Slotted aloha Time is divided into"slots"of one packet duration E. g, fixed size packets When a node has a packet to send it waits until the start of the next slot to send it Requires synchronization If no other nodes attempt transmission during that slot, the transmission is successft Otherwise“ collision” Collided packet are retransmitted after a random delay Success Collision Success Eytan Modiano
Slotted Aloha • Time is divided into “slots” of one packet duration – E.g., fixed size packets • When a node has a packet to send, it waits until the start of the next slot to send it – Requires synchronization • If no other nodes attempt transmission during that slot, the transmission is successful – Otherwise “collision” – Collided packet are retransmitted after a random delay 1 2 3 4 5 Success Idle Collision Idle Success Eytan Modiano Slide 6
Slotted Aloha assumptions Poisson external arrivals ● No capture Packets involved in a collision are lost Capture models are also possible Immediate feedback Idle(o), Success(1), Collision(e) If a new packet arrives during a slot, transmit in next slot If a transmission has a collision, it becomes backlogged and retransmitted after a random delay Let n be the number of backlogged nodes Eytan Modiano
Slotted Aloha Assumptions • Poisson external arrivals • No capture – Packets involved in a collision are lost – Capture models are also possible • Immediate feedback – Idle (0) , Success (1), Collision (e) • If a new packet arrives during a slot, transmit in next slot • If a transmission has a collision, it becomes backlogged and retransmitted after a random delay – Let n be the number of backlogged nodes Eytan Modiano Slide 7