Pure ALOHA is the original form of the ALOHA protocol. In this approach, A station transmits a frame whenever it has data to send. Since there is only one shared channel, there is always a risk of collisions between frames from different stations.
Note: To ensure reliable communication, the receiver sends an acknowledgment (ACK) for each correctly received frame. If the sender does not receive an ACK within a time-out period, it assumes a collision has occurred and retransmits the frame.
Key Mechanism
Transmission: A user sends a packet immediately when it is ready.
Acknowledgment: The receiver responds with an ACK if the frame is received correctly.
Collision Handling: If no ACK is received, the sender assumes a collision occurred.
Random Back-off: Before retransmitting, the sender waits for a random back-off time to reduce the chance of repeated collisions.
Features of Pure ALOHA
Random Access: Devices can send data anytime without waiting for a time slot.
Uncoordinated Transmission: No central authority or coordination mechanism exists.
Simplicity: Easy to implement and suitable for low-traffic systems.
Persistent Retransmission: Devices retransmit after a random delay if a collision occurs.
Contention-Based: Collisions are inherent and resolved using acknowledgment and retransmission.
Vulnerable Time in Pure ALOHA
Collisions in Pure ALOHA occur if packets overlap at any point.
Let each packet take 1 time unit (tp) to transmit.
If a user starts sending a packet at time t0,
Any packet generated in the interval (t0, t0 + tp) will collide with the beginning of the original packet.
Any packet generated in (t0 + tp, t0 + 2tp) will collide with the end of the original packet.
Note: The vulnerable period for a packet in Pure ALOHA is. If another transmission starts within this period, both packets will be destroyed.
Throughput Analysis of Pure ALOHA
The efficiency of Pure ALOHA can be calculated using probability.
Throughput (S):
where:
throughput (successful transmissions per packet time).
average number of packets generated per packet time.
Maximum Throughput: Throughput is maximized when, . This means that only of the channel capacity is effectively used for successful transmission, while the rest is wasted due to collisions.
Advantages
Simple to design and implement.
Suitable for low-traffic scenarios.
Works well in environments with unpredictable transmission times.
Disadvantages
Very low efficiency (maximum).
High probability of collisions under heavy traffic.
Unsuitable for real-time applications requiring guaranteed delivery.
