ALOHA in computer networks is a family of random access protocols for sending data over a shared channel without a central controller. ALOHA falls particularly in data-link layer multiple access protocols, and its name comes from the Hawaiian word ‘Aloha’, but in networking, it refers to the protocol, not a greeting. Developed at the University of Hawaii in the 1970s by Norman Abramson and his associates, the basic principle behind ALOHA is that each station transmits whenever it has data available to it.
But in this case, if a collision occurs (two or more frames or data packets sent at the same time), the frames get corrupted and retransmitted after a random delay. The fundamental problem ALOHA addresses here is the collision. ALOHA in computer networking uses a simple acknowledgment and retransmission system to deal with these conflicts. In this article, we will learn about what is ALOHA in computer Networks, its types, and its historical significance and role in modern networking.
Table of Contents:
What is ALOHA in Computer Networks?
“ALOHA is a simple communication protocol in computer networks that allows devices to transmit data anytime and handles collisions through retransmissions.“
The multiple access protocol ALOHA (Advocates of Linux Open-Source Hawaii Association) is used to send data over a public network channel. It functions within the Open Systems Interconnection (OSI) model’s Medium Access Control (MAC) sublayer.
How ALOHA Handles Collisions?
Transmission: A station sends a frame whenever it has data ready to transmit.
Acknowledgement (ACK): The sending station waits for the acknowledgment from the receiver to confirm that the transmission of the frame was done successfully.
No ACK Received: If the ACK is not received within a specific set duration, the sender assumes a collision has occurred and the frame was destroyed.
Random Delay: The sender waits for a random amount of time, known as the “back-off” period, prior to retransmitting the frame. This delay is important to prevent the same frames from colliding repeatedly.
Types of ALOHA
Two types of ALOHA are Pure ALOHA, where transmission is continuous, and Slotted ALOHA, which divides time into discrete slots to reduce collisions.
Pure ALOHA
Transmission: In pure ALOHA, the stations can transmit data in a continuous stream at any time.
Collision risk: The probability that a collision may occur is higher here because a frame can be destroyed by any other frame that starts sending data during a vulnerable period twice the length of one frame’s transmission time.
Efficiency: As collisions occur here frequently, the maximum theoretical efficiency is only 18.4%.
Slotted ALOHA
Transmission: In slotted ALOHA, time is split into discrete intervals called slots, with a station only allowed to transmit at the start of a slot. All stations in this must be synchronized to these slots.
Collision risk: The risk of collision here is less because frames that start in different time slots have less chance to collide with each other. Collisions can only occur if multiple stations are transmitting in the same time slot as others, which is rare. As a result, the vulnerable period is reduced to half as compared to Pure ALOHA.
Efficiency: As the chances of collision are reduced, the maximum theoretical efficiency doubles up to 36.8%.
Legacy and Relevance of ALOHA
ALOHA was a foundational concept that influenced the development of many other networking technologies but it is highly inefficient compared to modern protocols.
Influence on modern protocols: The principle of random access and collision handling lays the foundation for the more advanced Carrier Sense Multiple Access (CSMA) protocol, which can be seen in early versions of Ethernet.
Historical applications: ALOHA was originally developed for connecting networks in the Hawaiian Islands wirelessly. But all its principles were also used in early satellite communication networks.
Conclusion
ALOHA marked a major milestone in the evolution of computer networking by introducing the concept of random access communication. Its simple yet powerful idea, allowing multiple devices to share a common communication medium without centralized control, became the foundation for many modern network protocols. Though Pure and Slotted ALOHA are not widely used today due to their limited efficiency, their principles directly inspired the development of advanced protocols like CSMA/CD and CSMA/CA, which power Ethernet and Wi-Fi networks. In essence, ALOHA’s legacy lives on as the starting point of today’s wireless and distributed communication systems, proving that even simple ideas can revolutionize technology.
Frequently Asked Questions
1. What is the main drawback of ALOHA?
The biggest drawback of ALOHA is high collision probability, as multiple devices may send data at the same time, leading to data loss.
2. Where is ALOHA protocol used today?
ALOHA concepts are used in Wi-Fi, satellite communication, and RFID systems, where multiple devices share a common channel.
3. How does the ALOHA protocol work?
In ALOHA, each device sends data whenever it has something to transmit. If a collision happens, the device waits for a random time and tries again.
4. What is the efficiency of ALOHA?
Pure ALOHA has a maximum efficiency of 18.4%, while Slotted ALOHA improves it to about 36.8% because it reduces the chances of collisions.
5. What problem does ALOHA solve?
ALOHA helps multiple users share a single communication channel without needing a fixed schedule, making data transmission easier and more flexible.