Slotted ALOHA

What
Slotted ALOHA is in the class of random access protocols and allows us to support multiple sending nodes on a single channel.

Assumptions

 * All data frames are the same size
 * Time is divided into equal slots
 * Nodes are synchronized
 * Nodes begin to transmit only at the beginning of slots
 * If multiple nodes transmit, nodes detect collision

Operation

 * When node gets fresh data, transmit in the next slot
 * No collision -> success
 * Collision: Node retransmits at probabililty p

Efficiency
$$Np(1-p)^{N-1}$$. As N is taken to infinity, the max efficiency of this model goes to .37 or about 1/3