ALOHA
/ uh-LOH-hah /
ALOHA was the first random-access protocol, built in the 1970s to connect the Hawaiian islands by radio, and its rule is gloriously simple: when you have a frame to send, just send it. There is no listening before talking and no coordination. If two stations happen to transmit so that their frames overlap in time, they collide and both are garbled; each sender, hearing no acknowledgment, waits a random time and tries again. Simple to build, and the conceptual ancestor of Ethernet and Wi-Fi.
The cost of that simplicity is collisions, and the original (pure) ALOHA is fragile because a frame can collide with any other frame that starts at any time during a window twice as long as one frame's transmission. The fix was slotted ALOHA: divide time into slots exactly one frame long and require every station to begin transmitting only at the start of a slot. Now two frames either miss each other entirely or overlap completely; partial overlaps vanish. That single rule roughly doubles the best-case efficiency.
The honest numbers tell the story. Pure ALOHA tops out at about 18% channel utilization (its maximum throughput is 1/(2e) of the channel); slotted ALOHA roughly doubles that to about 37% (1/e). Both fall apart under heavy load as collisions snowball, and both leave most of the channel unused even at their best. ALOHA matters today less as a protocol you would deploy and more as the origin of a powerful idea — let nodes transmit freely and recover from collisions — that, refined with carrier sensing and collision detection, became the basis of the networks we actually use.
Two stations in pure ALOHA each fire a frame at slightly different times; the tail of one overlaps the head of the other. Both are corrupted, both get no ACK, and both back off for a random interval before retrying — hopefully not at the same moment again.
Send-whenever simplicity, paid for in collisions and low utilization.
Pure ALOHA peaks near 18% utilization and slotted ALOHA near 37% — both far from full, and both degrade under load. ALOHA's lasting value is the idea (free transmission plus collision recovery), not the efficiency.