8.7.2 Slotted ALOHA
Slotted ALOHA improves upon pure ALOHA by introducing time synchronization. The channel is divided into discrete time slots, each equal in duration to one packet transmission time. Users are permitted to begin transmission only at the start of a slot.
By constraining transmissions to slot boundaries, the vulnerable period is reduced. A packet is now vulnerable only during its own slot, rather than during an interval extending before and after its transmission start. As a result, the vulnerable period is reduced from 2T to T.
The throughput expression for slotted ALOHA becomes:
where G again represents the offered load in packet attempts per slot.
This expression shows that maximum throughput occurs at G = 1, yielding
Thus, the maximum theoretical efficiency of slotted ALOHA is approximately 36.8%, which is exactly twice that of pure ALOHA. The improvement arises solely from reducing the vulnerable period through time alignment.
Slotted ALOHA therefore represents a hybrid between completely uncoordinated access and fully deterministic time division. Although users are not assigned dedicated slots, they are synchronized to a common slot structure. Collisions remain possible when multiple users select the same slot, but the probability of collision is reduced compared with the unslotted case.
As traffic load increases beyond the optimal operating point, throughput again declines because most slots contain either multiple transmissions (collisions) or no transmissions at all. Effective performance therefore depends on keeping the offered load within a moderate range.
While slotted ALOHA improves efficiency, it still suffers from substantial collision probability under moderate to heavy load. More sophisticated contention-based techniques attempt to further reduce collisions by sensing channel activity before transmitting. These methods are examined next.
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