If E is significantly greater than the actual number of nodes N, then the actual offered load will be too low and the slots will all be empty. In either case, no messages will be received by anyone. Providing two slots for each value of E thus will yield better than .96, and three slots achieves .993. In either case, once E is close to N, additional probes using larger numbers of slots can determine N to whatever precision is desired, and the master can send messages to the individual unique addresses returned in the successful response messages. Probabilities of successful message transmission, collisions, and empty slots for Slotted Aloha channel access, plotted as a function of the logarithm of the ratio of estimated number of nodes to actual number.
Empty slots occur to the right of the peak, collisions to the left.
Severe power limitations will probably preclude the use of actively powered transmitters over any significant distance, including the display of lights proposed below. In addition, the passive nodes do not require a precise pointing system to achieve this high signal gain. Pister and his students at UCLA have explored the fabrication of such devices.