ALOHA is a random access protocol used in wireless communication networks, particularly for data transmission between multiple devices over a shared medium. It was first introduced in the 1970s by the University of Hawaii for their radio network, hence the name ALOHA. The protocol allows devices to transmit data at random times without prior coordination with other devices. If a collision (when two devices transmit at the same time) occurs, the devices wait for a random time before retransmitting the data.

There are two primary types of ALOHA protocols:

1. Pure ALOHA:

• Overview: In Pure ALOHA, devices transmit data whenever they have data to send without checking if the channel is free or busy.
• Collision Handling: If two devices transmit data at the same time, a collision occurs, and both devices must retransmit the data after waiting for a random time interval.
• Efficiency: Pure ALOHA has a low channel efficiency due to a higher chance of collisions. The maximum efficiency of Pure ALOHA is approximately 18.4%, meaning only 18.4% of the channel bandwidth is used effectively.

2. Slotted ALOHA:

• Overview: Slotted ALOHA improves upon Pure ALOHA by dividing the time into discrete slots. Devices can only begin transmitting at the start of a time slot.
• Collision Handling: If two devices attempt to transmit in the same time slot, a collision occurs, and both devices must retransmit their data after a random delay.
• Efficiency: Slotted ALOHA is more efficient than Pure ALOHA because collisions only occur if two devices attempt to transmit in the same time slot. The maximum efficiency of Slotted ALOHA is around 36.8%.

Comparison:

• Pure ALOHA: No time slots, higher collision probability, and lower efficiency.
• Slotted ALOHA: Time slots, reduced collision probability, and higher efficiency.

Both ALOHA variants laid the foundation for more advanced multiple access protocols, such as Carrier Sense Multiple Access (CSMA), used in modern networks like Ethernet and Wi-Fi.