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 uccess Collision Success
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, node becomes backlogged while backlogged, transmit in each slot with probability qr until successful Infinite nodes where each arriving packet arrives at a new node Equivalent to no buffering at a node(queue size 1) Pessimistic assumption gives a lower bound on Aloha performance
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, node becomes backlogged – while backlogged, transmit in each slot with probability qr until successful • Infinite nodes where each arriving packet arrives at a new node – Equivalent to no buffering at a node (queue size = 1) – Pessimistic assumption gives a lower bound on Aloha performance Eytan Modiano Slide 7