Frequently Asked Questions on Pure ALOHA

What is ALOHA?

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....

Pure ALOHA

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....

Throughput of Pure ALOHA

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:...

Maximum Throughput of Pure ALOHA

The maximum throughput occurs when G=0.5...

Key Features of Pure ALOHA

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....

Conclusion

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....

Frequently Asked Questions on Pure ALOHA – FAQs

What is the full form of ALOHA?...