Editing ALOHAnet (section)

=== Packet Reservation Multiple Access ===

Packet reservation multiple access (PRMA) is an implicit reservation scheme.
Some fixed number of slots form a frame.
After each frame, the satellite broadcasts the status of each slot from the previous frame, which indicates the reservation status of the corresponding slots of the next frame.
All ground stations wishing to transmit compete exactly like slotted ALOHA during any "free slot" of that next frame (i.e., either no one transmitted in that slot of the previous frame, or there was a collision when multiple ground stations transmitted in that slot of the previous frame).
If exactly one ground station happens to transmit during a "free slot", that ground station succeeds in reserving that slot of a frame -- the corresponding slot is implicitly reserved in all future frames.
From then on, the satellite broadcasts that that particular ground station has reserved that slot of the frame, and that ground station can continue transmitting with a guaranteed data rate during that slot of the frame; other ground stations are careful *not* to transmit during that slot of the frame, so there are no collisions during reserved slots.
When a ground station with a reserved slot has nothing to send, it simply stops transmitting, which gives up its reservation; the satellite notices its reserved slot is idle in one frame, and broadcasts that fact, which indicates that that slot will be a "free slot" in the next frame.<ref>
Alex Brand; Hamid Aghvami.
[https://www.google.com/books/edition/Multiple_Access_Protocols_for_Mobile_Com/NWvzGU12vh0C "Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond].
2002.
p. 77.
</ref><ref name="ppgk" >
Syed Imran Patel, Dr. M. Prasad, Dr. Ankur Goyal, Shivkant Kaushik
[https://www.google.com/books/edition/Wireless_Networks_and_Mobile_Computing/OPXJEAAAQBAJ "Wireless Networks and Mobile Computing"].
2021.
p. 33.
</ref>

Maximum channel efficiency for slotted ALOHA is 36%; PRMA improves maximum channel efficiency to 80%.<ref name="ppgk" />