What is Pure ALOHA?

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

1. Transmission: A user sends a packet immediately when it is ready.
2. Acknowledgment: The receiver responds with an ACK if the frame is received correctly.
3. Collision Handling: If no ACK is received, the sender assumes a collision occurred.
4. 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 (t_p) to transmit.
• If a user starts sending a packet at time t0,
• Any packet generated in the interval (t0, t0 + t_p) will collide with the beginning of the original packet.
• Any packet generated in (t0 + t_p, t0 + 2t_p) will collide with the end of the original packet.

Note: The vulnerable period for a packet in Pure ALOHA is 2 × t_p. 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): S = G \times e^{-2G}

where:

• \text{S = }throughput (successful transmissions per packet time).
• \text{G = }average number of packets generated per packet time.

Maximum Throughput: Throughput is maximized when\text{G = 0.5}, S_{\text{max}} = 0.5 \times e^{-1} \approx 0.184. This means that only 18.4\% 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 18.4\%).
• High probability of collisions under heavy traffic.
• Unsuitable for real-time applications requiring guaranteed delivery.