Pure ALOHA

Pure ALOHA is one of the earliest random access protocols developed for sharing a common communication channel. It was developed at the University of Hawaii in the early 1970s as part of the ALOHA system, which was designed to establish communication between the Hawaiian Islands using radio transmission.

What is Pure ALOHA?

Pure ALOHA is a simple medium access control (MAC) protocol that allows devices to transmit data whenever they have data to send, without checking if the channel is free or coordinating with other devices. If a collision occurs (when two or more devices transmit simultaneously), the devices involved wait for a random amount of time before retransmitting.

How Pure ALOHA Works

The operation of Pure ALOHA can be described as follows:

1. Transmission: When a device has data to send, it transmits immediately without checking if the channel is free.

2. Acknowledgment: After transmitting, the device waits for an acknowledgment (ACK) from the receiver.

3. Collision Detection: If the device does not receive an ACK within a specified timeout period, it assumes that a collision has occurred.

4. Backoff and Retransmission: After detecting a collision, the device waits for a random amount of time (backoff) before retransmitting. This random backoff helps reduce the probability of repeated collisions.

5. Repeat: The device continues this process until it successfully transmits its data or reaches a maximum number of retransmission attempts.

Vulnerable Time in Pure ALOHA

In Pure ALOHA, a collision occurs if another device starts transmitting during the vulnerable time, which is twice the frame transmission time. This is because:

• If another device starts transmitting during the current frame transmission, a collision occurs.
• If another device started transmitting before the current frame but is still transmitting when the current frame starts, a collision also occurs.

Therefore, the vulnerable time in Pure ALOHA is 2 × T, where T is the frame transmission time.

Throughput Analysis of Pure ALOHA

The throughput of Pure ALOHA can be analyzed using the following assumptions:

1. All frames have the same length.
2. Frames arrive according to a Poisson process with an average of λ frames per frame time.
3. The number of devices is infinite (to simplify the analysis).
4. The probability of a frame being involved in a collision is independent of its transmission history.

Given these assumptions, the throughput S (the average number of successful transmissions per frame time) can be calculated as:

S = G \times e^{-2G}

Where G is the offered load (the average number of transmission attempts per frame time).

The maximum throughput occurs when G = 0.5, giving a maximum throughput of:

S_{max} = 0.5 \times e^{-1} \approx 0.184

This means that the maximum channel utilization in Pure ALOHA is about 18.4%.

Example Scenario

Let's consider a simple example to understand how Pure ALOHA works:

1. Device A has data to send and transmits immediately.
2. While Device A is still transmitting, Device B also has data to send and starts transmitting.
3. The transmissions from Device A and Device B overlap, causing a collision.
4. Neither Device A nor Device B receives an acknowledgment within the timeout period.
5. Device A waits for a random time t₁ before retransmitting.
6. Device B waits for a random time t₂ before retransmitting.
7. If t₁ and t₂ are different enough, the retransmissions will not collide, and both devices can successfully transmit their data.

Advantages of Pure ALOHA

1. Simplicity: Pure ALOHA is very simple to implement, requiring minimal coordination among devices.

2. Decentralized: There is no central controller, making the system more robust to single points of failure.

3. Dynamic: Devices can join or leave the network without affecting the protocol's operation.

4. Low Delay under Light Load: When the network load is light, devices can transmit immediately without waiting, resulting in low delay.

Disadvantages of Pure ALOHA

1. Low Efficiency: The maximum channel utilization is only about 18.4%, which means that more than 80% of the channel capacity is wasted due to collisions and idle time.

2. Unstable under Heavy Load: As the network load increases, the number of collisions increases exponentially, leading to a decrease in throughput.

3. No Collision Detection: Devices do not check if the channel is free before transmitting, which increases the probability of collisions.

4. No Fairness Guarantee: There is no mechanism to ensure fair access to the channel, so some devices may dominate the channel while others starve.

Improvements to Pure ALOHA

Several improvements have been made to Pure ALOHA to address its limitations:

1. Slotted ALOHA

Slotted ALOHA improves upon Pure ALOHA by dividing time into discrete slots and requiring devices to transmit only at the beginning of a slot. This reduces the vulnerable time from 2 × T to T, doubling the maximum throughput to about 36.8%.

2. Carrier Sense Multiple Access (CSMA)

CSMA improves upon ALOHA by having devices listen to the channel before transmitting. If the channel is busy, the device waits until it becomes idle. This significantly reduces the probability of collisions.

3. CSMA with Collision Detection (CSMA/CD)

CSMA/CD, used in traditional Ethernet, adds collision detection to CSMA. Devices listen to the channel while transmitting and stop immediately if they detect a collision. This further improves efficiency by reducing the time wasted in collisions.

4. CSMA with Collision Avoidance (CSMA/CA)

CSMA/CA, used in wireless networks like Wi-Fi, tries to avoid collisions by using techniques like random backoff before transmission and the Request to Send (RTS) / Clear to Send (CTS) mechanism.

Applications of Pure ALOHA

Despite its limitations, Pure ALOHA and its variants have been used in various applications:

1. Early Packet Radio Networks: The original ALOHA system was used to establish communication between the Hawaiian Islands.

2. Satellite Communication: ALOHA-based protocols have been used in satellite communication systems, where the propagation delay is significant.

3. RFID Systems: Some RFID systems use ALOHA-based protocols for tag identification.

4. Low-Power Sensor Networks: In some low-power sensor networks, simplified versions of ALOHA are used to minimize energy consumption.

Conclusion

Pure ALOHA is a simple and pioneering random access protocol that allows devices to transmit data whenever they have data to send, without coordination. While it has limitations in terms of efficiency and stability under heavy load, it laid the foundation for more advanced protocols like Slotted ALOHA, CSMA, CSMA/CD, and CSMA/CA, which are widely used in modern networks.

The simplicity of Pure ALOHA makes it a good starting point for understanding more complex MAC protocols, and its principles are still relevant in certain applications where simplicity and low delay under light load are more important than maximum channel utilization.