Working procedure of Pure ALOHA

                +-------------------+
                |  Station has a    |
                |  frame ready?     |
                +-------------------+
                          |
                         Yes
                          v
                +-------------------+
                | Transmit the      |
                | frame immediately |
                +-------------------+
                          |
                          v
                +-------------------+
                | Wait for ACK      |
                | (for max RTT +    |
                |  processing time) |
                +-------------------+
                          |
              +-----------+-----------+
              |                       |
         ACK received            No ACK / timeout
              |                       |
              v                       v
    +------------------+   +------------------------+
    | Transmission      |   | Collision assumed.     |
    | successful. Done. |   | Wait a RANDOM backoff  |
    +------------------+   | time (to avoid repeated |
                           | collisions)              |
                           +------------------------+
                                       |
                                       v
                           +------------------------+
                           | Retransmit the frame    |
                           +------------------------+
                                       |
                          (loop back to "Wait for ACK")

Explanation

1. Whenever a station has a frame ready, it transmits immediately, without checking if the channel is busy and without waiting for any time slot (unlike slotted ALOHA).
2. After transmitting, the station waits for an acknowledgement from the receiver, allowing enough time for the frame to arrive and an ACK to return (round-trip time).
3. If the ACK is received within this window, the transmission is considered successful.
4. If no ACK arrives (implying the frame likely collided with another station's simultaneous transmission), the station assumes a collision occurred.
5. To avoid the colliding stations repeatedly retransmitting at the same moment (which would cause an infinite collision loop), each station waits a random backoff period before retransmitting.
6. The frame is then retransmitted, and the process repeats from the "wait for ACK" step until it eventually succeeds.

Because stations transmit with no coordination whatsoever, a collision can occur if another station starts transmitting anywhere within one frame-time before or after this station's own transmission — giving pure ALOHA its characteristic 2 x Tfr vulnerable time and correspondingly low maximum throughput (~18.4%).