Semi-synchronous ALOHA

A multiple-access protocol that operates according to a combination of the pure (unslotted) and slotted ALOHA access protocols is presented. The system is slotted so that the slot length is two packet transmission times, while access can take place asynchronously within the first half of the slot. The feedback information is in terms of time measurements. The capacity of this protocol is 0.5, and it is stable under the condition of infinite resolution of time measurements. The effect of the finite resolution of time measurements is studied. It is shown that the capacity achievable with a practical implementation of this protocol can be very close to the capacity obtained with infinite resolution. The author provides throughput and delay analysis for the case of a finite number of users, each equipped with a single buffer. It is shown that under this protocol, the delay is much higher at light load than at heavy load if the rescheduling rate is very small. However, the throughput also approaches 0.5 with a finite number of users