Access Methods (Contention & Token Passing)
Background of Access Method
An access method is a set of rules governing how the network nodes share the transmission medium. The rules for sharing among computers are similar to the rules for sharing among humans in that they both boil down to a pair of fundamental philosophies:
1. First Come, First Serve.
2. Take Turns
These philosophies are the principles defining the two most important types of media access:
Contention:
In its purest form, contention means that the computers are contending for use of the transmission medium. Any computer in the network can transmit at any time (first come, first serve).
Token Passing:
The computers take turns using the transmission medium.
Contention (CSMA/CD & CSMA/CA)
In pure contention-based access control, any computer can transmit at any time. This system breaks down when two computers attempt to transmit at the same time, in which case a collision occurs. Eventually, when a network gets busy enough, most attempts to transmit result in collisions and little effective communication can take place.
Mechanisms, therefore, usually are put into place to minimize the effects of collisions. One mechanism is Carrier Sensing, whereby each computer listens to the network before attempting to transmit. If the network is busy, the computer refrains from transmitting until the network quiets down. This simple "listen before talking" strategy can significantly reduce collisions.
Another mechanism is Carrier Detection. With this strategy, computers continue to listen to the network as they transmit. If a computer detects another signal that interferes with the signal it's sending, it stops transmitting. Both computers then wait a random amount of time and attempt to retransmit. Unless the network is extremely busy, carrier detection along with carrier sensing can manage a large volume of transmissions.
Carrier detection and carrier sensing used together form the protocol used in all types of Ethernet: Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD limits the size of the network to 2,500 meters. At longer distances, the broadcast-sensing mechanisms don't work -- a node at one end can't sense when a node at the other end starts to broadcast.
Apple's LocalTalk network uses the protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Collision avoidance uses additional techniques to further reduce the likelihood of collisions. In CSMA/CA, each computer signals a warning that says it is about to transmit data, and then the other computers wait for the braodcast. CSMA/CD adds an extra layer of order, thereby reducing collisions, but the warning broadcasts increase network traffic, and the task of constantly listening for warnings increases system load.
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