Pure ALOHA vs Slotted ALOHA: Differentiation, Throughput, and Worked Examples

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Nov 4, 2026

ALOHA is a foundational multiple-access scheme used to motivate collision-based wireless networking. Two classic variants are Pure ALOHA and Slotted ALOHA.

The key difference is timing structure, which changes the vulnerable period during which a new transmission can collide with an ongoing one. In Pure ALOHA, any overlap in time can cause a collision; in Slotted ALOHA, aligning to slot boundaries reduces the overlap cases.

type="tip" title="Pro Tip" content="When comparing ALOHA variants, always compare the vulnerable time window (how long an incoming attempt would overlap an existing frame). That single timing change drives the throughput difference."

Core timing difference (the heart of the protocols)

In Pure ALOHA, a station may begin a frame at any instant.
In Slotted ALOHA, a station may begin a frame only at times aligned to a common global slot clock.

This difference implies:

Pure ALOHA vulnerable period: if a frame of duration $T$ is in progress, another transmission starting up to $T$ before it begins or up to $T$ after it begins may overlap it. Hence the vulnerable interval is $2T$ .
Slotted ALOHA vulnerable period: transmissions can start only at slot boundaries, so the second transmission can only collide if it starts in the same slot. Hence the vulnerable interval corresponds to $T$ rather than $2T$ (fewer collision scenarios).

Worked examples: When collisions happen

1
Step 1
Let a frame duration be $T$ . For Slotted ALOHA, assume all nodes start frames exactly at times $0, T, 2T, \dots$ .
2
Step 2
Node A starts at time $t=0.3T$ and transmits for duration $T$ (until $1.3T$ ). Node B starts at time $t=0.9T$ . Since B’s frame overlaps A’s from $0.9T$ to $1.3T$ , the receiver sees a collision.
3
Step 3
Node A starts at $t=1.0T$ and ends at $2.0T$ . Node B starts at $t=0.2T$ and ends at $1.2T$ . They overlap from $1.0T$ to $1.2T$ , so this is also a collision.
4
Step 4
Assume Node A and Node B both begin exactly at slot boundary $t=2T$ and transmit until $3T$ . Since their frames overlap fully, the slot results in a collision.
5
Step 5
Node A begins at $t=2T$ (ends at $3T$ ). Node B begins at the next boundary $t=3T$ (ends at $4T$ ). They do not overlap, so this is collision-free.
6
Step 6
Pure ALOHA allows many partial-overlap collision cases due to arbitrary start times; Slotted ALOHA restricts starts to boundaries, eliminating all but same-slot overlap.

Differentiating statements (summary table)

Aspect	Pure ALOHA	Slotted ALOHA
Start time	Any instant	Only at slot boundaries
Vulnerable period (concept)	$2T$ overlap window	$T$ overlap window
Collision pattern	Many partial-overlap collisions	Collisions occur only when multiple transmissions pick the same slot
Practical requirement	No time synchronization required	Requires loose synchronization to slot boundaries

These distinctions are what lead to slotted ALOHA achieving higher maximum throughput than pure ALOHA under typical traffic assumptions (see throughput section below).

Throughput intuition using offered load $G$

To reason quantitatively, network analysis often models packet/frame arrivals as a Poisson process and uses offered load $G$ as the expected number of transmission attempts per vulnerable window.

Let:

$T$ = frame transmission time
$G$ = average number of attempts per frame time (normalized)

Then the probability that no one else transmits during the vulnerable window controls the probability of a successful transmission:

For Pure ALOHA, vulnerable window is $2T$ , so “no other transmission” corresponds to probability $e^{-2G}$ (conceptually: zero arrivals in a window twice as long).
For Slotted ALOHA, vulnerable window is $T$ , so success corresponds to probability $e^{-G}$ .

This yields classic success/throughput forms:

Pure ALOHA throughput: $S_{\text{pure}}(G) = G e^{-2G}$
Slotted ALOHA throughput: $S_{\text{slotted}}(G) = G e^{-G}$

From these curves, the maximum throughput for Slotted ALOHA is about twice that of Pure ALOHA (at the $G$ values that maximize each expression), reflecting the shorter vulnerable period in the slotted design.

Throughput vs Offered Load (Intuition)

Shows why slotted ALOHA can outperform pure ALOHA: success probability decays with $e^{-2G}$ vs $e^{-G}$ .

Visual comparison of collision windows

Common edge questions

How a packet attempt proceeds in ALOHA

Queue non-empty

1) Backlogged data exists

A node has a frame ready to send."

Pure vs slotted decision

2) Choose transmission time

Pure starts immediately; slotted waits for the next slot boundary."

Competing transmissions

3) Possible collision

If another node transmits during the vulnerable window, the receiver cannot decode."

ACK/timeout

4) Feedback

Successful reception yields ACK; otherwise the sender times out and retransmits."

Knowledge Check

Question 1 of 4

Q1Single choice

In Pure ALOHA, a collision can occur if another transmission starts within which time window relative to a given frame of duration $T$ ?

A vulnerable window of $T$ before and $T$ after (total $2T$ overlap risk)

Only at exactly the same starting instant

Only $T$ after the frame starts

Only when the receiver is off-line

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