ALOHA

ALOHA is a random access MAC protocol: a station transmits whenever it has data; if the frame collides it waits a random time and retransmits.

Pure ALOHA vs Slotted ALOHA

Basis	Pure ALOHA	Slotted ALOHA
Time	Continuous; stations send any time	Divided into slots of frame time $T$; stations send only at slot start
Synchronization	Not needed	Required (common clock)
Vulnerable time	$2T$	$T$
Throughput	$S = G e^{-2G}$	$S = G e^{-G}$
Maximum efficiency	$1/(2e) \approx$ 18.4% at $G=0.5$	$1/e \approx$ 36.8% at $G=1$
Collisions	Partial overlaps possible	Only full overlaps

How Slotted ALOHA improves efficiency

1. Stations may start only at slot boundaries, so frames either collide completely or not at all.
2. This halves the vulnerable period from $2T$ to $T$.
3. Fewer collisions mean fewer retransmissions, so maximum throughput doubles from 18.4% to 36.8%.

The cost is the need for clock synchronization and a slot-wait delay.