What is slotted ALOHA in computer networks?

ALOHA is a random access protocol used in packet switching networks for managing how multiple stations share a common communication channel. The protocol is named after the Hawaiian greeting "Aloha" and was developed at the University of Hawaii.

There are two main variants of the ALOHA protocol:

• Pure ALOHA − Stations can transmit data at any time
• Slotted ALOHA − Stations can only transmit at specific time intervals

What is Slotted ALOHA?

Slotted ALOHA is an improved version of Pure ALOHA that reduces collisions by organizing transmission times into discrete time slots. In this protocol, the shared channel time is divided into fixed-duration intervals called slots, where each slot corresponds to the time required to transmit one packet.

How Slotted ALOHA Works

The operation of Slotted ALOHA follows these key principles:

• Time synchronization − All stations are synchronized to the slot boundaries
• Slot-based transmission − Stations can only begin transmission at the start of a time slot
• One packet per slot − Each slot can accommodate exactly one packet transmission
• Collision handling − If multiple stations transmit in the same slot, a collision occurs and packets are retransmitted later

Advantages and Disadvantages

Performance Analysis

The maximum throughput efficiency of Slotted ALOHA can be calculated mathematically. The vulnerable period (time during which collisions can occur) is reduced to one slot duration.

The maximum throughput is achieved when:

S_max = 1/e = 0.368 (approximately 36.8%)

This means that under optimal conditions, Slotted ALOHA can achieve a maximum channel utilization of approximately 37%.

Conclusion

Slotted ALOHA improves upon Pure ALOHA by organizing transmissions into discrete time slots, reducing collision probability and doubling the maximum throughput from 18% to 37%. However, it requires precise time synchronization among all stations participating in the network.