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.
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.
Pure ALOHA procedure
- 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.
Vulnerable period for packet A
- 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.
Key Features of Pure ALOHA:
1. Random Access: Devices can send data whenever they have something to transmit, without needing to wait for a predetermined time slot.
2. Uncoordinated Transmission: Devices do not coordinate with each other before transmitting. They simply attempt to send data whenever they have data to send.
3. Simple Implementation: Pure ALOHA is straightforward to implement, making it suitable for early network experiments and scenarios with low traffic.
4. 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.
5. 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.