The ALOHA system was constructed to allow
radio communication between machines in scattered places on the
Hawaiian Islands. It is one of five classes of
protocols used on the multiple access (uplink) channel of a communication
satellite. The rest are
FDM,
TDM,
CDMA, and
polling.
Channel efficiency on ALOHA is only 18%. Using slotted ALOHA doubles this but then you have the problem of station synchronization, so all stations know when each time slot begins. The reference station, however, transmits a signal that is used by all ground stations as their time origin. Periodic resynchronization is necessary to the clocks, which run at slightly different rates, in phase.
A standard ALOHA system has one downlink channel and one uplink channel. A second uplink channel could be added to increase utilization. If both uplinks (in the dual uplink scheme) successfully transmit frames, the satellite can buffer a frame and transmit it during a later idle slot. A satellite with two uplink and one downlink channel slotted ALOHA channels can achieve a downlink utilization of 0.736, given an infinite amount of buffer space.