Access Methods

Access Methods

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 and 2) take turns. These philosophies are the principles defining the two most important types of media access methods:

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.

As you can imagine, contention-based access methods can give rise to situations in which two or more of the network nodes try to broadcast at the same time and the signals collide. Specifications for contention-based access methods include procedures for how to avoid collisions and what to do if a collision occurs. This section will introduce the CSMA/CD and CSMA/CA access methods.

On most contention-based networks, the nodes are basically equal. No node has a higher priority than other nodes. A new access method called demand priority, however, resolves contention and collisions and in so doing accounts for data type priorities. This section also describes demand priority access.

Further Information