The need for medium-access control

In wireless and certain wired networks, multiple users share the same physical medium. Data communication rates in networks can often be improved by using medium-access control (MAC) protocols, whereby multiple users share the medium in a controlled manner such that the adverse effects of their interfering signals is reduced. A general treatment of this topic can be found in Reference [21]. The main reason for improved data rates with medium-access control is that communication in noise typically tends to be at much higher data rates than communication in interference if the data rates are a function of the signal-to-interference-plus-noise ratio (SINR).

Earlier in the book, we introduced multiple-access schemes such as frequency-division-multiple access (FDMA), time-division-multiple access (TDMA), code-division-multiple access (CDMA) and space-division-multiple access (SDMA). Each of these multiple-access schemes attempts to reduce interference by ensuring that multiple links operate in orthogonal or approximately orthogonal spaces, such as by time or frequency division. We did not, however, describe in much detail how the assignments of frequency bands, time slots, or spatial dimensions to users are made.

In cellular telephone networks, the assignments of links to time slots, frequency bands, or codes can be made by the base station, which controls the behavior of the mobile units in its own cell. The network topology (where there is a central control node) and the connection-oriented nature of telephone links where links stay operational for long periods (seconds or minutes) make this an attractive approach.