Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? Explain your answer.

Question

Accepted Answer

**Answer: pure ALOHA has the lower delay at low load.**

### Reasoning

**Pure ALOHA:** a station transmits **immediately** when it has a frame. At low load collisions are very rare (the probability of a collision is about 1 − e^(−2G), which is close to 0 when G is small). So the frame is nearly always delivered on the first attempt, and its delay is just the transmission time (plus propagation time).

**Slotted ALOHA:** a station must **wait until the beginning of the next time slot** before transmitting. Even when there is no collision, every frame suffers an extra waiting time, which is on average **half a slot (T/2)**, and up to one full slot.

### Comparison at low load (G → 0)

| | Pure ALOHA | Slotted ALOHA |
|---|---|---|
| Wait before sending | 0 | about T/2 on average |
| Retransmissions | almost none | almost none |
| Delay | ≈ T + Tₚ | ≈ T/2 + T + Tₚ |

Because retransmissions are negligible in both, the **slot-synchronisation wait** is the only difference, and it makes slotted ALOHA slower.

**Note:** at higher load the situation reverses. Slotted ALOHA has fewer collisions (vulnerable period T instead of 2T), so its delay grows more slowly and it becomes better than pure ALOHA.

Reasoning

Comparison at low load (G → 0)

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