What is Pure ALOHA?

The ALOHA protocol was first developed at the University of Hawaii in the early 1970s for packet radio networks. However, it can be used in any situation where multiple devices share a common communication channel. This protocol allows devices to transmit data at any time, without a set schedule. This is known as a random access technique, and it is asynchronous because there is no coordination between devices. When multiple devices attempt to transmit data at the same time, it can result in a collision, where the data becomes garbled. In this case, each device will simply wait a random amount of time before attempting to transmit again. The basic concept of the ALOHA protocol can be applied to any system where uncoordinated users are competing for the use of a shared channel.

ALOHA is an early computer networking method created at the University of Hawaii in the early 1970s. It’s a straightforward way to send data over a shared medium, like a wireless or wired network. The main idea of ALOHA is how it handles collisions, which happen when two devices try to send data at the same time, causing interference.

There are two main versions of the ALOHA protocol:

In this article we going to discuss pure ALOHA in detail.

• Pure ALOHA refers to the original ALOHA protocol. The idea is that each station sends a frame whenever one is available. Because there is only one channel to share, there is a chance that frames from different stations will collide.
• The pure ALOHA protocol utilizes acknowledgments from the receiver to ensure successful transmission. When a user sends a frame, it expects confirmation from the receiver. If no acknowledgment is received within a designated time period, the sender assumes that the frame was not received and retransmits the frame. 
• When two frames attempt to occupy the channel simultaneously, a collision occurs and both frames become garbled. If the first bit of a new frame overlaps with the last bit of a frame that is almost finished, both frames will be completely destroyed and will need to be retransmitted. If all users retransmit their frames at the same time after a time-out, the frames will collide again. 
• To prevent this, the pure ALOHA protocol dictates that each user waits a random amount of time, known as the back-off time, before retransmitting the frame. This randomness helps to avoid further collisions.

• The time-out period is equal to the maximum possible round-trip propagation delay, which is twice the amount of time required to send a frame between the two most widely separated stations   (2 x Tp).
• Let all the packets have the same length. And each requires a one-time unit for transmission (tp). Consider any user to send packet A at a time. If any other user B has generated a packet between time (to), and (to + tp), the end of packet B will collide with the beginning of packet A. Since in a pure ALOHA packet, a station does not listen to the channel before transmitting, it has no way of knowing that the above frame was already underway.

• Similarly, if another user wants to transmit between (to, +tp) and (to +2tp) i.e. packet C, the beginning of packet C will collide with the end of packet A. Thus if two packets overlap by even the smallest amount in the vulnerable period both packets will be corrupted and need to be retransmitted.

The probability of successful transmission (S) can be derived from the probability that no other packets are sent during the vulnerable time period. This is given by:

S = G × e^-2G

where:

• S is the throughput (the average number of successful packet transmissions per packet time).
• G is the average number of packets generated by the system in one packet time .

The maximum throughput occurs when G=0.5

S_max=0.5 × e^-1 ≈ 0.184

This means the maximum throughput of Pure ALOHA is approximately 18.4%. In other words, only about 18.4% of the time is used for successful transmissions, and the rest is lost due to collisions.

• Random Access: Devices can send data whenever they have something to transmit, without needing to wait for a predetermined time slot.
• Uncoordinated Transmission: Devices do not coordinate with each other before transmitting. They simply attempt to send data whenever they have data to send.
• Simple Implementation: Pure ALOHA is straightforward to implement, making it suitable for early network experiments and scenarios with low traffic.
• Persistent Approach: Devices continue to attempt transmission even after a collision, using a form of exponential backoff. This means they introduce random delays before retrying, which helps reduce the chances of repeated collisions.
• Contention-Based: Since devices transmit without coordination, collisions may occur if two or more devices transmit simultaneously. Collisions are detected through feedback from the receiver or by the transmitting device itself.

For more details please refer Derive the efficiency of Pure ALOHA protocol and Differences between Pure and Slotted Aloha article.

Pure ALOHA is an early and simple method for sending data over a shared network, where devices transmit whenever they have data to send. It doesn’t check if the channel is free, leading to frequent collisions when two devices send data at the same time. This results in a maximum efficiency of about 18.4%, meaning many transmissions are lost due to these collisions.

What is the full form of ALOHA?

The full form of ALOHA is Advocates of Linux Open-source Hawaii Association.

What is ALOHA and its type?

ALOHA is a basic method for sending data over a shared network. It was created to manage how data is transmitted and to deal with collisions when two devices send data at the same time.

There are two types of ALOHA:

• Pure ALOHA
• Slotted ALOHA

What is the use of ALOHA?

ALOHA is used to transmit data over a public network channel.