ALOHA ALOHA protocol is developed for packet radio networks, but applicable to any shared transmission medium a number of stations share the transmission medium two or more simultaneous transmissions will cause a collision Sender When station has frame it sends Station listens for an amount of time If its hears an aCK, fine. If not, it retransmits the frame after a random time If no ACK after several transmissions, it gives up Frame check sequence can be used for error detection Receiver If frame is ok and address matches receiver, sends acK Otherwise, ignores this frame and does nothing Frame may be damaged by noise or by another station transmitting at the same time(collision). Overlap of frames also causes collision ALOHA is simple, but very inefficient Assuming random traffic, the maximum channel utilization is only about 18%4 ALOHA • ALOHA protocol is developed for packet radio networks, but applicable to any shared transmission medium. — A number of stations share the transmission medium. Two or more simultaneous transmissions will cause a collision. • Sender — When station has frame, it sends — Station listens for an amount of time — If its hears an ACK, fine. If not, it retransmits the frame after a random time — If no ACK after several transmissions, it gives up — Frame check sequence can be used for error detection • Receiver — If frame is OK and address matches receiver, sends ACK — Otherwise, ignores this frame and does nothing • Frame may be damaged by noise or by another station transmitting at the same time (collision). Overlap of frames also causes collision. • ALOHA is simple, but very inefficient — Assuming random traffic, the maximum channel utilization is only about 18%