-Passing Access Methods."

Carrier sensing methods tend to be faster than token-passing methods, but collisions can bog down the network if it is heavily populated with devices. Token ring does not suffer from collision problems, but the current throughput rates on token ring networks are restrictive when compared to new high-speed Ethernet networks.

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