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    With slotted ALOHA, the best channel utilization that can be achieved is 1/e. This low result is hardly surprising, since with stations transmitting at will, with- out knowing what the other stations are doing there are bound to be many collis- ions.

    In LANs, however, it is often possible for stations to detect what other sta- tions are doing, and thus adapt their behavior accordingly. These networks can achieve a much better utilization than 1/e. In this section, we will discuss some protocols for improving performance.

    Protocols in which stations listen for a carrier (i.e., a transmission) and act accordingly are called carrier sense protocols. A number of them have been proposed, and they were long ago analyzed in detail. For example, see Kleinrock and Tobagi (1975). Below we will look at several versions of carrier sense proto- cols.