Pure ALOHA: Flowchart and Working Procedure
Pure ALOHA (unslotted ALOHA) is a random-access channel access method where stations transmit whenever they have a packet, without waiting for time slots. A transmission is successful only if no other station transmits during the vulnerable period of that packet (for pure ALOHA, the vulnerable period spans , where is the packet transmission time). Packets that experience collisions are typically retransmitted after a random backoff interval. 2
Key ideas you will use to draw and explain the working procedure:
- keywordUnsynchronized random access where transmissions start at arbitrary times.
- keywordTime window in which any overlap with another transmission causes a collision.
- keywordTwo or more overlapping transmissions at the receiver so the receiver cannot decode.
Mermaid overview of the working loop (per station):
Footnotes
-
ALOHA protocol (overview of pure ALOHA and vulnerable period concepts) - https://en.wikipedia.org/wiki/Aloha_(protocol) - Explains unslotted/pure ALOHA and vulnerable period reasoning. ↩
-
Throughput and vulnerability window for ALOHA (success condition tied to overlap in vulnerable period) - https://en.wikipedia.org/wiki/ALOHA - Provides collision/vulnerable period context and performance discussion for ALOHA. ↩
ALOHA (Pure and Slotted) overview
From flowchart to procedure (what each step means)
In pure ALOHA, each station behaves independently. There is no global frame/slot boundary; the vulnerable period is determined purely by packet duration . If another transmission begins up to earlier or up to later than the start of your packet, their transmissions overlap for at least part of your packet, creating a collision. 2
This is why the working procedure can be explained in terms of:
- Immediate transmission (no slot waiting),
- Observation of success/failure (often via acknowledgment, or inferred failure),
- Retransmission with random backoff (to reduce repeated collisions).
Footnotes
-
ALOHA protocol (overview of pure ALOHA and vulnerable period concepts) - https://en.wikipedia.org/wiki/Aloha_(protocol) - Explains unslotted/pure ALOHA and vulnerable period reasoning. ↩
-
Throughput and vulnerability window for ALOHA (success condition tied to overlap in vulnerable period) - https://en.wikipedia.org/wiki/ALOHA - Provides collision/vulnerable period context and performance discussion for ALOHA. ↩
Working procedure of Pure ALOHA (per station)
- 1Step 1
A station gets a new packet to send (or has data waiting). There is no requirement to align to a time slot.
- 2Step 2
The station starts transmitting right away. Let the packet transmission time be .
- 3Step 3
Your packet is vulnerable if any other station transmits such that their transmissions overlap within your vulnerable period of length .
- 4Step 4
The station determines whether the transmission succeeded (commonly: it receives an ACK; if not, treat as a collision/unsuccessful reception).
- 5Step 5
If the packet failed, select a random waiting time (backoff) and then retransmit.
- 6Step 6
If successful, stop retransmitting; return to waiting for the next packet.
Pro tip: How to draw the flowchart correctly
Make the decision node depend on whether any other transmission overlaps during . In the diagram, that’s the logic behind the “Successful reception?” branch.
Common mistake: confusing pure ALOHA with slotted ALOHA
In pure ALOHA there are no discrete slots; packet start times can be anywhere in time. That changes the vulnerable period to (whereas slotted ALOHA uses a shorter vulnerable window).
Visual flowchart you can copy into notes
This is a clearer, exam-friendly flowchart showing what happens at the transmitting station.
Timeline intuition (packet vulnerable period in pure ALOHA)
Station S begins transmitting
t = 0Start of packet transmission duration ."
Overlaps if others start during first half
0 < t < TIf another station begins at any time in this range, transmissions overlap."
Overlap can still happen after your packet has finished
t = TBecause another start within the next can overlap with the tail portion during reception."
Vulnerable period ends
t = 2TIf no other transmission overlaps during , reception is successful."
Success probability intuition (why collisions dominate at high load)
Pure ALOHA success probability is tied to avoiding any interfering transmission in the vulnerable period.
Clarifying details
Pure ALOHA quick checks
Knowledge Check
In pure ALOHA, stations transmit without waiting for __________.