Idle Sense Multiple Access (ISMA) is a centralized access method for WLANs that works according to the collision method
. With this technique, the base station controls carrier sensing and collision determination. For this purpose, the base station then transmits an idle signal as soon as the carrier frequency is occupied. The individual stations use the idle signal for their synchronization and try to access the carrier signal according to a random mechanism. If two or more stations access the carrier signal at the same time, there is a collision, as with CSMA
, which is detected by the base station. The more stations are connected to a base station, the higher the probability of collision.
To prevent this from becoming too high, the ISMA access technique can be combined with a reservation procedure. The procedure is called R-ISMA, where the "R" stands for reservation.