8.7.1 Pure ALOHA
Pure ALOHA is the simplest form of contention-based multiple access. In this scheme, a user transmits immediately whenever it has a packet to send, without regard to the activity of other users. If a collision occurs, the user waits a random time interval before attempting retransmission.
Because transmissions are unslotted and may begin at any instant, a packet is vulnerable to collision during an interval extending one packet time before its start and one packet time after its start. This interval, known as the vulnerable period, has duration 2T, where T is the packet transmission time.
The throughput of pure ALOHA can be expressed as
where G is the offered load measured in packet transmission attempts per packet time. The exponential term represents the probability that no other transmissions occur during the vulnerable period.
This expression shows that throughput initially increases with offered load, reaches a maximum, and then declines as collisions become dominant. Differentiating the expression with respect to shows that maximum throughput occurs at G = 0.5, yielding
Thus, under ideal conditions, pure ALOHA can utilize at most approximately 18.4% of the channel capacity for successful transmissions. Beyond this operating point, additional transmission attempts primarily increase collision probability rather than successful throughput.
Although pure ALOHA is inefficient at high load, it performs reasonably well when traffic is light and sporadic. Its simplicity makes it suitable for systems in which coordination is difficult or where traffic is highly bursty.
The efficiency of ALOHA can be improved by introducing time synchronization, as described in the next section.
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