The Slotted Aloha Protocol

April 1996 / Reviews / E-Mail Without Wires / The Slotted Aloha Protocol

One of the greatest engineering challenges facing radio modems is keeping the peace on the airwaves. A radio modem one mile east of the central tower might not hear another modem one mile west of the tower. But if they both transmit at the same time, the central tower hears both signals. Worse, radio modems always try to use the minimum power needed to reduce interference.
One solution is called the Slotted Aloha protocol. The central tower pauses every n milliseconds, asks who wants to broadcast, then listens. Each modem waits a random amount of time and asks for a slot. Some signals get through, others collide. The tower determines who won and broadcasts a list of modems and their assigned time slots. The losers try again next time.
With few modems and light usage, polling periods are short and time slots long. As demand increases, the polling period grows so everyone can make their request. The time slots shrink and, with heavy demand, the modems may have to skip rounds.

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Matthew Wilson
My approach to software engineering is far more pragmatic than it is theoretical--and no language better exemplifies this than C++.

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