To understand the role of pure ALOHA and slotted ALOHA, it is important to consider the role of MAC protocols. MAC or Medium Access Control is a mechanism that is used to assign a unique address to a piece of network hardware. This is accomplished by the data link layer to provide each communicating device across a network with a unique identifier. This is essential in the case of both pure ALOHA and slotted ALOHA, since both these algorithms are utilized in instances where each node has random access to the communication link, and the probability of data collision is relatively high.
Slotted ALOHA, as the name suggests, uses slots to detect and manage collisions. When a node receives data, it is emitted to transmit in the next slot. If there is no collision, the node transmits in this slot, but if a collision occurs, the data is transmitted in the subsequent slot. This process continues until the node successfully transmits. The main disadvantage is there is a lot of wasted time through idle slots. However, a single node can transmit at the maximum transmission rate of the channel when no collision occurs. It is most useful when the channel is used for transmission almost 37% of the time it is active.
Pure ALOHA is also an algorithm that does not incorporate any listening, and as soon as a node receives a frame, it can transmit immediately. It is easy to observe that in this mode, the collision rate is increased. This is especially because the amount of data sent by nodes are unlikely to be fixed; hence there is often an overlap between one transmission and the next. The efficiency of transmission in this mode is even lower than in the case of slotted ALOHA. Another assumption in this mode is that when a collision occurs, all nodes will detect the collision.