Introduction to ALOHA Protocol
The ALOHA protocol is a simple communication scheme developed in the 1970s at the University of Hawaii to connect computers across the Hawaiian Islands. Named after the Hawaiian greeting "Aloha," it was one of the earliest random access protocols and forms the basis for many modern networking technologies like Ethernet and WiFi.
ALOHA operates on a "transmit now, apologize later" principle where stations send data whenever they have it, without checking if the channel is busy. If a collision occurs, the station waits a random time and retransmits.
How ALOHA Works
- Transmission: Any station with data to send transmits immediately (Pure ALOHA) or at the next time slot boundary (Slotted ALOHA).
- Collision Detection: If two or more stations transmit simultaneously, their signals collide and become corrupted.
- Acknowledgment: The sender waits for an acknowledgment from the receiver. If not received within a timeout period, it assumes a collision occurred.
- Retransmission: After a collision, stations wait a random period before retrying to avoid repeated collisions.
Key Concepts
- Vulnerable Period: The time during which a frame can suffer a collision (2×frame time for Pure ALOHA, 1×frame time for Slotted ALOHA)
- Throughput: The successful transmission rate (Pure ALOHA: 18.4%, Slotted ALOHA: 36.8% at optimal load)
- Backoff Algorithm: After a collision, stations wait a random time before retransmitting
- Channel Load (G): The total offered traffic including retransmissions