The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

7.1. Why a MAC?

Why is a MAC algorithm needed at all? Remember, MAC stands for Media Access Control. The purpose of a MAC is to allow multiple stations to decide among themselves which one gets to use the channel at any given time when each has data to transmit. Such a procedure is necessary when multiple stations share a common underlying physical channel and can all offer traffic simultaneously. That is, a MAC is needed only when there is a possibility that two or more devices may wish to use a common communications channel; the MAC algorithm provides a set of rules by which the devices negotiate access to that shared channel.

But if there is no common, shared channel, then there is no need for a MAC algorithm at all! Consider a point-to-point link with separate paths for communicating in each direction (for example, RS-422 over twisted pair, as shown in Figure 7-1).

Figure 7.1. Point-to-point communications

Assuming that the receiver at each end is always enabled, either device can transmit to the other at any time. There is no need for any sort of arbitration for use of the channel because there is never more than one device wishing to transmit on a given wire pair. Note that there are two independent communications channels here, one in each direction between the two stations. Such a channel can simultaneously support communication in both directions, and is referred ...