Pure ALOHA

In pure ALOHA, a station transmits a frame the moment it has one ready, without checking whether the channel is idle or coordinating with other stations in any way. If two stations' transmissions overlap in time even partially, both frames are destroyed (collision), and each station must wait a random backoff time before retrying.

• Vulnerable time = 2 x Tfr (frame transmission time), since a collision can happen if any other station starts within one frame-time before or after this station's own transmission begins.
• Maximum throughput ≈ 18.4% (at G = 0.5), where G is the offered load.

Example: Station A sends a frame at t=5ms. If Station B starts transmitting anytime between t=4ms and t=6ms (assuming Tfr=1ms), both frames collide and are lost.

Slotted ALOHA

Slotted ALOHA improves on pure ALOHA by dividing time into discrete slots, each exactly equal to one frame transmission time. A station is only permitted to begin transmission at the start of a slot — it must wait if its data becomes ready mid-slot.

• Vulnerable time = 1 x Tfr, since collisions can now only occur if two stations pick the exact same slot (partial overlaps across slot boundaries are eliminated by the synchronization).
• Maximum throughput ≈ 36.8% (at G = 1) — exactly double pure ALOHA's maximum.

Example: With 1ms slots, if Station A transmits in the slot starting at t=5ms, a collision only occurs if Station B also chooses to transmit in that same [5ms, 6ms) slot; B cannot start mid-slot at, say, t=5.5ms.

Key contrast

Aspect	Pure ALOHA	Slotted ALOHA
Transmission timing	Immediate, unsynchronized	Must align to slot boundaries
Vulnerable time	2 x Tfr	1 x Tfr
Max throughput	~18.4%	~36.8%
Needs global clock sync	No	Yes

Halving the vulnerable window by requiring slot-synchronized transmission is what allows slotted ALOHA to double pure ALOHA's maximum achievable throughput, at the cost of needing all stations to share a common time reference.