Differences between Pure Aloha and Slotted Aloha

The Aloha Protocol allows several stations to send data frames over the same communication channel at the same time. This protocol is a straightforward communication method in which each network station is given equal priority and works independently.

Aloha is a medium access control (MAC) protocol for transmission of data via a shared network channel. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel.

There are two types of Aloha protocols ? Pure Aloha and Slotted Aloha. In this article, we will highlight the major differences between these two protocols and understand their working mechanisms.

What is Pure Aloha?

Pure Aloha is the basic form of Aloha contention mechanism, in which demand-driven data frames from numerous stations are sent through a shared channel. It was first used at the University of Hawaii in 1970, under the direction of Norman Abramson.

• Continuous transmission − Stations can transmit at any time when they have data to send.

• Collision handling − If frames collide, senders wait for a random backoff time before retransmitting.

• Acknowledgment mechanism − Stations wait for acknowledgment and retransmit if not received within timeout period.

Due to the lack of coordination between stations, collision probability is high as stations transmit independently without checking channel availability.

What is Slotted Aloha?

Slotted Aloha was introduced in 1972 by Roberts as an improvement over Pure Aloha. It divides time into discrete slots to reduce collision probability.

• Time synchronization − Time is divided into discrete intervals called slots, each corresponding to one frame transmission time.

• Transmission rules − Stations can only transmit at the beginning of a time slot and must wait for the next slot if they miss the current one.

• Collision reduction − Collisions can only occur at slot boundaries, reducing the vulnerable time by half.

If multiple stations attempt to transmit at the beginning of the same slot, collision still occurs, but the overall collision probability is significantly reduced compared to Pure Aloha.

Key Differences

Efficiency Analysis

The throughput efficiency of Pure Aloha is given by S = G × e^(-2G), where G is the offered load. The maximum efficiency occurs at G = 0.5, yielding 18.4% efficiency.

For Slotted Aloha, the efficiency is S = G × e^(-G), with maximum efficiency of 36.8% at G = 1. This represents a doubling of channel utilization compared to Pure Aloha.

Conclusion

Slotted Aloha significantly improves upon Pure Aloha by introducing time synchronization and discrete transmission slots. While both are simple random access protocols, Slotted Aloha doubles the maximum throughput efficiency from 18.4% to 36.8% by reducing collision probability.