Aloha means "Hello". Aloha is a multiple access protocol at the datalink layer and proposes how multiple terminals access the medium without interference or collision. The protocol was developed by Norman Abramson and his colleagues at the University of Hawaii. The protocol allows every system to send a frame if it ready to send. But when a collision occurs the node will wait for a random amount time and then send the frame again. The process continues till the node has sent all the frames. Since the nodes send their frames without sensing the medium there is a high probability for collisions to occur. The maximum success rate or throughput that can be achieved with Aloha protocol is only 18%.
In 1972 Roberts developed a protocol that would increase the capacity of aloha two fold. The Slotted Aloha protocol involves dividing the time interval into discrete slots and each slot interval corresponds to the time period of one frame. This method requires synchronization between the sending nodes to prevent collisions.