The Medium Access Control (MAC) sub-layer is closely associated with the physical layer and defines the means by which the physical channel (medium) may be accessed. The three basic modes used in MAC are Round Robin, Reservation and Contention.
Round Robin – With this access, each station in turn is given the time slot to transmit data with the condition of maximum amount of data transmitted or time per turn. The station has to queue again for the next turn if it exceeds its given transmission time. This is to ensure efficiency and a fair opportunity is given to each station in the network. However, not every station has data to transmit; in this case, they will just simply pass the turn to others, consequently causes a considerable amount of overhead in passing the turn from station to station. Most common example is token ring LAN.
Reservation – A station wishes to transmit data reserves future slot for an extended or even an indefinite period.
Contention – This mode has no control is exercised to determine which station has priority to transmit data; all stations compete for the time slot in a way that can be. The advantage is that they are simple to implement and under light to moderate load. The contention mode is most common in local area networking especially the bus topology.